Incorporating the effect of gas in modelling the impact of CBM extraction on regional groundwater systems

Abstract

Production of Coalbed Methane (CBM) requires extraction of large quantities of groundwater. To date, standard groundwater flow simulators have mostly been used to assess the impact of this extraction on regional groundwater systems. Recent research has demonstrated that predictions of regional impact assessment made by such models may be seriously compromised unless account is taken of the presence of a gas phase near extraction wells. At the same time, CBM impact assessment must accommodate the traditional requirements of regional groundwater modelling. These include representation of surficial groundwater processes and up-scaled rock properties as well as the need for calibration and predictive uncertainty quantification. The study documented herein (1) quantifies errors in regional drawdown predictions incurred through neglect of the presence of a gas phase near CBM extraction centres, and (2) evaluates the extent to which these errors can be mitigated by simulating near-well desaturation using a modified Richards equation formulation within a standard groundwater flow simulator. Two synthetic examples are provided to quantify the impact of the gas phase and verify the proposed modelling approach (implemented in MODFLOW-USG) against rigorous multiphase flow simulations (undertaken using ECLIPSE∗). ECLIPSE simulations demonstrate convergence towards a time-asymptotic relationship between water saturation and pressure. This relationship can be approximated using a slightly modified van Genuchten function. Where this function is employed in combination with the modified Richards equation strategy to accommodate near-well desaturation, errors in predicted drawdown are reduced significantly, including in cases where complexities such as sloping coal layers are introduced to the model domain (the latter promoting buoyancy-driven movement of gas). Sensitivity analyses further indicate that only the general properties of the employed desaturation function need to be respected to significantly reduce errors in regional drawdown predictions that would arise if the presence of the near-well gas phase was ignored. These properties can be inferred from reservoir properties and from the outcomes of reservoir model simulations that are available at local CBM operation sites.