Experimental, algorithmic, and theoretical analyses for selecting an optimal laboratory method to evaluate working fluid damage in coal bed methane reservoirs

Abstract

The existing laboratory methods have limitations that make it very difficult to select the most suitable method for obtaining precise permeability measurements for evaluating reservoir formation damage. To date, there has been no research critically addressing this issue. In this study, the permeabilities of core or cuttings samples taken from No.15 Reservoir in the Qinshui Basin before and after damage caused by drilling and fracturing fluids were randomly measured using six general laboratory methods. Their average permeability values, as well as the extent of absolute and relative formation damage, were calculated and analyzed using a simple ranking method, statistical screening algorithm, and theoretical method. The results show that, first, there is a regular distribution of absolute reservoir damage measured by the laboratory methods; and second, the application priority of permeability measurement methods for formation damage evaluation in coal bed methane reservoirs is as follows: cuttings pulse decay method > constant flow rate method > nuclear magnetic resonance method > plunger pulse decay method = pressure oscillation method = constant pressure method. The cuttings pulse decay method is therefore recommended for measuring core matrix damage, while the constant flow rate method is suggested for measuring overall damage to core samples.