Coiled-Tubing Underbalanced Drilling in the Lisburne Field, Alaska

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 108337, "Coiled-Tubing Underbalanced-Drilling Applications in the Lisburne Field, Alaska," by Mark Johnson, SPE, BP; Patrick Brand, SPE, Blade; Sam French, SPE and Greg Sarber, SPE, BP; Dave Hildreth, SPE, Orbis; Bob Harris, SPE, Baker Oil Tool; Pedro Rangel, SPE, Schlumberger; Udo Cassee, SPE, Nordic; and Jimmy Clark, ASRC, prepared for the 2007 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Galveston, Texas, 28–29 March. The paper has not been peer reviewed. In 2005, BP Alaska began evaluating the application of underbalanced-drilling (UBD) technology as a method for drilling multilateral wells in the Lisburne field. The evaluation process was enacted as a response to slow rate of penetration (ROP) through the hard carbonate Wahoo formation; frequent total losses of drilling fluid when drilling conventionally; and poor understanding of the orientation, frequency, and effect of fractures on production. Introduction The Lisburne carbonate reservoir has approximately 2 billion bbl of original oil in place. The Wahoo formation, at a depth of approximately 8,900 ft true vertical depth (TVD), is tight, fairly thick (400 ft), and highly consolidated, with thin interbedded mudstone layers. Reservoir fluid is mainly oil with a gas cap covering a portion of the field. The gas contains approximately 60 ppm H2S and 12% CO2. Previous drilling in the reservoir was overbalanced. Lisburne is a hard-rock carbonate with unconfined compressive strengths from 15,000 to 25,000 psi. Drilling problems include low ROP when drilling overbalanced and instability of the mudstone layers when exposed to water-based muds (WBMs). Production performance of the reservoir in general has been disappointing, with an oil recovery to date of only 8% because of low matrix permeability and excessive gas production. It is apparent that the formation is fractured and that intersection of effective fractures is critical to well performance. Wells that have intersected significant fractures have been among the better performers. In a bid to intersect more of the fractures, BP Alaska had attempted through-tubing coiled-tubing (CT) sidetracking, placing single horizontal laterals in the carbonate sections. Production-performance delivery of these wells has been 50% successful, prompting consideration of other alternatives. One of the main alternatives considered was through-tubing CT-UBD sidetracks. The idea was to place multiple laterals from a single main wellbore within the carbonate sections, thus increasing exposure and avoiding the potentially unstable mudstones. The main motivation for UBD was to improve ROP and eliminate drilling problems associated with overbalanced operations.