Obtaining the Minimum Horizontal Stress From Microfracture Test Data: A New Approach Using a Derivative Algorithm

Abstract

Summary This paper presents a new approach to the interpretation of pressure-decline data from microfracture tests. The objective is to determine a representative value for the fracture closure pressure, which we relate to the minimum horizontal stress, σHmin. The method uses a derivative algorithm to identify different flow regimes that may occur during a pressure decline (e.g., wellbore storage and infinite- and fmite-conductivity fracture flow). High-quality data are a prerequisite for this method, but it is simple to use and the necessary computations can be performed easily with modern well-test-analysis software. This method has been applied successfully to data from the North Sea for a number of rock types with permeabilities ranging from effectively zero (claystone) to several darcies (sandstone). Results from different pressure declines for the same interval agree well, and a value often can be quoted for closure pressure to within ±0.5 bar. Several examples are included to demonstrate the advantages of this method. Guidelines for microfracture testing highlight what is important for obtaining test data for this method.