Underbalanced Drilling Of Fractured Carbonates In Northern Thailand Overcomes Conventional Drilling Problems Leading To A Major Gas Discovery

摘要

In the Phu Horm Field in Udon Thani Province, NorthernrnThailand, conventional overbalanced drilling of fracturedrncarbonates had presented severe difficulties. The first 2 wellsrndrilled in the field, Phu Horm 1 and 2, were sub-economic.rnTherefore, the Phu Horm 3 location was chosen to maximizernthe likelihood of encountering an extensive fracture networkrnthat would increase productivity and deliver economic flowrnrates.rnAmerada Hess commenced drilling the Phu Horm 3 wellrnconventionally using a water-based mud. The well was drilledrninto the top of the Pha Nok Khao Limestone reservoir,rnwherein severe fluid losses were encountered. The loss ofrnhydrostatic head resulted in a gas kick with 1200 psi atrnsurface. After pumping large quantities of loss circulationrnmaterial in an attempt to block the fractures, the bottomholernassembly (BHA) became stuck. The drillstring wasrnexplosively severed, 71m of BHA were cemented in the hole,rnand operations were suspended.rnThis paper will discuss how a sidetrack was drilled onrnPhu Horm 3 using underbalanced drilling techniques in orderrnto overcome the problems of lost circulation and safelyrnmaintain pressure control. The well was flow drilledrnunderbalanced by injecting fresh water as the underbalancedrnfluid and producing gas, water, solids and condensate back torna four-phase separation package. The well was successfullyrncompleted barefoot and underbalanced. The paper will discussrnthe planning and execution of the underbalanced drillingrnoperation, including some of the challenges, and learningsrngained for future operations.rnOn Phu Horm 3 the primary objective of using underbalancedrndrilling technology was to enable the well to be drilled safelyrnwhile maintaining pressure control and minimizing thernpotential of severe fluid loss. These problems had beenrnexperienced while drilling the over-pressured reservoir sectionrnof the original well. Other objectives were to reducernformation damage and evaluate productivity and reservoirrncharacteristics during the drilling operation.rnThe Phu Horm 3 underbalanced project is categorized as arnLevel 5B5 well in the IADC UB classification system due torncirculating wellhead pressures in excess of 3000 psi and gasrnflow rates of 50 MMscfd being seen during the course ofrnoperations. The well was drilled without any safety orrnenvironmental incidents, or loss or damage to equipment.rnThis underbalanced drilling project verified the applicabilityrnof underbalanced drilling in the Phu Horm field, as well asrndiscovering major gas reserves.