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