CHAPTER-19 A Workover.pdf

CHAPTER-19 WORK-OVER RIG AND OPERATIONS INTRODUCTION Works over operations are basically remedial measures that are carried out in a well to add/ restore the production from the well. The work over performance is gauged through Work-Over-Index which is defined as average number of wells worked over per rig per year.

NEED FOR WORK-OVER Several problems contribute to a decrease in productivity from a well such as: 1. Well bore and reservoir problems: a. Reservoir problems – Low reservoir pressure & small pay zone thickness b. Fluid related problems – water & gas coning, formation of precipitates & scale, flow of heavy viscous oil c. Around well bore problems – accumulation of formation fines in well bore & plugging of perforations 2. Casing damage/ leak or wellhead component/ seal failures 3. Production of sand, paraffin and scale deposition in flow conduit 4. Failure of completion or A/L equipment. 5. Bad cementation and channeling behind casing resulting in production of undesired water and gas from other layers 6. Loss of hydrocarbon of present layer to other layer. All the above problems cause a loss in production. In addition, the casing damage and well head or completion equipment failures lead to unsafe well conditions that need earliest possible mitigation. Works over jobs are necessary to restore/ increase production in a safe manner.

DIFFERENT TYPES OF WORK-OVER RIG: WORKOVER RIGS: Land-based work-over rigs (commonly referred to as well servicing rigs), there are truck-mounted mast with winches, cables and sheaves capable of pulling tubing, as well as performing the other functions of a WORKOVER. It consists of a mobile carrier, engine, draw-works and a mast. The primary function of a work-over rig is to act as a hoist so that pipe, sucker rods and down-hole equipment can be run into and out of a well. Land-based work-over rigs are easier to move between well sites and different geographical areas of operations than drilling rigs. Typically, the rigs are self-propelled and have less auxiliary equipment to move. Because of size and cost considerations, work-over rigs are used for these operations rather than the larger drilling rig. Fig 19-01 &19-02 shows land based work-over rigs before Mast-up & after Mast-up. These rigs are moved from one location to the other in mast down condition. PLATFORM RIGS or MODULAR RIG: A platform is a stationary offshore oil and/or gas production facility. Platform rigs or Modular Rigs are designed to provide offshore work-over, drilling and re-entry services at these facilities. These rigs have drilling and/or work-over equipment and machinery arranged in modular packages those are transported to, and assembled and installed on, fixed offshore platforms. Fixed offshore platforms are steel-like structures that either stand on the ocean floor or are moored floating structures. The top portion, or platform, sits above the water 353

level and provides the foundation upon which the rig is placed; it is shown at Fig 1903. JACK-UP RIGS: Jack-up rigs, Fig19-04, are mobile, self-elevating, offshore drilling and work-over platforms equipped with legs that can be lowered to the ocean floor until a foundation is established to support the hull, which contains the drilling and/or work-over equipment, jacking system, crew quarters, loading and unloading facilities, storage areas for bulk and liquid materials, helicopter landing deck and other related equipment. Many of our jack-up rigs are of cantilever design, a feature that permits the drilling platform to be extended out from the hull, allowing it to perform drilling or work-over operations over adjacent, fixed platforms. The water depth limit of a particular rig is determined by the length of the rig’s legs and the operating environment. Moving a rig from one drill site to another involves lowering the hull down into the water until it is afloat and then jacking up its legs with the hull floating. The jack-up rig is then towed to the new drilling site. INLAND BARGE RIGS: Inland barge rigs, Fig19-05, are mobile, self-contained, drilling and/or work-over vessels. When moved from one location to another, the barge floats; when stationed on the drill or work-over site, the barge is submerged to rest on the bottom. Typically, inland barge rigs are used to drill or work-over wells in marshes, shallow inland bays and offshore where the water covering the drill site is not too deep.

COMPONENTS OF A DRILLING/ WORK-OVER RIG: Fig19-06 shows different components of a drilling/ work-over rig. There may be a few components those may not be the part of a work-over rig due to the specific operation being carried by that rig. Following is the descriptions of the different components those certainly be there at the drilling rig, but may be optional at the work over rig based on its application, please refer Fig19 -06: 01. Mud Tank: A mud tank is an open-top container, typically made of steel, to store drilling fluid on a rig. They are also called mud pits, because they used to be nothing more than pits dug out of the earth. 02. Shale Shakers: Shale shakers are devices that remove drill cuttings from the drilling fluid that is used for boring/ drilling/ servicing the holes into the earth. 03. Suction Pipe: A suction line is the pipe work linking the mud tanks /pits with the mud pumps. This may be gravity fed or charged by centrifugal pumps to provide additional volumetric efficiency to the mud pumps. 04. Mud Pump: A mud pump is a reciprocating piston/ plunger device designed to circulate drilling fluid under high pressure down the drilling string and back up the annulus. 05. Power Source: Power source can be engine of the prime mover or a separate electrical motor. This gives power to the draw works which in tern up & 06. Hose: These are the flexible hoses to carry mud to goose-neck or swivel/ top drive, to hole. 07. Draw-Works: A draw-works is the primary hoisting machinery. Its main function is to provide a means of raising and lowering the travelling blocks. The wire-rope drilling line winds on the draw-works drum and extends to the crown block and traveling blocks, allowing the drill string to be moved up and down as the drum turns. The wire-rope enters the sheaves of the crown block and is makes several passes between the crown block and traveling block 354

pulleys for mechanical advantage. The line then exits the last sheave on the crown block and is fastened to a derrick leg on the other side of the rig floor. This section of drilling line (wire-rope) is called the "dead line". 08. Stand-pipe: Stand-pipe is a thick metal tubing, situated vertically along the derrick, that facilitates the flow of drilling fluid and has attached to it and supports one end of the kelly hose. 09. Kelly Hose: A Kelly hose (also known as a mud hose or rotary hose) is a flexible, steel reinforced, high pressure hose that connects the standpipe to the kelly (or more specifically to the goose-neck on the swivel above the kelly) and allows free vertical movement of the kelly while facilitating the flow of drilling fluid through the system and down the drill string/ tubular into the hole. 10. Goose-neck: Goose-neck is a thick metal elbows connected to the swivel and standpipe that supports the weight of and provides a downward angle for the kelly hose to hang from. 11. Travelling Block: A Travelling Block is the free moving section of a block and tackle that contains a set of pulleys or sheaves through which the drill line (wire rope) is threaded or reeves and is opposite (and under) the crown block (the stationary section). 12. Drill line: The drill line is a multi-thread, twisted wire rope that is threaded or reeves through the travelling block and crown block to facilitate the lowering and lifting of the drill string into and out of the wellbore. 13. Crown Block: A Crown Block is the stationary section of a ‘block and tackle’ that contains a set of pulleys or sheaves through which the drill line (wire rope) is threaded or reeves and is opposite and above the travelling block, refer Fig1907 A & B. 14. Derrick: A derrick is the support structure for the equipment used to lower and raise the drill string into and out of the wellbore. 15. Monkey board: The Monkey board the structure used to support the top-end of the stand of drill pipe vertically situated in the derrick. 16. Stand: Stand is the sections of 2 or 3 joints of drill pipe or tubings connected together and stood upright in the derrick. When pulling out of the hole, instead of laying down each joint of drill pipe, 2 or 3 joints are left connected together and stood in the derrick to save time. 17. Pipe Rack: A Pipe rack is a set of steel framed structures (typically triangularly shaped) over which drill pipes, tubings or other tubular are stacked on for storage prior to running-in or after pulling-out of the hole. Pipe rack also refers to the wooden pad on the drill floor where the stands of the drill pipe/ tubings are vertically stacked when tripping pipe (pulling pipe out of or running pipe into the wellbore). 18. Swivel: A Swivel is a mechanical device used on a drilling rig/ work-over rig that hangs directly under the travelling block and directly above the kelly, that provides the ability for the kelly (and subsequently the drill string) to rotate while allowing the traveling block to remain in a stationary rotational position (yet allow vertical movement up and down the derrick) while simultaneously allowing the introduction of drilling fluid into the drill string. 19. Kelly drive: A kelly drive refers to a type of well drilling device on an oil drilling rig/ work-over rig that employs a section of pipe with an outer surface that is square, hexagonal or octagonal, which passes through the kelly bushing and rotary table. This bushing is rotated via the rotary table and thus the pipe and the attached drill string turn. When drilling, the drill bit is attached at the end of the drill string and thus the kelly drive provides the means to turn the bit (assuming that a down-hole motor is not being used). 355

20. Rotary table: A rotary table is a mechanical device on a drilling rig that provides clockwise (as viewed from above) rotational force to the drill string to facilitate the process of drilling a borehole or performing milling/ drilling operations. 21. Drill floor: The Drill Floor is the heart of any drilling rig/ work-over rig and is also known as the pad. This is the area from where the drill string begins its trip into the earth. It is traditionally where joints of pipe are assembled, as well as the BHA (bottom-hole assembly), drilling bit, and various other tools. This is the primary work location for work-over or drilling crew and driller/ tool pusher. The drill floor is located directly under the derrick. 22. Bell nipple: A Bell-nipple is a section of large diameter pipe fitted to the top of the blow out preventer that the flow lines attaches to via a side outlet, to allow the drilling fluid to flow back over the shale shakers & or to the mud tanks. 23. & 24. Blow Out Preventer (BOP) – Pipe-Rams & Blind-Rams: Blow out Preventer (BOPs) are devices installed at the wellhead to prevent fluids and gases from unintentionally escaping from the wellbore. This can be the annular type (often referred to as Hydril named after a manufacturer), and the pipe rams and blind rams type, but the object in both cases remains the same. During drilling or well interventions, the BOP may be closed if overpressure from an underground zone causes formation fluids such as oil or natural gas to enter the wellbore and threaten the rig. By closing BOP (usually operated remotely via hydraulic actuators), the drilling crew/ work-over crew can prevent uncontrolled pressure release, thus regaining control of the down-hole pressure, by means of increasing mud weight, mud circulation or any other appropriate action(s) required based on the reasons/ situation. Once this is accomplished, often the drilling mud density within the hole can be increased until adequate fluid pressure is being placed on the influx zone, and the BOP can be opened for operations to resume. Fig-08 shows two sets of Blow out Preventers (BOP). 25. Drill String: A drill string on a drilling rig/ work-over rig is a column, or string, of drill pipe that transmits drilling fluid (via the mud pumps) and rotational power (via the kelly drive or the top drive) to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit. The drill string is hollow so that drilling fluid can be pumped down through it and circulated back up the annulus (void between the drill string and the formation/ casing). 26. Drill Bit: A Drill Bit, Fig -09, is a device attached to the end of the drill string that breaks apart, cuts or crushes the rock formations when drilling a wellbore (water, gas or oil). The drill bit is hollow and has jets to allow for the expulsion of the drilling fluid at high velocity and high pressure to help clean the bit and help to break apart the rock (for softer formations). 27. Casing Head: A casing head is a simple metal flange welded or screwed on to the top of the conductor pipe or the casing and forms part of the wellhead system for the well. It is the primary interface for the surface pressure control equipment, such as the blowout preventers (for well drilling) or the Christmas tree (for well production). The casing head, when installed, is typically tested to very strict pressure and leak-off parameters to insure viability under blowout conditions, before any surface equipment is installed. 28. Flow line: A flow line, used on a drilling rig, is a large diameter pipe (typically a section of casing) that is connected to the bell nipple (under the drill floor) and extends to the possum belly (on the mud tanks) and acts as a return line, (for the drilling fluid as it comes out of the hole), to the mud tanks.

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WORK-OVER OPERATIONS As mentioned above there can be several reasons to carry out a work over operations. Some of them are: WATER SHUT OFF (WSO) During the producing life of a field, a well may start producing oil along with high percentage of water. This causes loss of revenue in terms of less amount of oil produced and requires larger process facilities for removing water. The high water cut has been a major problem in Mumbai offshore due to preferential depletion of high permeability layers in a set of high and low permeability layers perforated together and subsequent preferential flooding (water injection) of theses layers for enhanced recovery. Various techniques are used for WSO (Water Shut Off) such as:


Cement Squeeze through the water-producing layer. In most cases this requires the retrieval of completion string.




Use of through tubing run bridge plug that is set on top of water producing layer to isolate it. Cement is then dumped on top of bridge plug using wire line dump bailer. This method can be successfully used in case the bottom most layers in a well is contributing water and can be resorted to without pulling out the string.




In case any other layer is producing water then a bridge plug and cement retainer are used in conjunction to isolate the water-producing zone.




Increased water production through channeling behind casing is remedied through block cementation jobs




WSO using gels, wherein gel polymers are pumped into the well. The gel, after coming in contact with water, forms a pancake type barrier to the flow of water owing to cross-linking of polymer molecules and thus stops water production.

GAS SHUT OFF (GSO) The excess and unwanted flow of gas from the reservoir causes a loss of reservoir energy that may ultimately affect the reservoir recovery. GSO is carried out in wells producing with high Gas to Oil ratios by:



Squeezing cement between oil and gas producing layers. Gel application as in the case of WSO

LAYER TRANSFER A layer transfer job is done to change the layer of production or injection based on requirement. RECOMPLETION Such operations involve change in completion type such as from single to dual completion or vice versa, recompleting a well with gas lift etc.

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The single completions are recompleted dually either to avoid cross flow from one layer to other owing to unequal pressure depletions over the producing life or to control injection rates in individual layers in an injection well. In dual completions with gas lift, gas injection optimization is most essential to avoid one of the strings starving for gas with the resultant decreased production. PROFILE MODIFICATION IN A WATER INJECTION WELL An injection well is used to inject water into the reservoir for maintaining its pressure and ensure better recovery of oil. Work over job for profile modification is carried out in water injection wells when some of the layers are taking either more or less amount of water than desirable. A water flow log is used to record the injection profile of all the layers. The layers taking more than desired amount are squeezed off with cement. The layers taking less water are re-perforated or some more layers added to them. SERVICING Servicing jobs are generally safety related and involve operations such as replacement of gas lift (G/L) valves, Sub-surface safety valves (SSSV), tubing and packer etc. Removal of tubing blockage due to sand, scales, wax & paraffin depositions and stimulation jobs also form a part of servicing work over jobs. WELL DEEPENING A well is deepened to include additional producing layers in the well. If the well has been on production before deepening, the existing layers are subdued and isolated through cement squeeze job. The well is then deepened, logged, cased, cemented and completed in the deepened horizon for production. In some instances, production from shallow well may be affected adversely by offset production from nearby deeper wells. The shallow well is then deepened to prevent offset drainage. SIDE TRACKING Side tracking is a very commonly used method to abandon or bypass the lower part of an existing well and add new layers. Many reasons exist for side tracking a well such as damage or collapsed casing, irretrievable junk in the hole, a damaged production zone in the old well, access to remaining/ new drainage area, drilling of drain holes/ laterals and horizontal wells etc. The well is side tracked by cutting a window in the casing and drilling a new hole through it. The conventional method of sidetracking involves placement of a cement plug immediately below the desired window depth to facilitate the section milling of the casing. This method requires several cement plug jobs before a window can be cut successfully. Also, it requires the cutting and retrieval of several casings to enable sidetracked hole to reach the targeted depth. The side tracking is of two types: Long Drift Side Track (LDST): In LDST, the 9 5/8” casing is retrieved first. A window is cut in 13 3/8” casing below 20” casing shoe and a new hole is drilled. The LDST enables drilling a well with a 358

horizontal drift in excess of 1000 m. The well is generally completed in 5” liner against the pay zone. Short Drift Side Track (SDST): In SDST, a window is cut in 9 5/8” casing below 13 3/8” casing shoe and a new hole is drilled. The SDST is used when the horizontal drift required is less than 1000 m. The well is generally completed in 7” liner against the pay zone. A present method for side tracking uses a kickoff tool or a whip stock packer that is set at the proper depth. Drilling is then directed out of the hole and to the desired location by setting the tapered whip stock at different points to change the route of the new hole. When the desired depth and target are reached, the new hole is logged and a liner is run and cemented in place. Completion is then carried out in the normal manner with a packer and tubing. The use of whip stock does not require the cutting & retrieval of casing thereby saves costly rig time. Side tracking using whip stock has been extensively employed for drilling horizontal drain holes in Mumbai offshore. FISHING: Fishing refers to the application of tools, equipment and techniques for the removal of junk, debris or fish from a well bore. The fishing operations are carried out to remove unwanted material from the well for the purposes of completion. Fishing forms a major part of work over operations. PLUGGING & ABANDONMENT: Plugging & abandoning job involves abandonment of wells that have outlived their economic producing lives for ensuring safety and environmental protection in future.

COMMON EQUIPMENT USED DURING WORK OVER: Most of the work over jobs involve operations like perforating, cement squeeze jobs, cement clearing, scraping of casing/ liner, milling etc. Some of the other frequently used tools in work over are: Work-over string The string of pipe used during workover is called the work-over/ work string. In offshore drill pipes of sizes 2 3/8” to 5” are used as work-over/ work string. Casing Scraper A casing scraper, Fig19-10, is used to remove foreign substances such as scale and cement from inside the casing wall. The scraper is usually run above a bit and is reciprocated to scrape the walls of the casing. Spring-tensioned blades provide the scraping action against the casing wall. Junk and Boot baskets Junk and boot baskets, Fig19-11, remove milled or drilled material from a well. A junk basket is run at the bottom of the work string along with TCR bit below junk basket. Through application of reverse/ direct circulation, the junk is swept into an inner chamber of basket and recovered once the basket is pulled to surface. 359

Cement retainer A cement retainer, Fig19-12, is similar to permanent packer but has a check valve inside the bore. A stinger/ seal assembly is run during cementation job. The valve is opened when the stinger assembly is stabbed into packer during the job but closes as soon as the stinger is picked out of the retainer bore. The closed valve holds final squeeze pressure as the excess cement is circulated out. Casing roller A casing roller, Fig19-13, consists of several rugged, heavy-duty rollers mounted on different centerline on a mandrel so that as the tool rotates only one roller at a time contacts the wall of the casing. This eccentric motion restores collapsed, dented or buckled casing to its normal diameter and roundness.

WORK OVER PROCEDURE The procedure for working over a well (which is a G/L) in general, involves the following steps: 1. Lock open SSSV. 2. Stop gas injection and bleed off annulus gas through burner in case of gas injection wells. 3. Bulldoze string volume (twice) into formation. 4. Fill annulus with seawater. 5. Perforate 5 m above packer. 6. Circulate and kill well. Under loss conditions, a LCM pill is placed and in case activity is observed ten brine of sufficient specific gravity is circulated. 7. The tubing condition can be assessed using Multiple Imaging Tool (MIT).
In case the tubing condition is very bad then cut tubing and fish out packer separately.
In case tubing condition permits over pull then try to release the packer and string directly. 8. POOH old completion string. 9. Make bit & scraper trip to required depth. 10. Re-perforate/ add layers and re-complete the well. 11. Activate through gas injection in case of gas injection well. 12. Stimulation job, if required. 13. Hand over the well to platform. FISHING OPERATIONS IN WORK OVER DEFINITION OF FISHING In oil field parlance, a “fish” is anything that is left in a well bore. Once the component is lost, it is referred to as "the fish." In open hole during the course of drilling, the fish may be anything from a part of or all of the drill or tubing (work) string, to smaller pieces of equipment such as bit cones, pieces of tools or any material accidentally dropped into the well. Similarly, in cased hole, there are various types and kinds of problems that occur which create the fishing jobs, such as objects being dropped into the well, packers to be retrieved, parted tubing, collapsed casing, dropped pipe and wire line tools either parted or stuck. 360

Fishing refers to the application of tools, equipment and techniques for removal of lost or stuck objects from the well bore. The term “fishing” is taken from the times of the earlier cable tool drilling when the crew simply put a hook on a line and attempted to catch the wire line when it would break so that the tool could be retrieved. Over the years, with advancement in drilling, completion and work over, fishing has also evolved greatly as an art and science of removing broken or stuck equipment or small non-drillable materials from the well bore. CAUSES LEADING TO FISHING OPERATIONS Some of the common causes that result in fishing operations are:
Human error in a majority of cases
Corroded tubing and equipments
Stuck packers. The stuck-up can be due to either differential or mechanical reasons.
Damaged casings above the packers that result in packer stuck-up during pulling out the string.
Work string stuck-up in open hole-Differential, Mechanical or key seating
Logging tool stuck-up in tubing/casing due to scale deposition or damage
Premature setting of cement during cementation CONSIDERATIONS FOR FISHING The key considerations for a fishing operation include: Knowledge of down-hole tool configuration: An understanding of the dimensions and nature of the fish to be removed is essential for designing a successful fishing operation. Typically, anything that is lowered into the hole is accurately measured and sketched so that appropriate fishing tools can be used if required to fish any item out of the hole. Well bore conditions: The well bore conditions need to be understood clearly to determine the cause of stuck-up. Based on Hooke’s law that stretch is proportional to strain, the free point/stuck point in the string are determined. Well profile: Successful fishing is much easier in a relatively straight well than in a highly deviated well bore. However, it is still quite possible to perform a successful fishing job in a highly deviated or horizontal well bore, if the proper approach is taken. When a highly deviated well does require a fishing job, most of the tools used in straight-hole fishing can be successfully run. Even wash pipe with specialized connections can be run in highly deviated wells. Because the pipe is still large and not very flexible, sections still have to be short in order to pass through high-angle doglegs. Jars, over shots, magnets and junk baskets can also be used successfully. There are several special considerations that should be taken into account when planning a fishing job in a high-angle deviated or horizontal well: •

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When a high-angle hole has been drilled by rotating drill pipe, a trough (channel) usually forms on the low side of the hole that is smaller in diameter than the drilled portion of the hole. This is a factor when fishing with an overshot or similar tool, as the fish will lie in the trough or smaller section. Broken or twisted-off pipe can fall under a ledge of a dogleg. 361

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Hole drag in horizontal or highly deviated wells inhibits good jarring action. Adding weight to the string for milling operations can be a problem. Drill collars are similar to wash pipe in that they cannot flex around high-angle doglegs and bends. They have to be run higher up in the more vertical section of the hole to be effective. (In a horizontal section, the drill collar is on the low side of the well bore, not adding any weight on mill). It is difficult to get the required torque down and around deviations when attempting to back off pipe in a highly deviated well.

Cost-Benefit analysis of fishing vis-à-vis side tracking: Fishing should be an economical solution to a problem in the well. The cost of the fishing job must be less than the cost to re-drill or sidetrack the well for it to make economic sense. The larger the capital investment in the well, the more time and expense can be devoted to a fishing solution. In the end, only experience, good judgment, a careful analysis of the problem and effective communication among all parties generally leads to a solution that will allow a return to normal drilling, completion or work over operations with the least amount of lost time and money. FISHING TOOLS: The key enabling technologies for successful fishing operations are cutting, milling, catching/engaging and pulling. Based on the kind of fishing operation, a variety of fishing tools are available. The use of appropriate fishing tool for any particular job will largely depend on the type of fish in the hole, whether the fish is stuck or free, whether it is in an open hole or in a cased hole, the condition of the hole at the site of break and the condition of the top of the fish. Each fishing job is unique but there are some basic tools such as safety joint, bumper sub, hydraulic jar and heavy weight drill pipes that are used in most jobs along with appropriate fishing tool and drill pipe work string. Based on intended application, the fishing tools can be classified into: -

EXTERNAL CATCH TOOLS: The external catch tools, Fig 19-15, engage a fish on its outside body. Some of the commonly used external catches fishing tools are: OVERSHOT: The overshot is one of the most widely used fishing tools. It is a highly versatile and efficient tool. There are several different types of over shots; however, each overshot is designed to engage a specific size of tubing, pipe, coupling tool joint, drill collar or smooth O.D tool. The over shots are designated by a series number that indicates their application for fishing certain types of fish e.g. Series 70. Over shots are used to release a fish with short space to engage the tool on it. Similarly, Series150 over shots are used to release fish with sufficient neck length and allow for circulation too. Over shots may be identified by one of the following strength types also:
Full Strength (F.S.) that is engineered to withstand all pulling, jarring and torsional strain.
Extra Full Strength (X.F.S.) that is engineered for extreme abuse.
Semi-Full Strength (S.F.S) that is engineered to withstand all pulling strain only.
Slim Hole (S.H) that is engineered to withstand heavy pulling strain only.
Extra Slim Hole (E.S.H) that is engineered for pick-up job only. 362

DESCRIPTION OF TOOL: The basic overshot, (Fig19-15), (from top down) consists of a top sub, a bowl, grapple, control and a guide. In addition to the basic components, some over shots can be dressed with either:
Spiral Grapple – used if the fish diameter is near the maximum catch of the overshot.
Basket Grapple – used if the fish diameter is considerably below maximum catch size (usually ½”) However, in operation the overshot functions in the same manner, whether dressed with spiral grapple or basket grapple. When the circulating pack off is not used, the fluid circulates down the drill pipe, around the top outside of the fish, thorough the slip or grapple assembly around the guide shoe and up the annulus. When the circulating pack off is used the annular space between the top outside of the fish and the inside of the lower part of the overshot is packed off, diverting the fluid flow down into the fish. If circulation can be diverted through the fish, it is easier to release and recover the fish. Pack offs usually are not high-pressure devices but will often withstand sufficient pressure to establish circulation through the fish. Both the grapples or slips and the pack-off can be easily damaged if the top of the fish is ragged, out of round, bent or damage. Some times over shots are used with extension subs that are installed between the top sub and the bowl of the overshot and extends the overshot bowl. Extension subs are used to either establish a longer hold on a fish that may be undersize at the top by having been pulled in two/ an overshot released several times or cover a bad section of pipe so that a tool joint can be caught. Extension subs will only cover a fish O.D. equal to the maximum catch of the overshot using a basket grapple and still remain full strength. DIE COLLAR The Die Collar, Fig19-17, is designed to retrieve tubular members from the well bore. The Die Collar is manufactured from high-grade alloy and specially heattreated. The hardened cutting teeth (wickers) are machined on a shallow taper (approximately 3/4 inch per foot) to provide an excellent grip and positive engagement. For operation, the tool is run to the fish top and minimum weight and sufficient rotation is applied to allow the wicker threads to become embedded in the exterior surface of the fish. A major disadvantage of die collar is that the disengagement of die collar, in case the stuck-up is not released, is extremely difficult and may further complicate the fishing operations. This is one of the reasons for which the die collar is typically used for mechanically backing off the string after just engaging the fish with die collar. CJ MILLING TOOL FOR PLUG & PACKER The CJ milling tool enables retrieval of packers and bridge plugs (bridge plug – Fig 19-14), used in the well. The tool, after milling the top slips of packer/ bridge plug, latches on to their collet for pulling them out. Mill out extension may additionally be required for packer. 363

TAPER TAP The Taper Tap, Fig19-16, operates in an exactly opposite manner to a die collar and is basically designed to retrieve tubular members from the well bore. It is the most economical tool of its kind for freeing fish. The Taper Tap is also manufactured from high-grade alloy and specially heat-treated. The basic Taper Tap is a single piece construction. The hardened cutting teeth (wickers) are carbo-nitrided and machined on a shallow taper (approximately 3/4 inch per foot) to provide an excellent grip for light duty pick-up jobs. For operation, the taper tap is run to the top of fish and rotated sufficiently to allow the wicker threads to get embedded into the interior surface of the fish. Above we have mentioned a few very commonly used fishing tools. Apart from these, there is several different kind of standard fishing tools used during the fishing operations. However there are some tools those are not used for the fishing operation but are used for some supportive operations, like Junk Cather Tools. JUNK CATCHER TOOLS The junk catcher tools, Fig 19-19 to19-21, are used to remove junk/ debris from the well bore prior to/during fishing/milling operations. Some of the commonly used such tools are:
BOOT BASKET
JET BASKET
JUNK BASKET
REVERSE CIRCULATING JUNK BASKET (RCJB) ACCESSORY TOOLS: Various accessories are used in the fishing string, Fig 19-22. Some of them are: A. WIRE CATCHER / WIRE-LINE SPEAR The wire line catcher is used to retrieve wire line. B. FISHING MAGNET The fishing magnet is used to retrieve all types of small objects having magnetic attraction from the borehole bottom. Objects such as bit cones, bearings, slips, tong pins and mill cuttings can often be retrieved only by magnetic attraction. C. LEAD IMPRESSION BLOCK (LIB) The LIB is used to determine the configuration of fish top and to locate its position in the well bore. The impression block is lowered on the end of the fishing string to approximately 5 feet above the fish. Circulation is used to clean the top of the fish and the string is then rapidly slacked and set on fish with 15,000 to 20,000 pounds of weight on the fish to get a good impression of fish top. D. & E. HYDRAULIC FISHING JAR The hydraulic fishing jar is used when a powerful upward blow is required to release the stuck fish.

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The hydraulic jar is placed directly below the drill collars in the fishing string. The intensity of each blow is controlled by the amount of stretch placed in the drill string. More the pull harder is the blow. The jarring effect is enhanced by placement of drill collars above the jar. F. BUMPER SUB Bumper sub is used below hydraulic jar to prevent transmission of impact generated by jar to tubing. G. SAFETY JOINT The safety joint provides a simple means of releasing and re-engaging fishing tools during many fishing operations. It is especially useful in operations requiring a fishing tool that is not normally easily releasable such as a tap or die collar. The safety joint can be released from the fish by applying a tension load to the string to shear the screws. Once the screws are sheared and the tension on the line is released, the string is picked up while applying slight left hand torque. The safety joint lugs will then shift into the release slot and the fishing sting is released from safety joint. MILLING & WASHOVER TOOLS: JUNK MILL Junk mills, Fig 19-23, are used to mill up almost everything that falls or becomes stuck in the hole. Pipe that has become cemented both inside and outside can only be milled with this tool. Loose or rotating junk can be pounded down to break it into smaller pieces and hold it in place so that the mill can cut it. A weight on the junk mill, as a good rule of thumb is 1000 pounds per O.D. inch of the mill. For use inside a casing, a mill with stabilizer pads and a smooth O.D. is used so that the casing will not be damaged. The O.D. of the mill should be same or slightly smaller than the drift diameter of the casing. In open holes, a mill without stabilizer pads in used. The O.D. of the mill should be about ¼ inch less than the hole-diameter with rough O.D. that will cut a small amount of side clearance. TAPER MILL Taper mills, Fig19-23, are used to ream partially collapsed or damaged casing to clean up ragged holes or windows and generally to enlarge and smooth rough and jagged surfaces. Milling out collapsed casing is done in increments of about ¼” i.e. a mill with an O.D. of about ¼” larger than the minimum collapsed pipe I.D. is run into the hole and the collapsed interval is then milled to about ¼” larger. Next, a second mill is run and the pipe I.D. increased to about ¼” again. This procedure is repeated until the pipe I.D. is back to full size. WASHOVER PIPE (W/O) In cases where the fish is a stuck pipe, the fish may be surrounded with settled mud, cement and other debris. This will not allow proper engagement of the fish with tool. The wash over pipe is used to clear the out-side area of fish. The wash over pipe, Fig 19-23, is run at the bottom of work string. The cleaning is done using circulation as the wash over pipe is lowered over the fish. 365

REMEDIAL & REPAIR TOOLS CASING ROLLER The casing roller, Fig 19-24, is used to restore buckled, collapsed and dented casing to its Original diameter. CASING PATCH The casing patch is used to make fast, economical repairs to damaged casing string without much reduction in its I.D. SAFETY REQUIREMENTS DURING WORK-OVER JOBS PRINCIPLES OF WELL CONTROL: A kick is an intrusion of formation fluids into the well bore that, if not controlled, will result in a blow out. The objective of well control procedures is to safely prevent or handle kicks and re-establish primary well control. During normal drilling operations, the primary well control is maintained through use of drilling fluid that provides sufficient hydrostatic head to overcome the formation pressures. Also, owing to the inherent resistance of liquids to flow, an additional pressure is applied to circulate the fluid during drilling to lift the formation cuttings to the surface. The pressure, so applied at surface, alters pressures in the pipe and well bore bottom. Thus fluid circulation creates additional over balance to contain the formation pressures. The total pressure at the bottom of the hole while circulating will be the sum of the hydrostatic head of the drilling fluid, the annulus friction circulating pressure and imposed surface backpressure. During drilling, this pressure must not be less than the formation pressure to avoid a kick and must not be greatly in excess of formation pressure to prevent fluid losses.

RECOMMENDED SAFE PROCEDURES & GUIDELINES FOR WORK OVER OPERATIONS: The recommended safe procedures & guidelines for work over & well stimulation operations have been issued by Oil Industry safety Directorate (OISD) vide OISDGDN-182. The OISD-GDN-182 lists out general safe operating practices and also recommended procedures & guidelines for Personnel individual responsibilities, various safety equipments such as Personal Protective Equipment, Life saving equipment, Fire fighting equipment etc. Some of them are given below: -

General Safe Operating practices for work over jobs are:






Good housekeeping on the job is essential for successful accident control and fire prevention. Keeping everything in its place promotes efficiency, quality and good work. Tools, equipment and working areas should be kept clean, neat and orderly. Thorough safety inspection should be made of all well servicing equipment before starting the job. Men on the rig floor should stand clear when rigging up or repair work is in progress overhead. 366











Unsafe or otherwise dangerous conditions, no matter how small, should be immediately reported to supervisor in charge for corrective action. Never leave well unattended unless it is safely shut-in. Upon completion of job, clean equipment and keep them neatly and safely in toolbox. All occupational injuries, no matter how small, will be reported promptly to the supervisor in charge. In the event of equipment, guy wires, lines etc. being present in a location frequented by personnel and thus creating a hazard, proper signs must be placed to warn the people. Control must be maintained over leaks and spills. However, if they occur, they should be cleaned up promptly to eliminate slipping hazard to personnel as well as fire hazard.

Well Subduing Well plan should be prepared well in advance and circulated to all concerned including driller in charge. It should contain the following information but not limited to:








Brief of earlier work over done Complications encountered during drilling/ work over Mud parameters during drilling/previous work over jobs Details of down hole equipment, tubing and X-mas tree Casing and cementation details Details of perforation Bottom hole pressure of existing well and expected BHP after work over job

Some of the checks that are essential during well subduing operations are:







The kill line must be tested to 1½ times the maximum expected shut-in pressure or its rated working pressures whichever is less. No hammering must be done on pressurized line to tighten the joints. The parameters of kill fluid must be regularly monitored. The parameters of return fluid must be monitored regularly during circulation to ensure that the well is killed and the fluid inside the well is properly conditioned. Improper returns or quick pressure build-ups must also be monitored to assess the conditions of loss/ activity. Before the X-mas tree is nippled down, Back Pressure Valve (BPV) must be installed in the tubing hanger. The SCSSV must also be kept closed. The BPV must be removed and SCSSV opened only after installing the BOP.

Following requirements apply during work over operations with X-mas tree removed:





Well control fluids, equipment and operations shall be designed, utilized, maintained and/or tested as necessary to control the well in foreseeable conditions and circumstances. The well shall be continuously monitored during well work over operations and shall not be left unattended at any time unless the well is shut-in and secured. When coming out of hole with drill pipe or work over or well completion string, the annulus shall be filled with well control fluid. 367

SUSTAINED CASING PRESSURE (SCP) DEFINITION Casing strings of oil and gas wells are isolated from formation behind casing with cement and a hydraulic isolation from one another by wellhead seals at surface. The production tubings are isolated from production casing by down-hole packers and tubing hanger seals at surface. The production casings are designed to withstand maximum expected reservoir pressure. Likewise, the tubings are also designed to withstand maximum expected reservoir/ stimulation pressures, type of fluids to be produced/ injected and their rates. Thus, wells are constructed to ensure hydraulic isolation of casing strings and completion tubing strings. Ideally, no casing head should have pressures except the tubing head. The casing head pressure, in case experienced, which rebuilds on bleeding off is termed as Sustained Casing Pressure (SCP). This, however, does not include casing head pressures deliberately applied as in the case of completions with gas lift, any down-hole pressures applied on down-hole tools or developed due to thermal expansion of annuli fluid. Due to production of hot fluids, under stabilized flow condition, the flowing tubing head temperature can be extremely high (depending on flow rate). This can cause heating of completion fluids in production casing and also of annuli-muds in isolation/outer casings leading to their expansion and pressurization of confined space. In Western offshore, in some of the high volume oil/ gas producers, fairly high pressures in production and isolation casings are experienced due to thermal effect. Thus, it is essential to confirm and nullify the pressures associated with thermal expansion. Only a small volume of fluid generally has to be bled if the pressure is caused by thermal effect. Under same stabilized flowing condition, on closing of needle valve, the casing pressure should remain zero. If not, the casing is having SCP. The injection pressure of lift gas introduced in production casing is not SCP if it remains confined to production casing. However, the pressure experienced in casings other than production casing is termed SCP. CAUSES OF SCP The various causes of SCP are: Loss of hydraulic isolation of tubing strings The loss of isolation can be from either down hole packer, sliding sleeve, expansion joint, tubing joints, tubing hanger seals, tubing leak etc. Such pressure experienced by production casing can leak to isolation casing from wellhead seals or casing joints, previous squeeze points, pin holes caused by corrosion etc. Loss of casing integrity The loss of casing integrity can again be due to leak in casing joints, previous cement squeeze points, pin holes caused by corrosion, casing shoe failures, inadequate casing cementation etc. Pressures from Shallow reservoirs Some shallow reservoirs above producing layers in a well that may not have any commercial significance can result in SCP, if casings are not adequately cemented against these reservoirs. 368

EFFECTS OF SCP The isolation and outer casings are neither designed to contain the maximum expected reservoir pressures nor are their threads gas tight. Generally, the Minimum Internal Yield Pressure (MIYP) of isolation casings are less than 50 % of MIYP for production casing. The casing shoe of the isolation and outer casings can fail if exposed to pressures higher than their leak off values. Failure of casing shoe and/or loss of hydraulic isolation in isolation casing can lead to migration of gas to shallow reservoir and eventually its pressurization. Such situations can lead to underground blow out resulting in loss of property, human life and environmental damage. If tubing leak develops and the problem remains unattended, the production casing being subjected to high pressures may leak or in worst case fail. In such an event, the next outer casing that is not designed to withstand high pressures will also fail thereby leading to a disastrous situation. Likewise outer casings cannot hold the pressure sustained from shallow inadequately cemented reservoir. The problem of SCP in an offshore environment is all the more critical since a number of wells are located on a single platform. The conductor and isolation casings of different wells are in close proximity to each other. Therefore, any undesirable event in one well can adversely affect the safety of remaining wells and that of platform itself.

MODULAR RIG FOR WORK OVER INTRODUCTION Offshore Modular Rigs, Fig 03, are basically a platform mounted work over rigs and consist of many lightweight components or modules that can be installed and configured on the well platform, piece by piece, allowing flexibility to deploy the rig on any of the platforms. This presents an exclusive advantage over jack up rigs, which are platform-specific due to the old pugmarks of previously visited rigs to those platforms. However, platforms have to be checked for their structural as well as space adequacy for installation of such rigs on top of platform. The flexibility of a modular rig allows fitting it on the available platform area with certain modifications and re-routing of well fluid lines. This fit-to-purpose feature of Modular Rigs makes them very versatile in use. The modular rig is equipped with hydraulic skidding system that allows it to reach all the wells on the platform, as and when required. In spite of the rig area being small, the operational features are not compromised and the rig is capable of all the conventional work-over operations. These rigs make use of unmanned platforms for accommodation of crew and crew boats for transfer & storage of material and piping. PRE-REQUISITES FOR DEPLOYMENT OF MODULAR RIGS Some of the pre-requisites for deployment of modular rigs are: Modifications to the platform Structural Modifications: A pair of strong beams supports the modular rig components on a platform. These beams, in turn, are supported by stub extensions connected to the deck legs, so that the rig loads are directly transferred to the deck legs without affecting the deck trusses.

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Provision of pedestals at the location of the four legs of the deck will be required to facilitate placement of the capping beams of the rig. The height of the pedestals shall be such as to help clear most of the obstructions on the main deck. Provision of a support arrangement for mounting the burner boom will be needed. The modifications to a platform can be categorized into two types: Topside modifications: Many obstructions exist on the main deck of the platforms that can hamper the deployment of the modular rig. Some of the obstructions can be overcome by selecting the height of stub extensions suitably. Some obstructions cannot be eliminated by this action. It is necessary to perform some modifications to piping, electrical and other facilities on the platform so that these obstructions can be eliminated. Such modifications are termed topside modifications. The required topside modifications vary from platform to platform. a) Dedicated Offshore Supply Vessels (OSVs) to transport various components/ modules from one location to another. b) Cranes of Multi-purpose Support Vessels (MSVs) to lift the modules from OSV to platform till the time the crane of modular rig is not installed. This is necessitated due to absence of cranes or availability of lesser capacity cranes on smaller unmanned platforms

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