Permeability Measurement of Organic-Rich Shale - Comparison of Various Unsteady-State Methods

Abstract

Abstract Accurate determination of organic-rich shale permeability is still a major challenge. Various methods have been proposed to measure the permeability on core plugs or crushed samples under various stress conditions using different fluids. Permeability obtained from core plugs and their crushed samples could differ by two orders of magnitude, potentially painting very different views of the reservoirs and resulting in differences in asset development workflows. This situation only reinforces the need for considerable additional focused work to quantify tight rock permeability and better understand the measurement method dependence. This paper presents the experimental comparison of three different unsteady-state transient methods for measuring the permeability of organic-rich shale plugs: pressure build-up, pulse-decay and oscillating pulse techniques. Permeability measurements are conducted isothermally using nitrogen gas on core plugs from the Barnett, Eagle Ford, Marcellus and Mancos formations at the same confining pressure and pore pressure. These plugs differ in mineralogy, total organic carbon (TOC), nuclear magnetic resonance (NMR) and helium porosity. Fractures are observed through the horizontal samples along the gas flow direction, while vertical samples do not have fractures. For each plug, the permeability tests begin with the pressure build-up measurement, and are followed sequentially by the pulse-decay and oscillating pulse methods whenever applicable. The plugs are then cut into smaller sizes for continuation of permeability tests and investigation of the permeability dependence on the plug size. For the samples analyzed, the permeability measured from the pulse-decay method is essentially identical to that from the oscillating pulse method. Compared to these two methods, the pressure build­up test generally gives a relatively higher value when requiring an independent porosity measurement to compute the permeability, while it gives a relatively lower value when no porosity is needed. However, the permeability difference among these methods is generally small. This indicates that the pressure build-up test can be used to perform permeability measurement on large core plugs that cannot be tested using the pulse-decay or oscillating pulse methods. There is no trend observed on the permeability dependence on the plug sizes used. Depending on fracture distribution and connectivity, the measured permeability of horizontal samples is randomly affected by the sample sizes, indicating multiple samples with various sizes may be required to obtain a representative permeability value for horizontal plugs. Vertical plugs show little permeability dependence on the sample sizes. Consequently, a small vertical plug can be used for very tight rocks enabling quicker matrix permeability measurements.