Underbalance Drilling Manual

SECTION 1 — CANDIDATE SELECTION

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SECTION 1—Candidate Selection TABLE OF CONTENTS

Page CHAPTER 1.1—Basic Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1-1 CHAPTER 1.2—Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2-1 WHERE UNDERBALANCED DRILLING HAS BEEN ATTEMPTED . . . . . . . . . . . . . . 1.2-1 THE EVOLUTION OF UB TECHNOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2-4 CHAPTER 1.3— Benefits of Underbalanced Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3-1 CHAPTER 1.4— Limitations of Underbalanced Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4-1 CHAPTER 1.5—Geological and Reservoir Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5-1 CHAPTER 1.6—The Well Candidate Vs. Specific Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SOLUTIONS TO DRILLING PROBLEMS WITH UB DRILLING . . . . . . . . . . . . . . . . . . SOLUTIONS TO COMPLETION PROBLEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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SECTION 1—REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ref. 1-1

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CHAPTER 1.1—Basic Definitions Underbalanced (UB) drilling is defined as deliberately drilling into a formation where the formation pressure or pore pressure is greater than the pressure exerted by the annular fluid or gas column. In this respect, “balanced” pressure drilling is a subcategory of underbalanced drilling because the annular pressure is expected to fall below the formation pressure during pipe movement.

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The term “pseudo-underbalance” has been used to describe conditions where the well is presumed to be drilled underbalanced, but in actuality, during pressure surges, connections, trips, or completion operations, the annular column pressure exceeds the formation pressure. For the purpose of this manual, that definition will not be used. Instead, it will be considered that the underbalanced condition was mechanically or hydraulically violated in pseudo-underbalanced operations. Underbalanced “mud” may be conventional drilling mud, water, oil, aerated systems (aerated mud or foam) or pure air with or without mist. “Air” or aerated systems may use air, natural gas, nitrogen, or a combination of gases. Within this manual, the term “gas” will be used to refer to air, natural gas, nitrogen or any other gas, except where noted. As a broad generalization, underbalanced drilling is undertaken for only three reasons: 1. To improve the drilling rate 2. To limit lost circulation 3. To protect the reservoir formation

Each of these three goals has numerous subcategories. However, if underbalanced drilling does not reduce the cost of drilling or improve production, it is of marginal value. The ultimate purpose, therefore, is to reduce the cost of producing the reservoir.

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CHAPTER 1.2—Historical Perspective Underbalanced drilling is not new. The first drillers to use underbalanced techniques were the cable tool operators in the early oil fields. With the advent of rotary tools, drilling muds were gradually developed to the point where well kicks and blowouts could be avoided. Underbalanced drilling with air was first revisited in hard rock areas in an attempt to increase penetration rate.

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Lyons (1984) states that the first oil and gas well using air as a drilling fluid was completed at Peters Point, Utah in 1953 by El Paso Natural Gas Co. However, there are references to a Hughes Tool Company technical document in 1952 that proposed volumes for air and gas drilling, and Brantley (1975) lists California wells drilled with natural gas in 1938. This would suggest that there were earlier UB drilling efforts. The first recorded aerated mud was used in Big Lake field, Regan County, Texas in 1934 (Brantley, 1971). They used natural gas in mud to avoid lost circulation. Aerated muds were used sporadically through the rest of the 1930s and 1940s. For example, Poettmann (1955) refers to aerated mud equipment in a service company catalog in 1945. Starting in 1956, the technical journals were full of air and gas drilling articles. Angel (1957) published the “Air Volume Requirements” tables and Goins (1961) discussed mist drilling. In the 1950s and 1960s, underbalanced drilling with aerated muds and foams was used to avoid or limit lost circulation in the mountainous areas of the U.S. and Canada. Foams were first commonly used by the U.S. Atomic Energy Commission (AEC) for drilling 12-ft diameter holes 3000–5000 ft deep in Nevada. The AEC developed “stiff foam” to lift the powdered rhyolite out of emplacement holes at French Man’s Flats in Nevada. Chevron in California used foam to clean sand out of production holes and wash over production liners. The same techniques were used with air and aerated muds in North Africa, Iran and other scattered areas. Aerated muds were well developed by 1963 (Rehm, 1963). In the following sections, most of the technical emphasis is on the use of air, gas, or nitrogen as a drilling medium or to reduce mud weight. However, since far more wells are drilled underbalanced by simply reducing mud weight, conventional muds that create an underbalanced situation (i.e., flow drilling) are also an important part of this discussion. WHERE UNDERBALANCED DRILLING HAS BEEN ATTEMPTED The first major use of underbalanced drilling procedures to protect the reservoir was in the Austin Chalk starting in 1988. Since 1988-1990, underbalanced drilling in the fractured Austin Chalk of Texas and Louisiana has become a standard practice. In the Austin Chalk, conventional water-base drilling fluids are used in the horizontal holes with the mud weight low enough to balance or Sec. 1

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underbalance the formation. Because of open fractures, there is no practical method to maintain mud density high enough to kill a potential kick. The long chalk fairway, which is up to 50 miles wide and 1000 miles long, stretches from Southwest Texas, northeast into Louisiana. With a total of about 7000 underbalanced horizontal wells, this area tends to dominate the statistics. However, there has

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been significant UB activity around the globe (Figure 1.2.1).

Figure 1.2.1. Underbalanced Drilling Areas

By 1990, serious efforts were also underway in Alberta and Saskatchewan, Canada to extend the benefits of underbalanced drilling to these reservoirs (Figure 1.2.2). While underbalanced drilling in limestone or chalk reservoirs is not unusual in the U.S., the greatest efforts in underbalanced drilling technology in sandstones have been in Canada.

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Figure 1.2.2. Number of UB Wells in Canada (Knoll, 1996 and Murch, 1998)

Geothermal underbalanced drilling is a special case that is most often undertaken to improve penetration rate and avoid lost circulation. Air (including air with steam) and aerated mud have been the most successful underbalanced fluids in geothermal drilling. High temperatures, which are often above the limits of chemical foam agents, tend to limit the use of foams. The use of underbalanced drilling and completion technology is expected to grow steadily over the next several years. Based on an industry survey sponsored by the U.S. Department of Energy, application of underbalanced drilling in the U.S. and around the world is predicted to grow rapidly (Figure x-1) in a manner reminiscent of horizontal drilling after its modern introduction in the mid1980s.

Figure 1.2.3. Projected Underbalanced Drilling in the U.S. (Duda et al., 1996)

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THE EVOLUTION OF UB TECHNOLOGY

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Gas drilling was an obvious idea (Figure 1.2.3), since U.S. law allowed free use of lease gas on the lease. It was popular starting in the 1950s. With the increase in value of natural gas, the use of gas drilling has decreased.

Figure 1.2.4.

Gas Drilling Blooie Line with 3 MMcfd

Air drilling became common in quarries in the 1940s, but large portable air compressors had to be developed before it could be applied in the oil field. Well Completions Co. of Denver, the first air drilling service company, started in 1954 with Gardener-Denver two-stage WEN and WEK compressors run by two-cycle diesel GM engines (Figure 1.2.4). Joy JN102 two-stage compressors were common by 1958. Boosters were available at the same time and allowed pressures up to about 900 psi. Larger compressors from Dresser (four-stage Clark CFB-4) and Ingersoll Rand 4HH3 (three stages and boosters) became available in 1966. The introduction of the air hammer to the oil field in that same period extended the use of air drilling into crooked-hole problem areas in West Texas. Air and aerated drilling are still conducted extensively in the Western U.S., Canada, and the steam fields at the Geysers in California.

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Figure 1.2.5. WEK Compressor Package (Western Air Drilling, 1954)

There were and still are technical problems with air drilling. There were major efforts to develop chemical squeezes during the 1960s and 1970s to squeeze off wet zones. This work was largely unsuccessful both technically and economically. Other problems included damp formations that were unstable when wet with mist, corrosion of the drill pipe, and downhole fires (burn-offs). Some newer technologies developed in the 1990s helped remedy these problems. New misting agents that inhibit sloughing shales are extending the application of air drilling. Corrosion has been largely controlled by inhibitors, and nitrogen defeated fires and oxygen corrosion problems.

Foam systems for drilling never became popular beyond certain specialized uses. These included AEC big holes, Arctic surface holes, and workover cleanouts. A resurgence of interest in foam occurred in 1990 in the Hugoton field in Kansas, Canada, and Oman and Yeman in the Middle East. Aerated muds for lost circulation likewise had problems with pressure surging (slug flow), corrosion in the drill pipe, and in general, unstable drilling fluid operations. Many of these problems have been overcome with better fluid technologies, the availability of nitrogen as a drilling fluid, and better modeling and/or understanding of flow regimes.

The modern resurgence in underbalanced drilling began in Canada. There has always been air drilling in the foothills and aerated systems in use elsewhere in the plains. In the late 1980s and continuing today, formal technical efforts were undertaken to extend aerated systems to drill reservoirs underbalance. The development of the closed surface system, nitrogen as a gas for drilling, better reservoir and rock strength analysis, and formalized techniques for maintaining constant hole pressure—all were derived or improved in Canada. It is now evident that there is wide potential application for underbalanced drilling in the U.S. and around the world (American Oil & Gas Reporter Staff, 1998). Benefits of the technology are becoming widely recognized and include: Sec. 1

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Formation damage is minimized

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Completion costs are reduced (often no need for stimulation)

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Lost circulation and differential sticking are prevented

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ROP is improved

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The formation can be analyzed during drilling operations in real time

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CHAPTER 1.3— Benefits of Underbalanced Drilling

The candidate well for underbalanced drilling must meet some simple evaluation criteria. Since a new underbalanced drilling project may involve extra costs and/or additional risk, there needs to be at least one firm economic and technical reason to change from the conventional overbalanced drilling program. It is also not realistic to assume that if a new project meets the evaluation criteria, it will be a technical or financial success. There is still the phenomenon of the learning curve. The evaluation of a single candidate well can be used to screen obviously inappropriate applications. Measuring success, however, requires a more statistical view provided by evaluating several wells.

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Underbalanced drilling can help resolve drilling cost problems or completion damage problems. The first step in screening a prospect is: does it appear that underbalanced drilling will provide a solution to one or more of the following technical problems?

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

PROBLEM SITUATIONS THAT CAN BE SOLVED WITH UNDERBALANCED DRILLING Slow drilling rate in hard rock Slow drilling rate in crooked hole Lost returns Drilling into depleted zones or in depleted fields Differential pressure sticking Limited water availability Skin damage from mechanical plugging Skin damage from “shale” hydration Fluid sensitivity Fractured reservoirs

Drilling situations that may benefit from the use of underbalanced drilling include: 1.

Hard Rock—Low Penetration Rate

Drilling penetration rate in sediments increases as the differential pressure between the wellbore and pore pressure in the rock decreases. The curves shown in Figure 1.3.1 shift on the axis for different formations and conditions but retain their general shape. If it is possible to reduce the differential pressure to the inflection point on the curve, a major improvement in penetration rate may be expected. Rate increases of ten-fold are not unusual.

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Figure 1.3.1. Drilling Rate vs. Differential Pressure in Two Alberta Fields (Plaxton et al., 1997)

Hard Rock—Crooked Hole

Drilling in hard, dipping formations results in low penetration rates because a low bit weight is a necessary part of the pendulum effect required to keep the hole straight. Reduced differential pressure (underbalanced drilling) allows a higher drilling rate at light bit weight. This is best performed using air or a light air/mist system. The air hammer, which requires very little bit weight, provides a spectacular drilling rate increase while allowing the hole to be kept straight. Straight holes with drilling rates of up to 20 times conventional are not unusual. 3.

Lost Returns

Drilling fluid is lost to the formation because the pressure exerted by the column of drilling fluid is greater than the fracture pressure of the formation, or there are open fractures, vugs, or channels in the formation that have less internal pressure than is exerted by the drilling fluid column. The obvious solution is to reduce the mud column weight rather than trying to plug the fractures and/or holes. 4.

Depleted Formations (Low Pressure Formations)

Pressure depletion in older oil fields leads to lost circulation in new wells, re-entries or workovers. Horizontal re-entries into depleted formations have become a common procedure in the last few years. The formation is subnormally pressured and lost returns or differential pressure sticking

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is to be expected. Mud column pressure needs to be reduced to below the formation pore pressure or reservoir pressure to prevent these problems. 5.

Differential Pressure Sticking

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A reduction in differential pressure sticking (Figure 1.3.2) is normally a bonus that is enjoyed when underbalanced drilling is applied for other reasons. While reduced sticking is a justification often mentioned in underbalanced proposals, the literature cited at the end of the chapter contains little information on the subject. Cited material is often anecdotal because sticking did not occur. It appears that underbalanced wells are not often drilled solely for the purpose of eliminating differential sticking. This problem was more often addressed with a slight reduction of mud weight to a minimum value or the use of an oil mud. 6.

Limited Water Availability

In desert or dry areas, limited water supplies have encouraged air drilling or the use of foam or aerated mud to combat lost circulation. 7.

Skin Damage Due to Mechanical Plugging of the Pore Throat

Skin damage, which involves the mechanical plugging of the pore throats (or permeability passages) in the immediate area around the wellbore (Figure 1.3.3), is one of the major constraints to production. The plugging may be on the surface of the wellbore or internal to the pore throat. There are several methods employed to avoid this problem:

Figure 1.3.2.

Differential Pressure Sticking

a. Ultraclean drill-in fluids, b. Non-invasive filter cakes, c. Underbalanced drilling. The approach with underbalanced drilling is to avoid flow into the reservoir and instead allow the reservoir to flow outward. This is the ultimate solution to avoid mechanical plugging of the face of the wellbore. A promising solution is to employ two or more of the above methods. This presents problems along with the potential benefits. It is impossible to keep an ultraclean drill-in fluid clean, although it may be kept clean enough to limit skin damage. The non-invasive filter cake may cause

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later problems with a completion using screens, and it is difficult to keep produced oil from emulsifying in the mud so that it can be separated and sold. Bennion (1996) argues, based on core testing in the laboratory, there is some glazing and pulverization of the wellbore from the drill pipe that may cause skin damage even in UB conditions. This is a real possibility but appears to be impossible to avoid (Table 1.3.1).

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Given no perfect solution for skin damage, UB drilling still seems to be an excellent method for limiting these problems.

Figure 1.3.3. Plugging of Pore Throats (Bennion, 1996)

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TABLE 1.3.1. Potential Formation Damage Mechanisms in Different Reservoir Types (Bennion, 1996)

8. Skin Damage due to Shale Hydration

“Dirty” formations containing shale or clay elements are another of the major elements of skin damage. There are a number of approaches to this problem: a. Underbalanced drilling, b. Inhibitive “mud” or circulating fluids, c. Special drill-in fluids. In this case, a combination of approaches is best. The main problems are that inhibitive muds may cause mechanical skin problems, and special drill-in fluids are expensive in both product cost and time. 9. Reservoir Fluid Sensitivity Some, or perhaps all, reservoirs are sensitive to fluid invasion (Figure 1.3.4). There may be oil in an oil-wet reservoir or water in a water-wet reservoir or some chemical or scale reaction

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in the pore space. In any case, one of the obvious solutions is to not penetrate the reservoir with the drilling and completion fluid, or to limit fluid penetration even with what appears to be a non-damaging fluid.

Figure 1.3.4.

Mechanism for Formation Blocking— Fluid Sensitivity (Bennion, 1996)

10. Fractured Reservoirs with Production from Fractures In fractured reservoirs like the Austin Chalk, the mud column balances against the fluid pressure in the fracture system, so it is really more properly described as a type of balanced pressure drilling (Joseph, 1995) (Figure 1.3.5). Modern seismic and logging techniques are demonstrating that more reservoirs produce from fractures than were previously considered.

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Figure 1.3.5. Fluid Losses and Gains (Bennion, 1997)

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CHAPTER 1.4— Limitations of Underbalanced Drilling

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There are important problems to consider when the mud column pressure against the formation is reduced (Figure 1.4.1). Most drilling procedures use the mud column pressure as a seal against well kicks or blowouts, heaving shales (geopressured shale), broken or fractured formations, general borehole instability due to techtonic stresses or weak formations, and salt. With deliberate underbalanced drilling, this type of protection is no longer available.

Figure 1.4.1. UB Drilling Problems

The technology to deal with reservoir protection is still evolving, but it is clear from the literature and field experience that underbalanced drilling and completions using current technology will not solve all problems of low well productivity. UB drilling is not a production enhancement technique or a panacea for all problems. If a reservoir will not produce without fracturing, it probably is not a good candidate for UB drilling. UB drilling can only solve skin damage and fracture plugging. New UB projects are often undertaken in wells where there is very little chance for success and therefore little risk of damaging the reservoir or incurring extraordinary costs. A poor well will never become a winner and, in the end, poor results or poor production will detract from the potential of a promising technology.

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The following is only a summary partial screening process for drilling a candidate well. If any of these conditions appear, they must not be in the open-hole section for underbalanced drilling, but behind casing. Thus, by definition, these require that underbalanced drilling be halted in that open-hole interval. However, very little is absolute and local conditions and practices may modify this screening process.

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THE “ABSOLUTE” RULE FOR UB OPERATIONS IT’S NOT WHAT YOU KNOW THAT HURTS YOU …IT’S WHAT YOU KNOW THAT ISN’T SO !!

THE NO-GO SCREEN If the following occurs within the open-hole section: NO, DON’T DRILL UNDERBALANCE 1. Weak formations will collapse—limit underbalance 2. Dipping fractured formations 3. Thick coal beds 4. Young geopressured shales POSSIBLE, BUT DRILL IT AND CASE IT QUICKLY 5. Thick shale section or older geopressured shales 6. Hard, thin salt beds EXPENSIVE 7. High pressure water flows 8. Hydrogen sulfide

The “Don’t Do It” Screen 1. Do not drill with extreme UB in weak formations. Weak formations will collapse unless supported by a mud column (Figure 1.4.4). Early work in the Gulf of Mexico indicated that the Miocene shale would collapse or slough at an underbalance of about 600 psi. Other formations have other limits. Literature cited at the end of the chapter mentions this problem (Bieseman, 1995, McLellan, 1994, McLellan, 1996, and McLellan and Pratt, 1995), but little specific data are given. What is clear from experience and recognized by many of the authorities, is that wellbore collapse

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does not follow simple rules and is not as common as rumored. Limit the UB to the minimum necessary differential pressure.

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2. Fractured dipping formations will cave into the wellbore unless supported by the mud column.

Figure 1.4.4. Underbalanced Drilling Problems (McLellan, 1996)

3. Thick coal beds. The coal beds in the Western U.S. and Canada are often fractured from techtonic activity and are very unstable. They will immediately collapse to their critical angle of repose. This is not necessarily true of massive unfractured coal beds, such as are common in the Eastern U.S. Large washouts or hole enlargements upset the otherwise delicate hole-cleaning ability of underbalanced drilling fluids. The result is cuttings left in the hole, heading of the fluid column, and pressure surges, all of which aggravate the problem. Sec. 1

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4. Geopressured shale or clay. These are a recipe for disaster. It is not always apparent that a shale is pressure sensitive. The Mancos Shale of Colorado, its equivalent Pierre Shale in Montana, and the Fort St. John Shale in Alberta, Canada will only stand against reduced borehole pressure for five to seven days. They then start to slough into the wellbore. In all cases with shale, even the small volume of water in mist drilling aggravates the problem. However, some of the new misting agents may stabilize these shales for longer periods. (These types of formations are often air or foam drilled, or drilled with aerated mud to increase the drilling rate; however, the hole will only stand open for a limited time.) 5. Thick shale sections. In general, thick shale sections cause problems with underbalanced drilling. They slough or cave into the hole. This is probably due to thick shale sections having some elements of laminating, geopressuring or sensitivity to water. In all cases, even the little water in mist drilling aggravates the problem. These formations need to be put behind casing within a few days.

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6. Thick salt beds. Salt will flow toward the point of least pressure. For underbalanced drilling, that is the wellbore. However, there are some anecdotal reports of drilling salt successfully with air. Potential Problems in Underbalanced Drilling:

7. Shallow high-pressure water flows or artesian flows make underbalanced drilling difficult and always expensive. When underbalanced drilling with water or mud, the water influx only dilutes the mud, which may be tolerable. It is possible, but expensive to drill underbalance with air, mist, foam, or aerated mud in the presence of an artesian water flow. 8. H2S poses a special problem for UB operations, but can be controlled with newly evolving closed systems.

9. Noncontinuous underbalanced conditions can be a significant problem and an indication that UB drilling may not be suitable. Greater damage than would be incurred with overbalanced operations may occur if UB conditions are not maintained coutinuously throughout drilling and completion operations. Real-time measurements (Figure 1.4.5) are essential to ensure success. Coiled tubing represents an important technological solution to avoid pressure pulses during connections.

Figure 1.4.5. Maintaining UB Conditions (Bennion, 1997) Sec. 1

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CHAPTER 1.5—Geological and Reservoir Perspectives Since the geological disciplines are often the basis for decisions about underbalanced drilling, the considerations of UB drilling need to be briefly reviewed on a geological basis.

UNDERBALANCED DRILLING DOES NOT NEED TO BE LIMITED TO HARD ROCK!

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1. Rock Types and Depositional Conditions (Borehole Stability). There is general agreement that limestones and hard sandstones are good candidates for underbalanced drilling. What is now apparent is that shales, clay shales (young shales) and many poorly consolidated sands are also excellent candidates for underbalanced drilling and completions. The hard Devonian Shales of the U.S. Eastern Provinces have produced gas and oil for decades. The Bakken Shale of North Dakota is a big producer from a relatively thin formation.

The soft Miocene sands and clay shales of the world’s marine basins have been drilled underbalanced and shown excellent results. A good example is the pressure-depleted Miocene fields in Lake Maracaibo in Venezuela, where aerated mud has reduced lost-circulation problems and improved production.

2. Pore Pressure (Formation Damage, Lost Circulation). Pore pressure is the major determinant for the borehole pressure from the mud column. Short of having actual production pressures, pore pressures from well logs are the main source of data on formation pressure. Before starting an underbalanced drilling project, pore pressure estimations should be available for all the open sections of the hole. 3. Fracture Pressure (Lost Circulation). Fracturing is a function of pore pressure. Fracture pressure based on calculations using pore pressure or field data is an important part of underbalanced drilling studies. 4. Fractures (Lost Circulation and Well Kicks). Open or partially open fractures always add to drilling and completion opportunities and problems. For a period of time, it was believed that open fractures would not be present at significant depths. Now it is apparent that fractures can be held open by the reservoir fluid. In open or loosely sealed fractures, the lost circulation pressure and the pressure at which the well flows are essentially the same. A well can be controlled under this condition, but it will always be flowing or losing fluid to some degree. 5. Permeability (as Reflected in Differential Sticking). Differential sticking occurs when drilling fluid leaks into the formation, leaving a fairly impermeable layer of solids on the wellbore. If the drill pipe or tubing is in contact with the wellbore, the filtrate can leak away from behind the pipe and create a low-pressure zone. The differential pressure over the area involved creates forces that

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cause pressure sticking or differential sticking of the pipe. This cannot happen if the well is underbalanced. Stuck pipe can be freed by changing the well condition to underbalance. 6. Water-Sensitive Shales (Borehole Stability). Water-sensitive shales are usually obvious. Shale samples soaked in water swell or dissolve. Some older shales (Cretaceous and older) are not obviously water-sensitive from a soak test, but are actually quite sensitive to water under wellbore conditions. An excellent example of such a shale is the Bakken Shale of the Williston Basin. Such shales can soak in water in the laboratory for months and not show any swelling or softening. In the wellbore, their reaction is quite different. So while not all shales are water sensitive, for underbalanced drilling, the assumption should be made that they are, and appropriate mud, foam, or mist properties need to be used. When laboratory data are used to determine shale stability, the proper questions need to be asked and all pertinent information needs to be given to the laboratory.

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7. Tectonic Stresses (Borehole Stability). This is a real example of “What you know that is not true will hurt you.” Considerable rock mechanics work has been focused on rock stability. Studies and field examples by McLellan (1994) show that elastic yielding of rock is quite complex and occurs at greater differentials and at lower rates than is normally assumed. This is important when looking at UB horizontal drilling in pressure depleted reservoirs where theoretical calculations based on the depleted pore pressure would lead to the conclusion that the wellbore structure is very fragile. In addition, Warren, McLellan, and Pratt (1995) in a study of stability in the unconsolidated oil sands in Peace River, Alberta, Canada, pointed out the importance of filter cake (from the drilling mud) in controlling the yielded zone. This is another area of underbalanced drilling where the seemingly obvious may not be true, and careful analysis and laboratory work need to be undertaken (Figure 1.5.1).

Figure 1.5.1. Wellbore Stability (Warren et al., 1995)

8. Rock Hardness (ROP). Apparent rock hardness in drilling is in part a function of the pressure from the borehole fluid column. Another major component is the strength of the rock itself. Finally, there is the problem of cleaning the bottom of the hole. In underbalanced holes, there is always the possibility of drilling faster than the bottom can be cleaned.

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CHAPTER 1.6—The Well Candidate Vs. Specific Procedures The final screening procedure in the study of an underbalance candidate is to go back to the original reason for considering the underbalance and relate it to the various procedures available. There may be governmental regulations that affect the decision process (see Section 12).

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When it is possible to use a straight liquid system, these have a significant advantage because they do not have the potential for major pressure surges and velocity surges possible when using gas or air. Pressure and velocity surges are the cause of most problems with compressible underbalance fluids.

Figure 1.6.1. Fluid Type Vs. Density

MIST

AERATED

FOAM

99.8%

500/1 100/1

98%

50/1 30/1 10/1

90% QUALITY

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Figure 1.6.2. Air Ratio Vs. System Type Sec. 1

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Underbalanced drilling procedures are compared below and are presented beginning with maximum pressure reduction.

ADVANTAGES

PROBLEMS

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1. DRY GAS: NATURAL GAS OR NITROGEN A. Maximum drill rate A. Wet formations B. No downhole fires B. Some hazard from natural gas on the surface C. Nitrogen is the most expensive but safest C. Cost, especially with nitrogen D. No corrosion D. Cost in large diameter holes E. Might be possible to drill deeper with E. No wellbore support nitrogen or natural gas because they are inherently drier than air. 2. DRY AIR A. Maximum drill rate B. Least expensive C. No corrosion inside the DP. Corrosion of the outside of the drill pipe possible with dampness (see Mist).

A. B. C. D.

Wet formations Possible downhole fires Cost with large diameter holes No wellbore support

3. MIST: AIR, NITROGEN, OR NATURAL A. A. Makes it possible to continue drilling in damp formations B. B. Generally represses downhole fires C.

GAS W/WATER, MUD & OIL Lower drilling rate than with dry conditions Requires more air or gas Dampness causes corrosion in the drill pipe with air D. Dampness may cause shale instability E. No wellbore support

4. FOAM: NITROGEN OR AIR A. Exceptional lifting capacity B. Variable wellbore pressure C. Wellbore pressure can be closely controlled D. Needs less gas than any other procedure

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A. Complex mixing system B. Wets the formation C. Corrosion in the drill pipe possible with air D. There may be foam disposal problem E. Reusable foams require more equipment at the surface F. No oil foam available

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5. NITRATED WATER OR OIL A. Little nor no danger of downhole fire B. Little or no corrosion in fresh water C. Nitrated oil (crude or diesel)—no corrosion

A. Pressure surges are major problems B. Potential for corrosion with formation or salt water C. Further potential for corrosion with H2S with water

6. AERATED MUD A. Can take advantage of mud properties (density, filter cake, inhibition) B. Easy to increase mud density or mud up if there is a problem C. Only limited special surface equipment

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A. B.

7. O IL OR INVERT EMULSION A. Simple but limited to about 7.5 ppg (0.9 SpG) with diesel oil or 8.0 (0.96 SpG) with invert emulsion B. No water wetting of the formation C. No pressure surges

8.

C.

Wets the formation Strong potential for drill pipe corrosion Pressure surges

A. B. C. D.

Expensive Environmental laws Disposal problems Oil or water will calcify in the mud

“WATER” OR LIGHTWEIGHT MUD A. Nondamaging filter cake possible A. B. No pressure surges B. C. Can use ultraclean drill-in fluid

Oil emulsifies in the mud Disposal problems

9. GLASS BEADS IN THE DRILLING FLUID (Medley et al., 1997) A. Simple, but limited to about 7 ppg A. Special equipment needed for solids (0.84 SpG) in mud control B. No pressure surges C. No corrosion 10. CONVENTIONAL MUD USED IN AN UNDERBALANCED CONDITION

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SOLUTIONS TO DRILLING PROBLEMS WITH UB DRILLING In all of the following areas, horizontal wells add a further dimension. 1. Hard Rock—Low Penetration Rate. If the candidate well passes the formation screening, there is an abundance of literature and experience to guide well design for air or mist drilling. Since penetration rate is affected by differential pressure in the wellbore, any method of reducing the mud column pressure should improve the drilling rate. The question then becomes economics, safety, and regulations. 2. Crooked Hole—Low Penetration Rate. Increasing the penetration rate is limited by low bit weights. The air hammer with air or gas is an ideal solution. The addition of a light mist represses penetration rate. Foam or other techniques have less effect on bit weight versus penetration rate and the economics of these techniques needs to carefully considered.

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3. Lost Circulation. This is a simple matter of downhole pressure. With lost circulation, the depth of the fluid column can be measured and the bottom-hole pressure calculated. It is always best to use noncompressible fluid solutions to avoid pressure surges in the hole. Greater pressure reductions are possible with foam, and maximum reduction with air or gas. Where other problems relating to mud properties are present, and some mud column pressure is required, it might be necessary to plan ahead to use a parasite string and aerated mud. 4. Depleted Reservoir. This problem combines lost circulation with differential pressure sticking and often slow drilling rates. The reservoir pressure needs to be balanced with the most suitable fluid. The added requirements to control reservoir flow and protect against reservoir damage require special care in defining the boundaries of the problem. Any pressure surge above the reservoir pressure will cause some damage from the drilling fines, if from nothing else. The best solutions are dry gas or nitrogen, possibly oil mist with nitrogen or one of the noncompressible light fluids. It is difficult to avoid pressure surges with aerated fluids and difficult with any UB system to complete the well without overbalancing the pressure. Drilling with coiled tubing may be an effective solution for these conditions.

5. Differential Pressure Sticking. The best solution for sticking is to determine the pore pressure of the permeable formation where the sticking is occurring and reduce the formation pressure to below that level. Oil might serve a dual purpose in this case. SOLUTIONS TO COMPLETION PROBLEMS

1. Skin Damage. It is probably impossible to avoid some skin damage when drilling into the reservoir even under balance. Bennion (1996) makes an excellent point that glazing of the formation from the pipe and fine solids, and capillary inbibition in low water saturation reservoirs may defeat the best UB plans. Nevertheless, it is apparent that skin damage to low pressure and low energy reservoirs, where there is a major problem with future production, can be limited by drilling into them underbalanced. Dry natural gas or nitrogen is a good solution if flow from the reservoir can be handled at the surface. A UB mud with the new type of nondamaging filter cake should be considered.

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INBIBITION—Capillary pressure in fine grained sandstones with permeabilities in the range of ± 1 md may be greater than the underbalance force. (Bennion, 1996)

2. Fluid Sensitivity, which here is used to define several problems relating to the chemistry of the fluids and geometry of the permeability, is important in UB considerations because often no thought is given to mist or foam water. Bennion (1995) notes that foam can be an excellent blocking material in some pore spaces. Emulsifiers found in foaming and misting agents can also cause emulsion blocking. True underbalanced drilling, when the completion is done underbalance, may not resolve all the reservoir damage problems relating to pore material chemistry and geometry. Laboratory screening for potential problems may help define the limits to damage with various fluids and pressures (Bennion, 1996).

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3. Depleted Reservoirs, which have some energy, can often clean up if not extensively damaged during the entry. Many of the same comments above apply for a depleted reservoir with the additional concern of avoiding the loss of whole mud to the well. If it is possible to handle surface flow, the sandstone zones are best drilled with gas. 4. Fractured Reservoirs. In fractured limes like the Austin Chalk of Texas, the fractures appear to clean up even from whole mud if large volumes of high-solids mud are not lost to the formation. Drilling with a continuous kick and overpressure surges during connections, which approximate an aerated mud, appear to do little long-term harm.

5. Horizontal Wells using open-hole completions or screens benefit from a stringent underbalanced drilling and completion regime. The primary problem is to avoid pressure surges which will overbalance the reservoir. CONCLUSION

There is no guarantee of a unique solution to underbalanced drilling problems. Air, gas, mist, and foam are all variations on a system. There is a gradation between aerated mud and foam. Procedures using light fluids overlap with some aerated mud techniques. The best approach might be to eliminate the negatives; whatever is left is the most appropriate solution.

“Examine all possible solutions and discard those that are incorrect. Whatever is left, however unlikely or irrational, is the best solution.” “Sherlock Holmes” Solution

Experience is a good teacher in downhole problems. It is generally impossible to calculate all the conditions and potential problems because the basic downhole parameters are not known. What remains then is a best guess.

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SECTION 1—REFERENCES

Allen, J.H., 1976: “A Review of Reverse Circulation Air Lift Methods for Big Hole Drilling,” World Mining, January. American Oil & Gas Reporter Staff, 1998: “UBD Holds Wide Potential Application,” The American Oil & Gas Reporter, April. Bennion, D. Brant, 1997: “Reservoir Screening Criteria for Underbalanced Drilling,” Hart’s Petroleum Engineer International, February. Bennion, D. Brant, 1996: “Selecting Proper Fluid Critical to Successful UBD Operation,” The American Oil & Gas Reporter, August.

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Bennion, D.B. 1996: “Screening Criteria Help Select Formations for Underbalanced Drilling,” Oil & Gas Journal, January 8. Bennion, D., and Thomas, F.B, 1994: “Underbalanced Drilling of Horizontal Wells: Does It Really Eliminate Formation Damage?” SPE 27352, February, 1994 Bennion, D.B., Brent, T.F., Bietz, R., Bennion, D.W., 1994: “Underbalanced Drilling, Praises and Perils—Lab and Field Experience,” SPE/CIM Conference Horizontal Wells, Calgary, Canada, November 21. Bieseman, T. and Emeh, V., 1995: “An Introduction to Underbalanced Drilling,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Burge, P., 1995: “Further Advances in Underbalanced Drilling and Completion,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Cuthbertson, Robert L., 1998: “Why Work Underbalanced?” Hart’s Oil and Gas World, October.

Deis, P.V., Yurkiw F.J., and Barrenechea, P.J., 1995: “The Developments of an Underbalanced Drilling Process: An Operators Experience in Western Canada,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Duda, John R., Medley, George H., and Deskins, W. Gregory, 1996: “Strong Growth Projected for Underbalanced Drilling,” Oil & Gas Journal, September 23. Essery, R.L. and Rogers, E.E. 1976: “Techniques and Results of Foam Redrilling Operation—San Joaquin Valley, CA,” SPE 5715. Falk, K. and McDonald, C., 1995: “An Overview of Underbalanced Drilling Applications in Canada”, SPE 30129, May. Francis, P.A., Patey, I.T.M., Spark, I.S.C, 1995: “A Comparison of Underbalance and Overbalanced Drilling-Induced Damage Using Reservoir Conditions Core Flood Testing,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Gedge, Ben, 1997: “Recent Developments in Underbalanced Drilling,” presented at the 1997 IADC European Drilling Issues Conference, Berlin, June 5-6.

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Gray, R. 1995: “Laboratory Evaluation of Underbalance Formation Damage Compared to Neutral and Overbalance Conditions,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Guo, B., Hareland and G., Rajtar, J.M., 1995: “Design of Aerated Mud Drilling Programs,” PDVol.65, Drilling Technology, ASME. Jokhoo, K., 1976: “Aerated Foam Drilling in Trinidad,” Petroleum Engineer, June. Joseph, Robert A., 1995: “Underbalance Horizontal Drilling, Part 1: Planning Lessens Problems, Gets Benefits of Underbalance,” Oil & Gas Journal, March 20. Kaitsios, E. et al, 1994: “Underbalanced Drilling Through Oil Production Zones With Stable Foam in Oman,” IADC/SPE Drilling Conference, Dallas, TX.

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Larsen, V. and Birkeland,R., 1995: “Underbalance Drilling Offshore,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Leising, L.J., Rike, E.A., 1994: “Underbalance Drilling with Coiled Tubing and Well Productivity,” SPE 28870. Lorenz, H., 1980: “Field Experience Pins Down Uses for Air Drilling Fluids,” Oil & Gas Journal, May 12.

Lorenz, H., 1980: “Why Air, Mist, or Foam Drilling?—Applications and Experience,” Drilling Technology Conference, March 18. Lunan, B. and Curtis, F., 1997: “An Integrated Team Approach to Underbalanced Drilling,” paper 97-75, presented at the 48th Annual Technical Meeting of the Petroleum Society, Calgary, June 8-11.

Lunan, B. 1995: “Under-Balanced Technique Yelding Positive Impact,” The American Oil & Gas Reporter, April. Lunan, B. 1994: “Underbalanced Drilling - Two Case Histories, Western Canadian Basin,” 6th Annual International Conference on Horizontal Well Technologies and Applications. Philip C. Crouse & Assoc. Houston, Texas, November. Lyons, William C., 1984: Air and Gas Drilling Manual, Gulf Publishing Company, Houston, Texas. MacDonald, R., 1995: “Winning with Underbalanced Drilling,” Paper No. 95-104, CADE/CAODC Spring Drilling Conference, Calgary, Canada, April. McCaffery, F.G., 1973: “The Effect of Wettability, Relative Permeability and Inbibition in Porous Media,” Ph.D. Thesis, University of Calgary, September. McLellan, P.J., 1994: “Assessing the Risk of Wellbore Instability in Horizontal and Inclined Wells,” SPE/CIM/CANMET. McLellan, P.J., Wang, Y., 1994: “Predicting the Effects of Pore Pressure Penetration on the Extent of Wellbore Instability,” SPE/ISRM 28053.

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Medley, George H. et al. 1997: “Field Application of Lightweight Hollow Glass Sphere Drilling Fluid,” SPE 38637, presented at SPE 72nd Annual Technical Conference & Exhibition, San Antonio, TX, October 5-8. Mullane, T.J. et al., 1995: “Benefits of Underbalance Drilling: Examples from the Weyburn and Westerose Fields, Western Canada,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October 5. Murch, Colin B., 1998: “Underbalanced Drilling: An Integrated Approach,” Drilling Contractor, July/August. Plaxton, B.L. et al., 1997: “Modeling Drilling Rate of Penetration in Underbalanced Horizontal Wells — A Case Study,” paper 97-122, presented at CADE/CAODC Spring Drilling Conference, Calgary, April 8-10. Pratt, C.A., 1995: “Modifications to and Experience with Percussion Air Drilling,” SPE/IADC 16166.

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Rehm, W., 1963: “Lost Circulation Your Problem? Don’t Overlook Aerated Mud,” Oil & Gas Journal, December 2.

Rommetveit, R., Vefring, E.H., Bieseman, T., Faure, A.M., 1995: “A Dynamic Model for Underbalanced Drilling with Coiled Tubing,” SPE 29363. Russell, B.A., 1993:“How Surface Hole Drilling Performance Was Improved 65%,” SPE 25766. Saponja, J., 1995: “Comparing Conventional Mud to Underbalanced Drilling in a Depleted Reservoir,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Scott, S.L., Wu, Y., Bridges, T.J. 1994: “Air Foam Improves Efficiency of Completion and Workover Operations in Low Pressure Gas Wells,” SPE Mid-Continent Gas Symposium, Amarillo, TX, SPE 27922, May. Scott, S.L., Yulin Wu, 1994: “Air Foam Improves Efficiency of Completion and Workover Operations in Low-Pressure Gas Wells,” SPE 27922. Springer, S.J., et al., 1994: “A Review of the First 1500 Horizontal Wells in Western Canada,” SPE/CIM Conference on Horizontal Wells, Calgary, Canada, November. Stone, R., 1995: “The History and Development of Underbalanced Drilling in the U.S.A.,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October. Surewaard, J. 1995: “Underbalanced Operations in Petroleum Development Oman,” 1st International Underbalanced Drilling Conference, The Hague, Netherlands, October.

Teel, M. 1995: “What’s Happening in Drilling: Pseudo-Underbalanced Drilling and Beyond,” World Oil, April. Warren, B.K., McLellan, P.J., Pratt, C.A. 1995: “Wellbore Stability, Drilling fluids Design, and the Drilling Performance of Horizontal Wells in Unconsolidated Oil Sands at Peace River, Alberta.” SPE/IADC. Wilson, G.E. 1981: “A General Overview of Air Drilling and Deviation Control,” SPE/AIME. Sec. 1

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World Oil Staff, 1995: “ Underbalanced Drilling, Concept and Considerations,” World Oil, June.

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Yee, S., Comeaux, B., Smith, R., 1995: “Recent Advances in Underbalanced Horizontal Well Drilling,” 7th Annual International Conference on Horizontal Well Technologies and Applications, Philip C. Crouse & Assoc. Houston, Texas, November.

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