In characterizing and modeling the performance of naturally fractured reservoirs (NFR), fracture length and fracture connectivity (permeability) are often assumed to be constant. A number of field tests done on Spraberry Trend wells suggest this assumption is not always true for NFR.
Pressure transient tests and decline analyses have been used to determine permeability, skin, fracture connectivity and average reservoir pressure for the Spraberry Trend reservoir of West Texas. It has been hypothesized by L. Elkins1 that fractures open and close due to pressure effects. In order to better understand these effects and fluid movement pathways, a number of specialized pressure transient tests have been conducted and analyzed in the E.T. O'Daniel Unit of Spraberry. This included constant pressure decline rate tests, build up tests, step-rate tests, falloff tests and a multi-well interference test, carried out so as to minimize the risk of induced fractures which can cause reduced sweep efficiency and fluid losses to other non pay zones. Results indicate that fracture systems are very stress sensitive in the Spraberry formation, with fractures opening and the effective permeability increasing at high injection rates (in addition, reservoir flow simulation has also confirmed that permeability is stress sensitive).