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In the Phu Horm Field in Udon Thani Province, Northern Thailand, conventional overbalanced drilling of fractured carbonates had presented severe difficulties. The first 2 wells drilled in the field, Phu Horm 1 and 2, were sub-economic. Therefore, the Phu Horm 3 location was chosen to maximize the likelihood of encountering an extensive fracture network that would increase productivity and deliver economic flow rates. Amerada Hess commenced drilling the Phu Horm 3 well conventionally using a water-based mud. The well was drilled into the top of the Pha Nok Khao Limestone reservoir, wherein severe fluid losses were encountered. The loss of hydrostatic head resulted in a gas kick with 1200 psi at surface. After pumping large quantities of loss circulation material in an attempt to block the fractures, the bottomhole assembly (BHA) became stuck. The drillstring was explosively severed, 71m of BHA were cemented in the hole, and operations were suspended. This paper will discuss how a sidetrack was drilled on Phu Horm 3 using underbalanced drilling techniques in order to overcome the problems of lost circulation and safely maintain pressure control. The well was flow drilled underbalanced by injecting fresh water as the underbalanced fluid and producing gas, water, solids and condensate back to a four-phase separation package. The well was successfully completed barefoot and underbalanced. The paper will discuss the planning and execution of the underbalanced drilling operation, including some of the challenges, and learnings gained for future operations. On Phu Horm 3 the primary objective of using underbalanced drilling technology was to enable the well to be drilled safely while maintaining pressure control and minimizing the potential of severe fluid loss. These problems had been experienced while drilling the over-pressured reservoir section of the original well. Other objectives were to reduce formation damage and evaluate productivity and reservoir characteristics during the drilling operation. The Phu Horm 3 underbalanced project is categorized as a Level 5B5 well in the IADC UB classification system due to circulating wellhead pressures in excess of 3000 psi and gas flow rates of 50 MMscfd being seen during the course of operations. The well was drilled without any safety or environmental incidents, or loss or damage to equipment. This underbalanced drilling project verified the applicability of underbalanced drilling in the Phu Horm field, as well as discovering major gas reserves. |
