oa Field Development Planning for Coal Seam Gas: Challenging the Paradigm

Coal Seam Gas (CSG), alternatively known as Coal Bed Methane (CBM) is one of the exiting new energy plays emerging in the Asia-Pacific as well as elsewhere. CSG reservoirs and reservoir behavior over time are very intrinsically different from conventional plays, therefore conventional field development planning workflows need to be modified to address the focus issues and decisions relevant for CSG: number and type of wells required to achieve the committed gas delivery, timing of development startup taking into account anticipated range in dewatering time, optimum development phasing, optimum facilities layout, mitigation strategy to cover the key downside risks, as well as flexibility required to cater for upside. To date, CBM developments have typically followed a ‘low tech’ approach of pattern drilling with limited emphasis on subsurface studies and instead, a strong focus on ‘on-the –fly’ optimization of the wells and completions concepts. We believe there is potential to overhaul the traditional ways of going about CBM developments by using modern subsurface study and uncertainty management techniques to aid in upfront concept optimization (Figure 1). Objective of this paper is to illustrate this potential and some of the workflows and concepts involved.<br>As with conventional developments, reservoir characterization and modeling are a key component of the FDP study workflow. A lot of the focus is on mapping the distribution and properties of the reservoir rock. However, in a CSG play coal is the reservoir and gas storage is not in a conventional pore system but adsorbed on a molecular scale onto the internal surfaces of the coal. The reservoir properties that matter for GIIP and UR calculation are therefore different from conventional and include net reservoir (= coal)<br>thickness, isothermal coal properties (which control the gas adsorption & desorption capacity of the coal), impurities content (i.e., ash and moisture), gas content (i.e., saturation) and permeability of the natural fracture system in the coal. CGS developments to date are typically onshore, shallow and therefore drilling intensive. The wealth of well data brought about by such intense appraisal and pilot development drilling opens the door to elaborate geostatistical analysis as an effective means to highlight spatial trends and variability, deliver multiple realization maps and 3D models of each of the relevant reservoir properties, as well as to recommend optimum appraisal strategy and location. Rigorous analysis of the fracture system via integration of fracture spacing/orientation data from core and scanner tools with welltest results and seismically mapped structures can reveal key clues on permeability and connectivity of the coals. Maps of fracture orientation combined with (sub)seismic faulting can also aid in optimizing well placement. Because GIIP and UR equation is different from conventional, adaptations to existing modeling tool functionality are required to facilitate CSG volumetric computation. Like their conventional counterparts, CSG static reservoir models are then upscaled and transferred to dynamic simulation to establish optimum well count and spacing, predict dewatering time and required water handling capacity, and determine the optimum balance between wells and compression. We have developed fit-for-purpose workflows and toolkits to facilitate data transfer from mainstream static modeling tools to the specialist dynamic modeling packages that can adequately forecast CSG production.