Understanding Well Test Responses in Tight Fractured Sandstone Reservoir

A well-known pressure transient derivative signature in dual-porosity fractured reservoirs shows stabilization preceded by a dip. Such signature allows in principle estimation of a dynamic matrix block size. In many case however, dual porosity fractured reservoirs do not display a clear dip in the pressure derivative. In this work we investigate the causes of the absence of characteristic dual medium signatures for tight gas reservoirs. A list of possible explanations includes dual medium signature hidden by wellbore storage, test too short to observe the dual medium signature, dual medium signature partially suppressed or smoothed by transient effects, and a matrix block size given by a probability density function. These hypotheses were all investigated using pressure transient analysis and reservoir simulation. It is concluded that most likely the presence of transient effects may explain the absence of a clear dual medium signature in the derivative. This may be enhanced by the presence of a distribution of block sizes. The understanding of the reasons behind the weak observed dip helps in applying the right PTA analysis model, and prevents estimating, for example, wrong fracture porosities that may result from calculation using storativity ratios obtain from wrongly used steady-state pressure transient models.