Pressure solution for sequential hydraulic tests in low‐transmissivity fractured and nonfractured media

Abstract

In the last decade, considerable effort has been applied to the characterization of low‐permeability, heterogeneous formations. A primary reason for the increased activity is the importance of estimating formation properties for use in safety assessments at proposed nuclear waste disposal sites, such as those in Switzerland, Germany, Sweden, and the United States. Methods for determining formation properties (e.g., transmissivity, static head, storativity, and flow boundaries) are varied, but hydraulic testing is commonly used. Hydraulic tests in low‐permeability media typically consist of a sequence of multiple test events such as slug, constant pressure, and pulse tests. Each single test event can significantly affect the measured pressure response of subsequent test events. The pressure response can also be affected by borehole mud pressures that occur prior to testing (i.e., pretest pressures) and other factors such as well bore storage, well bore skin, and well bore temperature. We present a new analytical solution that accounts for all of the aforementioned complexities. The solution technique treats a sequence of pretest pressures and multiple test events (slugs, pulses, and shut‐ins) as one test sequence, thereby accounting for the influence of one test event upon another. The solution is derived so that only a kernel function, the constant‐rate pumping test solution, is required for new flow models. Furthermore, the solution is presented for any flow dimension allowing for interpretation in fractured formations where linear, radial, and fractional flow may exist. We demonstrate the use of the solution by inverse modeling to estimate the flow model and parameters for an example hydraulic test conducted in Switzerland.