Drilling Extended-Reach/High-Angle Wells Through Overpressured Shale Formation
- G.J. Guild (Amoco UK Exploration Co.) | J.T. Jeffrey (Amoco Production Co.) | J.A. Carter (Amoco U.K. Exploration Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1994
- Document Type
- Journal Paper
- 161 - 166
- 1994. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.12.1 Measurement While Drilling
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This paper discusses Amoco U.K. Exploration Co.'s unsuccessful attempt to drill an extended-reach well in Arbroath field (U.K. Block 22/17) and how problems encountered were resolved on subsequent high-angle and extended-reach drilling operations in the field. The resolution of these problems over the course of drilling four high-angle wells and one extended-reach well involved (1) determining mud weight by use of rock mechanics principles, (2) evaluating hole conditions with a wellsite torque-and-drag program, (3) optimizing BHA performance, and (4) developing techniques for effective high-angle hole cleaning. By use of the methods outlined in this paper, improved drilling performance and significant cost savings are demonstrated.
The initial attempt to drill Arbroath Well T14, an extended-reach well with a maximum 70° inclination, was suspended because of caving shale formation and inadequate hole cleaning. In this field, shale can be mechanically unstable at high wellbore inclinations. Earlier field experience by an offset operator had indicated that wells in this area were limited to 65° inclination because of the in-situ stress found in the shale. After suspension of work on Well T14, an evaluation of problems encountered and possible solutions was conducted. The unstable nature of the shale as the wellbore inclination increased was confirmed to be the primary problem. Kwakwa et al. had previously demonstrated the importance of mud weight as a means to stabilize the Tertiary period shale in this area.
Another major problem was the difficulty in determining how well the hole was being cleaned during drilling. Johancsik et al. had previously developed a drillstring torque-and-drag model and Brett et al. had demonstrated the use of this model as a means of monitoring hole cleaning. Kwakwa et al.'s mud weight recommendations and Brett et al.'s hole-cleaning monitoring recommendations were successfully applied during the drilling of four high-angle wells and one extended-reach well together with (1) a backreaming program to clean the wellbore, (2) use of a variable gauge stabilizer to improve steerable bottomhole assembly (BHA) performance, (3) drilling of a 16-in. intermediate hole instead of a 17 1/2-in. intermediate hole, and (4) use of 6 5/8-in. drillpipe to improve hydraulics and hole cleaning. These methods resulted in improved drilling performance and significant cost savings.
Arbroath field is an oil development in North Sea U.K. Block 22/17. Its sandstone reservoir, which is normally pressured, is at 8,300 ft TVD. Fig. 1 shows that the reservoir is overlain by a massive section of Tertiary shale with a maximum 12.8-lbm/gal pore pressure. The shale requires both chemical inhibition to prevent it from swelling and a mud weight significantly higher than pore pressure to stabilize it mechanically. To drain the reservoir, development of the field outer edges was necessary. This required wells with a horizontal displacement of up to 15,000 ft and tangent angles of up to 74 . All well paths would pass through the overpressured Tertiary shale.
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