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Abstract Underbalanced drilling (UBD) technology has been gaining in popularity around the world because of its capability to reduce or eliminate formation damage, to increase production rates, and in some cases, increase the volume of recoverable reserves. The technology is applicable to fields where formation damage is a concern or where problems such as severe fluid loss, differential sticking, steering problems or slow drilling rates are encountered with conventional drilling.
With underbalanced drilling, the formation pressure is greater than the hydrostatic pressure, allowing hydrocarbons to flow into the wellbore during drilling. This prevents potentially damaging drilling fluids and drilled fines from penetrating the producing formation.
In previous experiences in the field using traditional drilling methods, considerable formation damage had been experienced, and although UBD had never been attempted in the area before, Brunei Shell Petroleum and its government partner decided to use this technology in a three-well trial in the Rasau field, located onshore in Kuala Belait, Brunei.
Of the three wells attempted during this UBD trial one well could not be drilled due to catastrophic borehole collapse. The other two wells were successfully drilled to depth using UBD techniques; however, multiple hole volumes of solids were produced during drilling and production testing, indicating borehole stability problems.
Production rates observed after drilling to TD and prior to running completions indicated zero formation impairment, with well productivity exceeding expectation; however, during the completion phase, mechanical problems occurred, and post completion well tests indicated productivity reductions of 60 to 70% compared with the pre-completion tests. This paper discusses the planning, drilling, results, highlights, and lowlights from this UBD trial, along with learnings and recommendations for future application of the technology. The execution of the program led to a sharp learning curve, and the development of recommendations that can be applied to future operations in this field. These primarily relate to well (construction) design, drilling procedures, equipment design, rig-up and rig-down optimization between wells, and completion design.
Introduction The Rasau field structure is a local accumulation in a major anticline ridge extending from Miri, Malaysia to Seria, Brunei. The accumulation is divided into two fault systems, one trending in a southwest to northeast direction and the other in an east to west direction. The Rasau field has been on production since 1983 and there are currently 27 producing wells. The sandstone reservoirs are generally of moderate quality (1–100 md), and are of shallow marine origin, resulting in a laterally extensive stacked sequence. The producing reservoirs, typically located at a depth of 4900ft - 6500ft, contain saturated light oil (40°API) with varying sizes of gas cap. The drive mechanisms are reservoir specific and are typically a combination of gas cap expansion, solution gas drive and aquifer influx. To date, there have been no secondary recovery schemes implemented in the Rasau field.
The 3-well trial project incorporated several major challenges that had to be addressed through the initial planning / design phase and risk/hazard assessment processes. To minimize formation damage and reduce cost, produced oil and gas were chosen as the drilling fluids. The wells were drilled at high angle through the target reservoirs, cutting through the many thin layers of pay to maximize formation exposure. Due to the layered nature of the reservoir, a moderate amount of shaley formation was expected within the planned trajectories (up to 50%), the stability of which was to be assessed as one of the trial objectives. The stability of this material was to prove a key factor during the underbalanced operations, and is thought to have been the cause of significant problems due to borehole breakout and collapse. Fig. 1 shows the location of the Rasau trial wells, and Fig. 2 shows the trial-well trajectory.
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