DefinePK

Abstract

Water influx is an important factor which needs to be quantified during early stages of reservoir development to justify project economics. For a CBM reservoir it is much more important to quantify degree of connection between Coal seams and aquifer (if any) as its production mechanism is based on efficient dewatering process. However, it is difficult to quantify connection factor values early in field life. Many traditional models ranging from simplest steady state model by Schilthuis and fetkovich utilizing material balance, to unsteady state solution of diffusivity equation by Van-Everdingen and Hurst exists for finding water influx but nearly all of them have inherit assumptions related to aquifer/reservoir boundray pressure or influx rate and requires accurate historical production data for estimation of correct influx which is often not available. However, during Appraisal/exploratory stage we do have accurate measurement of pressure at wellbore and production/injection rate when we conduct pressure transient testing. Well test analysis plays an important role in reservoir characterization and can aid in correct water influx calculations. Currently, pressure falloff test responses to quantify water influx in the reservoir with wells that exist near constant pressure boundary can be analyzed by either type-curve matching or non-linear regression analysis. The former is basically a trial-and-error procedure and the later can lead to incorrect/impractical results. So, we need a more robust and accurate methods for water influx calculation. In this work, a practical method is developed to interpret the injection-falloff test response for a CBM reservoir in connection with aquifer to quantify connection factor between aquifer and reservoir using pressure transient tests which are usually conducted during field appraisal/exploration phase. Besides complementing the conventional straight-line method for the determination the leakage factor, we also provide a solution using characteristic points found on the pressure, pressure derivative and second pressure derivative log-log plot of a ‘leaky aquifer’ reservoir model (Cox and Onsager, 2002) which allowed us to develop relationships for the accurate estimation the leakage factor. An extremely useful application of the second pressure derivative was also included for estimation the unknown reservoir permeability for cases in which radial flow regime is completely masked by other flow regimes. The provided interpretation methodologies were successfully tested with synthetic examples.