Although matrix permeability is an important control on the commercial viability of unconventional reservoirs, there is no consensus on the appropriate way to measure it in the lab. In previous work (Clarkson et al. 2012), we investigated the use of pressure- and rate-transient analysis (PTA/RTA) methods to analyze data obtained from a new core plug analysis procedure designed specifically to extract information (permeability and pore volume) from ultra-low permeability reservoir samples (core plugs). The new approach involved analysis of rate and/or pressure data from the core test analogously to larger-scale well-test/production data. Although this approach offers some advantages over conventional pressure-decay and pulse-decay test analysis procedures, there are some practical issues related to core measurements to support the analysis that need to be addressed. For example, injection or production rate measurements, which are difficult to perform, were required to implement some of the analysis procedures. Further, in the original work, constant injection rates for the injection/falloff portion of the test, and constant flowing pressures for the production portion of the test, were assumed. In this work, we demonstrate that accurate measurement of injection rates is not necessary to obtain an accurate estimate of permeability and that the analysis methods can be applied for variable injection rates and flowback pressures.
Several permeability estimates are possible during a single test cycle using the procedures described in this work. Permeability may be estimated from the injection/falloff cycle of the test by 1) identifying the end of transient (linear) flow from derivative techniques combined with the distance of investigation calculation and 2) applying a conventional straight-line analysis of linear flow. Two independent estimates may therefore be obtained, but the latter requires estimation of the injection rate history. Similarly, two permeability estimates may be obtained from the production (flowback) cycle of the test using the distance of investigation and straight-line approaches. Both falloff and production analysis procedures are tested by simulating constant and variable injection rates, and constant and variable flowing pressures, respectively. In some cases, particularly with variable rates/flowing pressures, it may not be possible to obtain permeability from all approaches, making multiple, redundant estimates desirable to avoid a failed core analysis. As with the original test procedure proposed by Clarkson et al. (2012), the unique properties of unconventional reservoirs may be accounted for in the new analysis procedure, such as adsorption and non-Darcy flow (slippage and diffusion), and heterogeneities may be detected and analyzed.
We believe this new technique for analyzing core data will considerably improve on current techniques for establishing permeability of unconventional reservoir samples.