BOREHOLE MEASUREMENT OF THE HYDRAULIC-PROPERTIES OF LOW-PERMEABILITY RACK

Hydraulic tests conducted in low-permeability media are subject to num erous influences and processes, many of which manifest in a nonunique fashion. To explore the accuracy and meaning of the interpretation of hydraulic tests conducted under such conditions, two semianalytical mo dels are developed in which variable well bore storage, variable tempe rature, and test method are considered. The formation is assumed to be of uniform permeability and uniform storativity in both models. To in vestigate uncertainty in the use of these models, a comparison is cond ucted to similar models that account for nonuniform formation properti es such as finite skin, double porosity, and fractional flow. Using th e models for a finite skin and double porosity as baseline cases, resu lts show that the interpretation of slug tests are normally nonunique when tests are conducted in material of low permeability. Provided tha t a lower bound is defined for storativity, the uncertainty in a given interpretation conducted with the model for a uniform medium can be e stablished by comparison with a fit to the data obtained using the mod el incorporating finite skin. It was also found that the degree of unc ertainty can be diminished by conducting the test using an open hole p eriod followed by a shut-in period (similar to a drill stem test). Det ermination of the degree of uncertainty was found to be case specific and must be defined by using at least a comparison between the model f or uniform media and that for finite skin. To illustrate the use of th e slug test model and determine the degree of uncertainty that will ac crue with the use of that model, a field example, potentially influenc ed by variable well bore storage, is presented and interpreted.