Challenges in Building 1D Geomechanical Model for CBM Reservoir-A Case Study from Onshore Sumatera,Indonesia

摘要

Coal bed methane(CBM)is considered to be an alternative source of energy in Indonesia.Methane gas is produced from the coal seam which acts as both the source rock and reservoir rock.Gas is stored within fractures,mesopores and micropores.Potential CBM reserves in Indonesia are quite large and are accumulated in several basins.The Sumatera Basin is one of them. This paper describes a geomechanical model built using available data from one offset CBM well(well X)in Babat Field,South Sumatra and also regional knowledge.The model will be used to optimise the drilling strategy for developing the field and is also a requirement of the fracture permeability analysis. The key challenge is the lack of sufficient calibration data(pore pressure,laboratory rock mechanical tests,etc.).Log data are available from the offset well,however these data are of poor quality due to severe hole enlargements.Those are mostly observed in the shallow interval above 300 metres and within coal beds interbedded with clastic rocks. The geomechanical modelling process involves constraining the overburden,pore pressure,rock mechanical properties,and the magnitude and azimuth of the two horizontal principal stresses.Drilling experiences from daily drilling reports from well X were evaluated to summarise the most relevant geomechanical and pore pressure events which were used to verify the model.There were no significant drilling problems encountered in this well except for tight spots at shallow depths that might not be geomechanically related.Based on the observations of wellbore failure in shale interpreted from electrical imaging data,modelling suggests that the stress regime in the field is strike slip stress.Fractures that are observed in all lithologies have average strike of 182°N and average dip angle of 53°. Mud weight planning was done for the proposed well using the geomechanical model and recommen- dations were made regarding the preferred well orientation,additional data gathering(cores for conduct- ing rock tests,extended leak-off tests,full suite of logs,etc.)to reduce uncertainty in the model.The well was drilled successfully and cores,additional logs were taken for further model update.