Production forecasting is required at all stages of coal seam gas (CSG) reservoir development. Depending on the stage of appraisal or development, different methodologies can provide the best fit for the forecasting objectives. This paper compares technical advantages and disadvantages of several forecasting approaches while considering potential accuracy, time required to construct the forecast and general fit for purpose. The basis of comparison is case studies of CSG projects in the Surat and Bowen basins.
In general, a very similar set of forecasting tools can be applied to CSG reservoirs as for the conventional oil and gas fields. A notable difference is gas desorption from the source rock, which needs to be included into numerical and analytical model and is typically described by adsorption isotherms. Some approaches, like Decline Curve and Pseudo Steady State Well Deliverability can be applied with minimal modifications. Flowing Material Balance and more detailed numerical reservoir simulations require changes to account for the gas desorption mechanism. However, these tools are already well established in the industry.
In addition to comparing the established approaches for CSG production forecasting, we propose a new hybrid method and compare its applicability to the other tools. The hybrid method uses output of the numerical reservoir simulation model and applies an analytical correction to adjust the predicted production rates to the actual observed data and to produce the forecast at possibly different bottom hole pressure compared to the original numerical model. The new hybrid method is recommended in cases where a quick forecast is required for fields with a large (hundreds and more) number of wells. The advantage of the new approach is that it provides a quick response while still maintaining the characteristics of the initial reservoir model.