Matrix permeability measurement from fractured unconventional source-rock samples: Method and application
Introduction
Source rock matrix permeability is one of the most important parameters for characterizing a source rock reservoir and for predicting hydrocarbon production (e.g. Clarkson et al., 2012; Clarkson, 2013). The overall production and the late period production of an unconventional reservoir depend, to a large degree, on the matrix permeability of the rock formation (e.g. Heller et al., 2014).
It is a technical challenge to measure meaningful permeability values of source rocks in a laboratory because the matrix permeability is extremely small and there could be induced fractures due to the retrieval of the samples from the reservoir depth to the surface, the transportation of source rock samples from well sites to laboratories, and sample handling in laboratories (Civan, 2017). The laminations of the source rocks make the samples highly susceptible to breaking up along the lamination surfaces. For a cylindrical plug sample of source rock with induced fractures, the fractures dominantly determine the permeability values measured in laboratory by the traditional methods including the steady state flow method (American Petroleum Institute, 1998; Mallon and Swarbrick, 2008) and the transient pressure pulse decay (PDP) method (Brace et al., 1968; Hsieh et al., 1981; Dicker and Smits, 1988; Jones, 1997; Alnoaimi et al., 2014). The measured permeability values, largely from the induced fractures, do not represent the true matrix permeability values under reservoir conditions and can be misleading to reservoir engineers.
To minimize the effect caused by the induced fractures, Luffel et al. (1993) proposed to use a pressure decay method on crushed source rock samples for the matrix permeability measurements, which is called the GRI (Gas Research Institute) method. The GRI method is based on the consideration that there is no fracture or microfracture in small rock particles with sizes on the order of 1 mm. It has gained popularity in the petroleum industry as it appears to have eliminated the effect of induced fractures and is relatively easy to use (Restech, 1995). Later studies, however, indicate that the GRI method generates results that are very sensitive to the particle size of crushed rock samples (Cui and Glover, 2014) and to the test condition, such as relative volume of the gas and solid (Tinni et al., 2012). The GRI method is also limited to the unconfined stress condition (Cui and Glover, 2014), and suffers from fluctuations in laboratory procedures and lack of standard (Sinha et al., 2012). Since the GRI method is generally conducted at relatively low gas pressure conditions, the impact of Knudsen diffusion is another concern with the method (Liu and Zhang, 2020).
Efforts have been recently made to deal with the problems that the GRI method faces. Realizing that crushing rock would alter the rock properties significantly, Peng and Loucks (2016) proposed to measure the matrix permeability on plug samples with a confining pressure and low pore pressures between 0 and 200 psi, though the correction of effects of Knudson diffusion and slippage flow is needed due to the low gas pressure used in their procedure. Gan et al. (2018) determined matrix permeability values using numerical simulation of the pressure decay tests on core plugs under low pore pressure based on an assumption that the permeability is uniform and isotropic in all directions. Nevertheless, their methods are still limited to low gas pressure conditions or the use of numerical simulation-based analysis approaches.
The focus of this study is on determining rock matrix permeability from fractured source rock plugs. To the best of our knowledge, Ning (1992) and Ning et al. (1993) conducted early work with similar objectives. They used numerical methods for historical matching of pressure pulse decay tests to determine the fracture and matrix permeability values from fractured plug samples. However, their procedures are complex and were developed for isotropic rocks while unconventional rocks are highly anisotropic, as will be discussed later. Thus, their methods have not been widely used in practice.
This work determines matrix permeability from pressure pulse decay test data of fractured source rock plugs based on observed gas pressure histories from different flow regimes. Unlike the previous methods mentioned above, this work employs an easily used analytical method. The method allows for measuring the permeability of unconventional source rock samples with minimal disturbance to the rock fabrics because it performs the measurements on whole plug samples without crushing. Both fracture and matrix permeability's dependence on the effective stress can be assessed with this method.
Section snippets
Method
As previously indicated, we estimate both fracture and matrix permeability values from a fractured core sample based on test data from different flow regimes observed from a pulse decay test. The test setup consists of an upstream gas reservoir, a core holder containing a core sample that is subject to confining pressure, and a downstream gas reservoir. They are connected to three different pumps to provide gas pressures and confining pressure, respectively. Initially in a pulse decay test,
Permeability measurement results
To validate the proposed method, we performed a pulse decay measurement on a non-fractured sample with low permeability. The measured permeability is considered the true matrix permeability of the sample. We then created artificial fractures by over-pressurizing the sample and measured the matrix permeability from the fractured core plug using our method. The comparison between permeability values measured before and after artificial fracturing can be used for evaluating the usefulness of our
Discussion
As previously indicated, the stress release of rock samples, when taken from the subsurface to the surface, will induce fractures in these samples. The transportation of the source rock samples from well sites to laboratories and plugging processes will further enhance the possibility of inducing fractures in the core samples to be used in laboratory for permeability measurements. Since source rock permeability is very low, the existence of the induced fracture would make laboratory
Conclusions
This work develops a practical method to measure the matrix permeability values from fractured source rock samples by extending a commonly used pressure pulse decay method. A source rock sample with fractures can be more accurately described by a dual-continuum system consisting of a fracture continuum and a matrix continuum. During the pulse decay test, the initial flow across the rock sample is dominated by the fracture continuum because it has a much higher permeability than that for the
Author statements
Jilin Jay Zhang, developed the algorithm, designed the experiments, and performed the data analysis. The main author of the manuscript.
Hui-Hai Liu, first initiated this idea and worked on the theoretical part of the project. Co-authored the manuscript.
Mohammed Boudjatit, examined the experimental design and participated in the data analysis. Co-authored the manuscript.
Declaration of Competing Interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in
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