Optimising Horizontal Coal Seam Gas Wells by Combining Reservoir Simulation and Transient Well Modelling

In 2017, APLNG drilled the first horizontal wells within the Surat Basin targeting the Walloon coal seam gas (CSG) measures. This reservoir is quite shallow with the potential for relatively low pressures. To address this uncertainty, a study was performed to identify an optimum operational strategy to maximise the cumulative gas production of a well over the first five years of production. This was achieved by using a Latin Hypercube sampler and a Genetic Algorithm optimiser to identify optimum reservoir simulation scenarios. The optimized simulation scenarios were then modelled within a multiphase transient simulation model, to better understand the flow regime behaviour within the wellbore. This predicted the flowing potential of the well whilst modelling flow assurance risks such as wellbore slugging. The result was an innovative workflow that identified optimum operational strategies whilst accounting for the uncertainties in reservoir pressure and the fluid hydraulics in the wellbore. After completing the reservoir optimisation studies, the optimised cumulative gas production showed increases between 3% – 6% compared to the base case. Other improvements included; higher peak gas production, higher peak water production resulting in earlier desorption of gas, shorter time to initial gas, and shorter time to peak gas. After running the optimised reservoir simulation cases through the transient models, it was found that the days to peak gas was reduced by 80-90%, whilst the slugging periods were reduced by 90-100%. The models were also used to quantify the impacts of changing operational/design parameters such as horizontal well length, casing sizes, pump speeds, and choke settings. APLNG used these results to design their well start-up and ramp-up strategies, and successfully kick off their horizontal wells. The results of this innovative workflow for reservoir and wellbore modelling in a CSG field highlights the new insights that can be gained by combining traditional reservoir simulation with mathematical optimization and transient well flow modelling. These workflows enhance our understanding of how to improve efficiencies and maximise production volumes within CSG fields.