Gas Production Potential Assessment of Individual Seams for a CBM During Formation Test Operation

Abstract This paper discusses how the challenges in Unconventional Coal bed methane (CBM) wells were resolved by execution and integration of advance logs to delineate seam-wise subsurface flow potential and preparation of data informed completion strategy. Due to commingled flow from multiple coal seams, CBM wells in this field suffers from major issues in prediction of reservoir/production engineering parameters like well deliverability, degree of undersaturation, mobile water saturation, desorption pressure and dewatering pattern of the coal seams. In this paper, we present a workflow on how Integration of advance logs acquired during early life of wells can be used to delineate reservoir properties of individual coal seams for exploitation of geologically complex coals. A comprehensive data acquisition program was carried out for individual coal seam to access individual coal seams production/dewatering behaviour, evaluate the hydro-fracture efficiency, and further develop understanding of individual coal seam flow behaviour. We used special data collection methods in three technical areas to gather information about the Coal seams: (1) Acoustic Imaging (2) Dual packer formation testing (3) Production profile using spinner logs. This paper describes workflow for planning, execution and interpretation of measuring downhole flow behaviour of multi-stage coal seams in a CBM field using formation testing and production logs. Formation Testing (Dual Packer) job conducted in six different seams having contrasting reservoir properties in terms of seam thickness, pressure, mobilities etc. Continuous recording of Flowing BHP at different flowrate, along with optical spectroscopy data (for fluid type identification) provided an opportunity to use this dataset for estimation of production potential properties of each coal seams and their relative depletion from initial conditions more accurately. Production Logging data acquisition in each seam aided to understand flow and dewatering behaviour of individual stages and layers. Timeseries normalized Flowing BHP and rate dataset were plotted alongside optical fluid analyser to obtain direct evidence of In-situ desorption pressure. For each coal-seams, flow rates were increased in steps for multi-rate testing and build-ups taken at regular interval for pressure transient analysis studies. These data were then used for estimation of various reservoir parameters like coal-seam diffusivity, deliverability and finally gas production potential. Estimation of Initial Pressure (P*) validated the relative depletion of coal seams as was observed in production logs. Production log evaluation also provided vital insights into coal seam production/dewatering performance, desorption pressure and Hydraulic Fracturing (HF) performance. Continuous liquid level monitoring, dewatering and optimization of artificial lift depths can help to increase dewatering rate and activate non-contributing intervals to achieve incremental gas flow. Comprehensive reservoir data acquisition plan for an unconventional CBM reservoir using production log and formation testing data provided deeper insights into reservoir parameters, well's drainage pattern and areas of effective dewatering. The high-resolution subsurface data acquisition from individual layers aided in efficient field development planning.