Well Spacing Determination in Unconventional Coal Bed Methane (CBM) Reservoirs by Novel Techno-Economic Analysis to Optimize Depletion Plan and Gas Recovery

Abstract Coal Bed Methane (CBM) fields are relatively tight and normally pressured unconventional reservoirs and hence requires a greater number of wells for optimum depletion plan and gas recovery. In this scenario, optimization of drainage area or well spacing becomes critical. The study aims to devise a methodology to determine the spacing for a particular reservoir by integrating both subsurface technologies and economics. The lateral length for horizontal well is standardized at 1000m and spacing of wells varied. Generally, CBM fields don't acquire high density data to capture detailed uncertainties due to inherent issues. Hence, single well simulation with average properties is the practical way for forecast with reasonable certainty. Various simulations run by keeping permeability constant at 5mD which is the mean permeability of major coal reservoirs and varying associated drainage area from 150 acres to 1200 acres. Three more cases run by varying permeability for optimizing the drainage area in sub-sector level. Economics (net present value - NPV10) is carried out for each forecast. Though 150 acres spacing for horizontal well gives more than 50% recovery factor in first 05 years for moderate to high permeability areas, but the number of wells require to achieve becomes double for 300acres. The recovery factor matrix shows 300 acres spacing to be a more optimized option compared to the other options. With reduction in permeability spacing needs to be reduced at least up to 150 acres. For all permeability cases, NPV and first 05 years recovery factor are plotted against different drainage areas. Well cost become half as number of wells required to develop field in 300 acre spacing becomes half if spacing increases to 600 acres. From plot it can be noted that 300 acres intersects NPV at optimum level with a good recovery factor of ~50%. The rest of the drainage area cases either gives less NPV or less EUR. As evident from all the result, sub-sectors in moderate to good permeability region should be developed with higher spacing and low permeable regions should go for lesser spacing. The optimum spacing may vary from reservoir to reservoir based on the changes in key properties. Generally complex multi well models don't capture the uncertainties to acceptable level as CBM fields don't acquire high density data for property population. Hence, a creative methodology is conceptualized by integrating single well simulation sensitivities with normalised costing and economics. These individual cases are then summarized in a creative plot which will enable quick and accurate decisions. The study may form a guidance to other fields while deciding this most important aspect.