(2.1.592) Volumetric Stripping With Gas Migration Shri R.K. Khanna, CE(D), WCS, IDT, ONGC, Dehradun, India Shri Debasish Pramanik, CE(D), WCS, IDT, ONGC, Dehradun, India - (Main Author) Shri K.K. Sethi, CE(D), WCS, IDT, ONGC, Dehradun, India Shri Sandeep Saksena, SE(D), WCS, IDT, ONGC, Dehradun, India

Description & Application of Work Kicks taken in a well-bore with string off-bottom require stripping-in while maintaining a constant BHP (bottom hole pressure). Stripping-in is the technique used to lower the drill string through the BOP stack with the well under pressure. In order to avoid over pressuring the well as the drill string is stripped-in, a volume of mud equal to the closed end displacement of the drill pipe must be bled off. If the influx taken is gas which is migrating up hole, an additional mud volume (which depends on the rate of migration) needs to be bled off so as to maintain a constant BHP. Therefore, the operation requires a precise monitoring of changing BHP as gas is migrating up & the drill string is being stripped-in, more so if the margin between the shut-in casing pressure & maximum allowable annular surface pressure (MAASP) is narrow. The procedure of ‘Volumetric Stripping with Gas Migration’ described in various available technical publications works well if the MAASP is comfortable & a considerable overbalance can be tolerated in the well-bore. However, if this procedure were to be followed in such cases where the MAASP is critical (which is a common scenario in deep water drilling), it is likely to result in exceeding the fracture strength below casing shoe & thus may lead to loss circulation / under ground blow-out. The proposed work suggests a detailed step by step procedure of volumetric stripping with gas migration while keeping minimum overbalance & finds application in cases where MAASP is critical. Observation & Analysis When a well kick is noticed with string considerably off-bottom, a full opening safety valve in open position is installed on the string. The FOSV is closed & the well is shut-in. An inside BOP is installed above the FOSV & the FOSV is opened. The shut-in casing pressure (SICP) is allowed to increase & recorded periodically till it stabilizes. When the SICP has stabilized the BHP has almost become equal to the pore pressure. The gain in the surface pit volume is also recorded which indicates the volume of the influx. An increase in SICP after stabilization is indicative of a gas influx which has started migrating up the hole & this is causing an increase in the BHP too. Standard Procedure of ‘Volumetric Stripping with Gas Migration’ Now, if the standard procedure of ‘Volumetric Stripping with Gas Migration’ described in various available technical publications is to be followed, the initial influx volume (at this stage the influx is below the bit i.e in the open hole section) is used to calculate the influx height when the string would have been stripped below the influx (i.e when the influx against the BHA). The loss of hydrostatic due to increase in influx height against BHA is calculated & the d/string is stripped-in without bleeding at surface, allowing the casing pressure to increase by the aforesaid loss of hydrostatic plus a safety margin (say 100 psi). After the casing pressure has increased by this value, it is maintained constant by bleeding at surface & the mud bled is collected in a measuring tank (trip tank) as the string is being stripped-in. If the influx is gas & is migrating, the volume bled & collected at the surface shall be more than the close end displacement of the length of the d/string stripped-in. After every stand of d/string stripped-in, the close end displacement equivalent of mud volume is shifted to another measuring tank (strip tank). The excess mud volume remaining in the trip tank is noted & when this volume reaches a pre-calculated value the casing pressure is allowed to increase by hydrostatic pressure equivalent of this volume in the open hole section of the well-bore. The above described standard volumetric stripping-in procedure has following limitations : a) The initial overbalance is much higher & this can be detrimental if the MAASP is critical.

b) The actual gas influx volume when the gas reaches against the BHA, due to expansion, shall be higher & shall actually result in more hydrostatic reduction when it is positioned against the BHA than what is considered in this procedure.

Proposed Procedure of ‘Volumetric Stripping with Gas Migration’ The equipment consisting of a calibrated trip & strip tank should be set up as shown in fig. 1 for volumetric stripping. A strip sheet as shown in fig. 2 may also be prepared. It should be ensured that the pressure gauges to be used while stripping are calibrated. API guidelines should be followed for use of BOP stack components for stripping e.g. if surface pressure is less than 1000 psi, annular BOP can be used for any length of stripping whereas if the surface pressure is more than 1000 psi a combination of BOPs has to be used. In case annular BOP is to be used, provision may be made for surge bottle & the BOP’s element shall be lubricated by dumping oil or diesel or viscosified water on the element. Precautions should be taken to slow down the stripping speed as tool joint is passing the annular BOP to avoid damage to its element. To avoid higher surge pressures, the stripping speed should not exceed 1 ft/sec.

1. After the shut-in casing pressure is stabilized the string is stripped in initially without bleeding at surface till the casing pressure increases by a pre decided safety margin (say 100 psi).

2. Thereafter, stripping is continued by slowly bleeding close end displacement volume of the

length of the d/string stripped-in in the trip tank. 3. After every stand of d/string stripped-in, the increase in casing pressure is noted. Since we

are bleeding only the close end displacement volume, the increase in casing pressure is an indication of gas migration & shall depend upon the rate of migration.

4. The stripping-in is continued in this manner till the casing pressure increases by a pre-

decided value (say 50 psi), the stripping is now halted & this excess casing pressure is neutralized by bleeding at the surface. The extra mud volume (over & above the close end displacement equivalent of the stripped-in length of d/string) is collected in the trip tank.

5. The procedure (from s.no.2 to s.no.4) is continued till the excess mud collected in trip tank

reaches a pre-calculated value (say equivalent of 50 psi hydrostatic in the open hole section). When this happens, the initial safety margin (100 psi) has decreased by 50 psi.

6. At this stage the casing pressure is allowed to increase by 50 psi (this shall again increase

the safety margin to initial value of 100 psi) by stripping-in & without bleeding. 7. The stripping-in is resumed again by bleeding only the close end displacement volume of the

length of the d/string stripped-in. 8. The procedure is repeated (from s.no.2 to s.no.7) till the bit reaches the influx top & starts

entering the influx. 9. As the bit & the BHA is being stripped-in in to the influx, it shall be indicated by a much higher

increase in casing pressure (due to hydrostatic reduction as the influx height now increases against BHA) for every stand stripped-in as compared to the casing pressure increase observed earlier when the influx was below the bit.

10. This excess increase in casing pressure (which can be easily determined since we have been

recording the increase in casing pressure for every stand stripped in while the influx was below the bit) is retained so as to keep the BHP constant.

11. At this stage the position of the influx against BHA can be verified by making up the Kelly &

recording SIDPP & SICP using the rig pump. The difference in SIDPP & SICP is a clear indication of a part of influx having moved against BHA.

12. The stripping is continued till the influx is completely against the d/string. As & when it

happens, the SIDPP & SICP can be recorded using the rig pump (a zero value of SIDPP in case of swabbed-in kick indicates that the influx is completely against the d/string annulus) & the influx can be removed from the well-bore using first circulation of driller’s method.

The tabulated strip sheet pertaining to an exercise that was run on DRILSIM-5000 simulator at IDT is shown in figure 2. and schematic diagram in figure 3(a) to figure 3(d). This exercise was run with a gas migration rate of 500 feet/hour. It may be noted that some parameters which are documented in the strip sheet e.g. gas bubble volume, gas top depth & BHP shall not be available in an actual field stripping operation. Conclusions & Recommendations The proposed procedure ensures maintaining a constant BHP while stripping in with gas migration with a much lesser safety margin & also takes into account the hydrostatic reduction in well bore as per actual position & volume of gas around the d/string annulus. Moreover, while following this method accurately, we do get a clear indication (a sharp increase in casing pressure as close end displacement mud volume is being bled) when the d/string enters the influx, which can be corroborated by recording SIDPP. This facilitates the application of conventional driller’s method for removal of gas influx once the influx is completely against the d/string annulus. The procedure has been tested on DRILSIM-5000 simulator at IDT & has given expected results. Hence, it ensures more realistic maintenance of BHP vis-à-vis the various methods described in available technical publications.

Figure. 1

MGS

NRV

Calibrated Trip Tank

Calibrated Strip Tank

Calibrated Strip tank

Well Details: Well Depth 5603 ft Bit Position 3991 ft Kick Volume 13.02 bbl SICP Initial 755 psi Mud Weight 10.5 ppg

Strip Sheet Bit depth Gas

Top depth

BHP Casing Pessure

Influx Volume

Trip tank

reading

Volume Bled

Excess Volume

Bled Fig A 3991 4633 3770 809 13.02 0

4075 4598 3786 825 13.01 2.13 2.13 4165 4555 3805 844 13.02 4.26 2.13

Fig B 4254 4518 3830 869 13.02 6.41 2.15 Bleeding 4492 3771 811 13.25 6.77 0.36 4345 4453 3798 839 13.22 8.95 2.18 4436 4416 3816 861 13.21 11.05 2.1 Bleeding 4406 3770 814 13.35 11.26 0.21

Fig C 4524 4276 3771 875 13.26 13.39 2.13 Fig D 4615 4135 3794 942 13.41 15.51 2.12

4704 4028 3842 977 12.94 17.65 2.14

Figure. 2

Well Shut In on a swabbed kick – Surface Pressureallowed to increase (as Safety Margin) by 50 psi

3991

4633

Figure. 3(a)

809

9

Figure. 3(b)

Sharp rise in Casing inside the influx.

86

Stripped In three stands, surface pressure increased by60 psi thus BHP also increased by 60 psi. CasingPressure required to be reduced by 60 psi, excessvolume bled to be measured. When cumulative excessvolume bled becomes equal to 50 psi equivalenthydrostatic head in open hole Casing is increased by 50

4518

4254

psi thereby maintaining Safety Margin of 100 psi

4276 4524

875

Pressure indicating Bit enteres

Figure. 3(c)

942

4135

4615

Sharp rise in Casing Pressure continued. The reading ofSIDP & SICP will give an indication of influx comingcompletely against the string annulus. In case of aswabbed in kick SIDP will be 0 psi.

Figure. 3(d) Significance & Usefulness of Work The described procedure results in comparatively much precise control over BHP while stripping-in with gas migration, is simpler to follow & finds application in wells where the MAASP is critical especially in deep waters. References

a) Randy Smith Advance Well Control Training Manual b) Advance Well Control by Robert de Grace c) ‘Volumetric Stripping Guidelines’ Kingdom Drilling, UK