In 2005, BP Alaska began evaluating the application of Underbalance Drilling (UBD) Technology as a method for drilling multilateral wells in the Lisburne Field. The evaluation process was enacted as a response to three key challenges at Lisburne:
Slow Rate of Penetration (ROP) through the Wahoo's hard carbonate formation,
Frequent total losses of drilling fluid when drilling conventionally,
Poor understanding of the orientation, frequency and impact of fractures on production.
The main objective of implementing underbalance technology in the Lisburne Field was productivity improvement. This was to be realized by improving ROP, thereby allowing for long laterals to be drilled, intersecting more fractures. Coiled tubing underbalanced drilling (CT-UBD) would also allow for drilling multi-lateral sidetracks from a single parent wellbore through the production tubing. In addition, it was felt that underbalanced drilling would eliminate losses to the formation, with a side benefit of potentially mitigating formation damage.
A two well, 5 lateral pilot project was approved. The 6th generation Alaska coiled tubing drilling rig, in continuous service since 2002, was adapted for use in an underbalance drilling mode. Due to the harsh North Slope winter climate and desire to minimize impacts to the environment, the UBD surface kit was designed to be small and to eliminate the need for flaring. This dictated that returned fluids be processed and sent to the production facility at or above pipeline pressure. A significant amount of front end loading was conducted to help ensure safe delivery of pilot objectives.
The pilot was completed during the summer of 2006 with excellent HSE performance. Although not without operational problems, the project demonstrated that underbalanced drilling could be used to increase rate of penetration and bit life. The pilot also demonstrated that underbalanced drilling could eliminate many of the hole problems associated with conventional drilling, making the drilling of long reach multilaterals feasible. The drilling rate more than doubled and a record Lisburne CTD horizontal length of 2,564 ft was achieved. The pilot also demonstrated the challenges and strengths of real time formation evaluation in identifying productivity features in the wells. Over 9,000 ft of lateral length was drilled and 14,000 barrels of oil were produced during the drilling operations. The cost of the project was 22% less than the cost per foot of overbalanced drilled wells in the same field. Additional underbalanced drilling is being contemplated for the future.
The Lisburne carbonate reservoir has approximately 2 billion barrels of OOIP. The Wahoo formation, at a depth of about 8,900 ft (TVDSS), is tight, fairly thick (400 ft) and highly consolidated, with thin inter-bedded 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 had been done overbalanced. Lisburne is a hard rock carbonate with unconfined compressive strengths from 15,000 to 25,000 psi. Drilling problems include low rate of penetration (averaging about 105 ft/ day) when overbalance drilling, and instability of the mudstone layers when exposed to water base muds. Production performance of the reservoir has in general, been disappointing, with an oil recovery to date of only 8% due to low matrix permeability and excessive gas production. Several strategies to improve oil production had been tried, including acid stimulation, vertical well hydraulic fracturing, etc., all with mixed results. 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 (as inferred from lost circulation) have been among the better performers.