X gas field is located in the Corridor block of Onshore South Sumatra, Indonesia and operated by ConocoPhillips Grissik Ltd. (CPGL). The field was first discovered in 1994 and started production in 2001. In terms of reserves, X field is the second largest in the Corridor. All production comes from fractured igneous (granite, granodiorite) and metamorphic (meta conglomerate, quartzite, phyllite and marble) rocks of the pre-Tertiary basement.
To date, eight wells have been drilled during the exploration and development program. The most recent development well was the X-8, which was drilled in 2017. The X-8 well was designed to enter the top of the reservoir structure at a location where the seismic amplitude exhibited dimming. Based on analysis of the seismic and well productivity, dimming amplitudes are indicative of faults and fractures.
Historically, exploration, appraisal and development wells drilled in the field did not experience any wellbore stability issues. During drilling of the X-8, wellbore stability and collapse caused significant problems and resulted in the sidetracking of the well three times. Prior to the drilling, it was recognized that depleted reservoir conditions may result in wellbore stability issues, but the rock strength was determined to be sufficient to overcome any wellbore collapse. It would only be necessary to overcome the pressure difference, and this could be done using managed pressure drilling (MPD). Although MPD was implemented successfully, the wellbore collapsed during drilling and before the completion string could be run. Ultimately, the well was completed using the drill string since standard completions were impossible under the time constaints.
An after-action review of X-8 was conducted, and hypotheses were generated to explain the wellbore stability issue in this well. The review included fault interpretation uncertainty, lithology competency, reservoir lithology and geomechanics. Based on this work, two explanations were suggested as the main cause of wellbore collapse. The first explanation is that the top of the basement consists of a weathered zone resulting from exhumation in the Mesozoic. The weathered zone consists of rock material generated by both physical and chemical weathering. The second explanation is shear failure due to reservoir depletion. Both explanations are supported by well data, seismic attributes and current reservoir pressure. In the end, with an understanding of the potential failure mechanism, the last sidetrack was completed by leaving the drill string inside the hole to enable the well to produce. The learnings suggest that drilling through fracture basement reservoir is challenging and with depletion, the reservoirs can be nearly impossible to drill. Casing or liner drilling may be the only solution to successfully drill these types of wells and produce the hydrocarbons.