Quantification of Relative Permeability in Coal Bed Methane Reservoirs Through Production Data Analysis and Material Balance for Reliable Forecasting, Evaluation, and Optimize Field Development
Authors: Parida Bibhu , Upadhyay Rajeev , Dixit Mukul , Sinharay R. , Mishra Aditya , Patel Aagam , Patel Shubham , Chand Jagadish , Fife Dustin 
Published: November 2024

Relative permeability is one of the most important properties in Coal bed methane/gas (CBM/CBG) fields for reliable reservoir modelling due to two-phase fluid flow. In CBM reservoirs, one of the biggest challenges remains with the high unreliability of laboratory experimental relative permeability data due to the inherent nature of coal. The scope of this study is to define the shape of the relative permeability curve using available production data analysis.

The interplay between water and gas within the coal cleats affects the mobility of both water and gas. In Sohagpur CBM blocks of India, there is enough production data associated with wells. In this study, a comprehensive production data analysis (PDA) was done on wells to quantify the relative permeability curve. Though gas and water rates are available from the production history, it becomes essential to determine the well static pressure at different times so that the pressure-dependent gas and water properties can be evaluated. Conventional Z function is modified by King (1990) for CBM to include sorption behaviour and known as Z* function which, when used with pressure and plotted with cumulative gas production (Gp), gives a straight line like conventional gas reservoir behaviour. The plot gives static well pressure for any known Gp in the production history of the well and gas water properties can be determined at every pressure using standard correlations.

Thus, the relative permeability ratio of gas and water krg/krw can be obtained at every pressure using the relationship described above. It is found that the application of material balance study which, when combined with Production Data Analysis (PDA), gives useful information on relative permeability. This helped quantifying relative permeability which is used for reservoir modelling and simulation. History matching exercises are carried out for the wells for part of the available production data and forecasts are generated for rest of the production history. The forecasts of both gas and water are matching with the history which acts as validation points and provides good confidence on reliability of the generated relative permeability curve. Further, forecast period of the existing wells are extended to more years and sensitivities are carried out with additional infill and/or new to optimize further development planning.

The present study brings uniqueness in the form of two innovative workflows integrating production data, compositional data, and pressure. This has been used along with existing conventional knowledge of material balance with some modification for coal reservoir adsorption phenomena which adds considerable value to the existing knowledge. These workflows can be applied in other reservoirs or basins for more reliable reservoir modelling and forecasting which may help both CBM evaluation and exploitation time and cost effective and optimize the field development.