Linear Impermeable Boundary in Geothermal Pressure Transient Analysis: a Reservoir Modelling Assessment

Abstract

There are several analytical models available in oil and gas theory for application to well tests. These include the standard homogenous infinite reservoir model as well as various boundary models. This includes a linear impermeable boundary model which is commonly used to represent an impermeable fault near the well. While all these models are of interest to the geothermal reservoir engineer, commonly they do not fit geothermal well test data. The models are based on analytical solutions with many assumptions that do not fully hold in the geothermal reservoir environment.
Advances are being made in modelling geothermal well tests using the TOUGH2 reservoir simulator rather than analytical models. A standard design for setting up TOUGH2 models has been developed and so to accompany this, an equivalent to the linear impermeable boundary model has been developed. This is achieved by modifying the block volumes of the radial grid as if they are cut by a planar feature at a specified distance from the well. Using these modified volumes to simulate the results of geothermal pressure transients yields derivative plot results which are similar in shape to the equivalent analytical model. The proximity of the boundary to the well is varied in order to investigate how proximal the boundary must be in order to affect the model results.