analysis after stuck pipe

7/29/2019 Analysis After Stuck Pipe

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Lino L Canado Novenber 7th, 1999 Page 1

Analysis of the stuck pipe situation on the second well and other problems in theCTD operations on the second Cerro Negro campaign:

Problems:

Very slow ROPs as compared to last campaign:Despite the fact that we were using more powerful motors and that the stratigraphic column on the area that weare drilling now do not have the same hard formations encountered before (Jobo and Yabo on the new area aresofter) average ROPs are a lot slower. We took 58.5h and 64.5h (drilling time only) respectively to drill the firsttwo wells to the top of the basement, compared to about 32h on some of the best wells on the last campaign(using a DBS 4 blade bit w/ 4x10/32s and a 3 -1/8 7:8 motor).

Stuck in hole while POOH:On the second well we got stuck with the BHA while POOH and after the well was TD.

Clay inhibition:Even though the mud system under use right now is exact the same we used on the last campaign it was notedhole swelling and the fishing BHA could not get to the top of the BHA stuck in hole. A subsequent cleaning tripwith motor and bit brought large amounts of cuttings (shale w/ sand) to the shale shakers.

Crew experience and experience in the areaBesides one crew supervisor and the WSS all the other people on location were debuting in CTD in Cerro Negro.Additionally to that only the Tool Pusher, WSS, and the crew supervisors (CT drillers) had CTD experience, beenthe tool pusher the only one with reasonable experience. It is very difficult to make these wells faster and safer ifwe cannot build continuous experience with the crews.

Swaco mud system:Swaco built a tank with a circulation system, which is different from what has been proposed. Suction anddischarge lines are not the same as in the proposition, besides that there are a few safety issues to be fixed in thetank, like electrical wiring and a the need of a toe board around the tank working floor.

Discussion of problems:

ROPsFirst thought to improve ROPs as compared to last campaign was the use of bigger motors to take advantage ofthe higher power and better endurance. Drill each well with only one trip was the target, and Anadrill was aimingto drill each two wells with the same motor to reduce cost associated with repairs. Because of this Anadrillsrecommendation was to run a 3 motor schemed down to 3-3/8 OD. Despite of their own recommendationmotors provided at location were 3 with a 1/8 kick pad (3-5/8 OD) and a 3-3/4 bit box (2-7/8 REG thread)These large ODs left a clearance of only of an inch or 1/8 of an inch on each side (assuming it is centralized)around the kick pad area. At the bit box the clearance was even smaller: 1/8 of an inch, which correspond to1/16 if the BHA is considered centered.As proven on the real life the 3 motors didnt work for the following reasons:

It was needed 8Klb of WOB to drill ahead (20 to 25 ft/h), which required to snub weight with the CT injector to

keep drilling (-3,000lb to5000lb weight indicator!). On the second well pumping pressure went to values as high as 5,000psi with 110gpm ~ 120gpm, and it was

around 4,000psi on the first well. These values are quite high if compared to the 3,200psi ~3,600psi seen onthe first campaign with the same mud, but using a bit with 4x10s and a 3-1/8 motor. On paper the pressuredrop around the motor was calculated to be in the range of 30psi. What might happen on real life is that theplastic clays encountered on the majority of the drilling of the first two wells build up around the motor andreduce flow path, causing the higher pressures encountered.

The build up of clays around the tight clearance and the bent nature of CT could have forced the motor againstthe well walls as weight had to be snubbed to drill ahead, which produced a stick -slip condition that slowed


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down ROPs even more. The high pressure drop caused by a restricted flow path might have contributed to float the BHA, which

granted to increase even more the need to snub weight wit h the CT injector and than worsen the effectscommented above. The fact that the bit sub was 3 was not take into consideration on the pressure droparound the motor calculation and could have contributed to increase pump pressure and floating effect a swell.

Second idea to increase ROPs were the increase of the bit HSI. Reason been the low HSI used on the firstcampaign as compared to what is recommended for the PDC bits, and reinforced by the bit balling experiencedon the first well of the second campaign. An increase on the HSI was partially accomplished with a bit speciallymade with 4 x 8/32s instead of the normal 4 x 10/32s provided by most of the bit manufacturers for the PDCs inthis hole size. According to the bit manufacturer they couldn t make a bit with 4 x 7/32s ports. Because of this theHycalog bit which has 4 x 8s had one port plugged on the first 500 of the second well.

It is fully agreed that a better HSI at the bit would help to increase ROPs but it is doubtful that this was theonly reason that was slowing down the penetration rates on the first two wells. As commented above theROPs in the last campaign were twice faster with a bit that had 4 x 10/32s (a lot lower HSI) as compared to 4x 8/32s in the first well and 3 x 8/32s on the second well (one port was plugged of).

Most likely all the advantages that could be gained with a better HSI were offset by the problems caused withthe bigger OD motor as commented above.

Suggest ions:

All the comments above about the 3 motor are to be proven. It is suggested to run a 3-1/8 motor in thenext well to compare the results and than decide the best way to go on these stratigraphic wells drilled withCT. As this report is been written we have on location a 3-1/8 motor which will be used to re-drill the currenwell (PeastN) after the side track is executed with a 2-7/8 motor.

Other alternative is to drill a 4-3/4 hole and use the same 3 motors. In this situation there will be enoughclearance and we still can take the advantage of the better endurance and power of the 3 motor. This ispossible as we now have a 5-1/8 BOP, which is been prepared at Dowells El Tigre base.

Alternatives to improve on the bit design have to be looked into more details. To increase HIS it can be useda bit with 3 blades and 3 ports, which may help with the bit balling problem as well.

Some Resul ts :

After the MOF in the second well (BHA stuck in hole and lost) a side track was made at 1,462 after a 1000 CMTplug was mixed at 16ppg and pumped on the top of the fish (2,441). The side track was made using a 2-7/8 5:6standard motor (Trudrill made) w/ a 1.15 deg bent housing. Time drilled (5ft in 2h) and than drilled 40ft of holeusing the motor with bent housing. Next trip a 3-1/8 7:8 motor (Black Max made) was run and the results were asfollowing: Average ROPs increase from 30~40 ft/h to 180ft/h in the shallower shales Pumping pressures were around 4500psi, but using the Hycalog bit that has 4 x 8/32s as compared to

5,000psi w/ the 3 motor when run with the DBS bit that has 4 x 10/32s. Cuttings at the shaker had a defined shaped and looked like the cuttings seen on the other campaign (look

like chunks cut by a PDC bit) as compared to clays clogged in mushy balls when drilling with the 3 motorBesides that it was possible to observe chunks of coal as well, which are common in Cerro Negro and werenever seen at the shakers when using the 3 motor. Probable reason for that is that the small clearance onthe 3 motor (3 at the bit box) did not allowed the bigger size cuttings to be carried and pass the bit box

(3 ). This forced the cuttings to be grind by the bit before they were reduced to a size that would passthrough the tight clearance. This could have been another effect that slowed down ROPs and contributed tobit balling seen in the first well.

The 4 x 8/32s jets at the bit and the higher HSI seemed to help on ROPs as drilling was smoother than in thelast campaign. Detrimental fact of the smaller TFA was higher pumping pressures, around 4,500psi (onbottom) as compared to 3,600psi on the last campaign, worsen by the fact that the CT used this time is a 2 x0.204 WT and before it was 2 x 0.175 WT.

Stuck pipe


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Most likely the stuck pipe condition happened because of the ultra small clearance between motor and well ID,added to the fact that this area has a lot longer length of the typical plastic clays that are observed in Cerro NegroThe clays might have swallowed a little bit and the motor came shaving them on the way up, resulting in packingaround the motor. The big motor OD also avoided the back reaming action of the bit as bit and motor were aboutthe same OD! (3-3/4 in the bit box X 3-7/8 bit).The long time to finish drilling the well (64.5h drilling) resulted in longer exposition time and than higher chancesto have problem with the clays. This same problem was noted on the third well of the first campaign when theBHA got stuck for about 2h after it stayed on bottom circulating for 18h w/o movement (during a CT unit repair).After investigation it was also found that the operator inexperience in OH worsen the situation as an over pull of35K was applied (45K nominal) before it was tried to push down harder (pushed down only with 3.5Klb beforethe over pull was applied).

Suggest ions:

Increase BHA (motor) to bit clearance. This can be accomplished in one of the two ways mentioned above:Either reducing the motor size back to 3-1/8 or increasing the hole size to 4 and keeping the 3 motors.

It is a must to keep using the same personnel on the CTD operations to be able to build local experience. Itwill be very difficult to keep improving in time per well if we cannot keep the same personnel in subsequentcampaigns. If experience is not build, we will never be competitive on turn key the Faja stratigraphic wells.

Clay Inhibition:

Mud system in use is the same as in the last campaign but a few facts might haven worsen the problem with tighthole due to clays swelling:

Slower ROPs caused a longer exposure time of the clay sections Longer clay sections have been observed on this area of Cerro Negro.

Suggest ions:

Drill the hole faster to reduce the exposure time, which might be accomplished by the use of smaller motorsor bigger bot as mentioned above.

M-I to make other suggestions to improve clay/shale inhibition

analysis after stuck pipe

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