Abstract
A review of coal bed methane (CBM) drill stem tests (DST) in Queensland has shown that complex reservoir behavior is common. In particular radial composite or multiple boundary effects have been diagnosed on log-log derivative plots. In this paper the treatment of stress dependent permeability and porosity (SDPP) in pressure transient testing and production forecasting is addressed. The application of a pseudopressure transform, which is the traditional method for tackling the problem, is described. The SDPP pseudopressure method presumes that the rock mechanics model is already known. In this study an alternative procedure termed pseudo model is proposed where the rock mechanics parameters can be determined by the nonlinear regression facility available in well test software. The application of this methodology is illustrated by a field example of an injection falloff from the US. One of the main effects of stress dependent permeability is a magnification of the effect of mechanical skin from formation damage and the origin of high total skin in CBM wells is attributed to this mechanism. Gas block due to the pressure falling below the critical desorption pressure is also a problem and typical gas-water relative permeability curves are used to assess this effect.
