GM-FP-BOP-01 - Stripping Operations Rev 00

GM-FP-BOP-01

Guidelines and Manuals

Stripping operations Exploration & Production

Rev: 00

Date:09/03/2004

Page: 2 of 24

TABLE OF CONTENT 1 INTRODUCTION.........................................................................................................3

2 PROCEDURE TO FOLLOW .......................................................................................3

3 PRACTICAL PROCEDURE........................................................................................4

3.1 Stripping with an annular blowout preventer ..........................................................................5

3.2 Stripping with two combined preventers...............................................................................13

3.2.1 Principle .............................................................................................................................13

3.2.2 Stripping with annular/ ram preventers ..............................................................................13

3.2.3 Stripping with dual ram preventers.....................................................................................14

4 RECOMMENDATIONS FOR STRIPPING ................................................................ 15

5 STRIPPING AND VOLUMETRIC CONTROL CALCULATION ................................ 21

This document is the property of Total. It must not be stored, reproduced or disclosed to others without written authorisation from the Company.

stripping operations rev 00.doc

GM-FP-BOP-01

Guidelines and Manuals

Stripping operations Exploration & Production

Rev: 00

Date:09/03/2004

Page: 3 of 24

1 INTRODUCTION A study of blow out frequency in the American waters of the Gulf of Mexico region showed that approximately 40% of all blowouts are a direct result of kicks in the well during tripping. The explanation for this relatively high percentage lies in the difficulty of controlling a well taking a kick when the drill bit is far above the bottomhole. In this domain, prevention is always better than cure, and therefore everything should be done to detect a kick; drilling teams must be constantly made aware of the risks of swabbing, and of the importance of the correct use of the trip tank during drillstring tripping operations. These guidelines describe the operations to be carried out and other procedures to follow, to run the drill string back down hole, under pressure below the closed blow out preventers.

2 PROCEDURE TO FOLLOW Standard procedure formerly used to consist of re-running the bit as far down hole as possible, as long as the flowrate was not excessive. The uncertainty arose when the time came to decide whether or not the flow rate was excessive. In the past, this uncertainty resulted in considerable kicks in the well, making well control more difficult or even impossible. One factor which is important, but often neglected, is the position of the kick in relation to the drillstring. If the drill bit and the Drill Collar have, run into a less dense fluid without the driller being aware of it , then there will be a considerable drop in hydrostatic pressure. This may result in overpressure loss in the producing formation, a rapid increase in kick volume and finally, high pressures at the surface. Consequently, the correct procedure is to shut in the well at the first sign of flow. If this procedure is followed to the letter, then pressures indicated at the surface will be minimal in all cases and therefore ensure more methods available for use in killing the well. The best way of running the drill bit down to the bottom again as quickly as possible, with pressure in the well, is to choose a combination of the stripping method and the volumetric method.

This document is the property of Total. It must not be stored, reproduced or disclosed to others without written authorisation from the Company.

stripping operations rev 00.doc

GM-FP-BOP-01

Guidelines and Manuals

Stripping operations Exploration & Production

Rev: 00

Date:09/03/2004

Page: 4 of 24

3 PRACTICAL PROCEDURE The most frequently heard objections raised against stripping drill pipes back to bottom, through an annular blowout preventer are: 1. Excessive pressure at the surface. 2. Excessive overpressure when the pipe is run in. 3. Difficulty in maintaining a correct bottomhole pressure due to the entry and/or expansion of the gas bubble. 4. Loss of hydrostatic stability when the drillstring is at the same level as the gas bubble. To avoid excessive pressure at the surface, the correct procedure for closing the well, as described above, should be followed, i.e. shut in the well as soon as the first sign of flow is detected. When the well has been shut in, if the pressure readings are low, stripping would appear to be the most efficient method. Bottomhole pressure can be controlled during stripping by combining it with volumetric control. Figures 1 to 5 illustrate this procedure: the drop in hydrostatic pressure that occurs when the drill collar enters the gas bubble can be compensated by maintaining a constant pressure at the bottom, reducing overpressures to a minimum and adding a safety pressure (S). The following hypotheses were adopted to simplify the procedure and facilitate its application in tripping operations: 1. The kick is assumed to be a trip gas bubble which forms in the drilling phase. 2. The kick remains in place as a continuous slug, entirely filling the annular section of the well. 3. To calculate the decrease in hydrostatic pressure, the bubble is assumed to migrate to a constant annular space between the wellbore/drill collar (the smallest annulus, although, this said, the wellbore/drill pipe annulus is unknown). The method is divided into three separate operations, again to make it easier to put into practice on site: 1. The driller runs in the drill pipes at constant speed, avoiding overpressures in the well, after first adjusting the closing pressure of the annular blowout preventer to prevent any leaks occurring when the tool joints pass. The closing pressure can be determined after stripping exercises or from the manufacturer’s information (see Fig.2). 2. A choke operator maintains a constant, predetermined pressure in compliance with the instructions given by the toolpusher. 3. One or two operators assigned to the trip tank/stripping tank continuously measure(s) the volume of returns in the trip tank and recover(s) from the stripping tank the volume of mud corresponding to each stand run in the hole. If necessary, add a measure of foam inhibitor to the mud in the trip tank to help obtain a sound interpretation of the levels in the two tanks.

This document is the property of Total. It must not be stored, reproduced or disclosed to others without written authorisation from the Company.

stripping operations rev 00.doc

GM-FP-BOP-01

Guidelines and Manuals

Stripping operations Exploration & Production

3.1 1.

Rev: 00

Date:09/03/2004

Page: 5 of 24

Stripping with an annular blowout preventer Once the well is shut in, calculate the volume of the kick and control the pressure at the surface at two-minute intervals. When the pressure has stabilized, it will be recorded every five minutes or at every “stripped” length.

2. Calculate the specific volume of the mud in the annular space wellbore/drill collar which corresponds to 1 bar of hydrostatic pressure.

Equivalent volume (in l/bar) =

Vas × 10,2 d1

Vas: Volume of annular space wellbore/drill collar (in l/m) d1: Initial mud density. 3. Select the value of the working pressure level ∆p : between 5 and 10 bars inclusive, it must be readable on the control manometer. 4 Convert this working pressure ∆p into a working volume corresponding to the wellbore/Drill Collar annulus.

∆v = (

Vas × 10.2 ) x ∆p d1

This is the volume of mud that will be used for the levels in the volumetric method (see figure4). 5. Calculate the additional pressure “S” which will compensate for the loss of hydrostatic pressure when the drill pipe and the drill collar are in the gas bubble (not to be confused with the trip margin). However, as the exact position of the kick in the well is unknown and it is therefore impossible to say exactly when the additional pressure should be applied, it is recommended that adequate safety measures be taken from the start of the stripping operation. It is essential to know the kick volume observed at the surface in order to calculate S: S=(

d1 - dgas Gain Gain )x Vas w - bore/DC V w - bore 10.2

Gain:

in litres

Vas wellbore/DC:

in l/m

V w-bore:

in l/m

d1 :

initial mud density

dgas :

gas density

This document is the property of Total. It must not be stored, reproduced or disclosed to others without written authorisation from the Company.

stripping operations rev 00.doc

GM-FP-BOP-01

Guidelines and Manuals

Stripping operations Exploration & Production

Rev: 00

Date:09/03/2004

Page: 6 of 24

6. Adjust the closing pressure of the annular BOP to an acceptable minimum; no leaks should be tolerated. Check that there is no flow above the annular casing during the adjustment of the closing pressure. 7. Start to lower (strip) the first length of drill pipes, already equipped with the full bore valve ¼ open and topped by the Gray type valve and allow the annular pressure to rise to a value Pa: Pa = Pa1 + S + ∆p Pa1: initial annular pressure S : additional pressure compensating for hydrostatic loss with bit/D.C. at the same level as the influx. ∆p : working pressure level NOTE: The bottom of the first stand is “stripped” with the choke closed until the required annular pressure (Pa) is reached. Only the remainder of the length, which is “stripped” at a constant pressure, should be taken into consideration in calculating the corresponding exterior volume to be bled off from the stripping tank. For example, if the annular pressure Pa is reached after “stripping” two singles in the first stand, only the volume of the 3rd single is recovered in the “stripping tank”. The same principle is also applied when a level ∆p is added. 8. Maintain this pressure level Pa constant throughout the stripping process The mud gain corresponding to the external volume (VE) of the lowered drillstring is recovered by purging from the trip tank, then transferred by gravity to the stripping tank once the length has been entirely run in to the well. This procedure means that any other volume increase in the trip tank is probably due to the gas bubble starting to expand and therefore the onset of hydrostatic pressure loss in the well. NOTE: If, due to an oversight during the stripping process, the bottomhole pressure dropped below the formation pressure (Pf