Contents lists available at ScienceDirect Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd Matrix permeability measurement from fractured unconventional source- rock samples: Method and application Jilin Jay Zhang a, ⁎ , Hui-Hai Liu a , Mohammed Boudjatit b a Aramco Services Company: Aramco Research Center – Houston, Houston, TX, USA b Saudi Aramco, Dhahran, Saudi Arabia ABSTRACT Shale matrix permeability is one of the most important parameters for characterizing a source rock reservoir and for predicting hydrocarbon production. The low permeability value and the presence of induced fractures during core retrieval and transportation make the accurate measurement of the true permeability values for source rocks a significant challenge for the industry. The steady state flow method and the transient pressure pulse decay method on core plug samples mainly measure the permeability of fractures when fractures are present. While the Gas Research Institute (GRI) method that uses pressure decay on crushed rock samples was designed to overcome this difficulty associated with the induced fractures, its measurement results are reported to be sensitive to the particle size of crushed rock samples and also need correction of Knudsen diffusion effect. Moreover, the GRI method is limited to the unconfined stress condition. This work develops a practical method to measure the matrix permeability values from fractured source rock samples by extending the 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 source rock sample is dominated by the fracture continuum because it has much higher permeability values than those for the rock matrix. Thus, the initial gas pressure signals from the test are used to estimate the fracture permeability. During the late- stage of a pulse decay test, the flow process within the rock sample is controlled by the rock matrix. The observed pressure signals at this stage are used for estimating matrix permeability. The method is based on the analytical solution to gas flow in the fractured rock sample and relatively simple to apply in practice. Both fracture and matrix permeability's dependence on the effective stress can be assessed with this method. 1. 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 un- conventional reservoir depend, to a large degree, on the matrix per- meability of the rock formation (e.g. Heller et al., 2014). It is a technical challenge to measure meaningful permeability va- lues of source rocks in a laboratory because the matrix permeability is extremely small and there could be induced fractures due to the re- trieval 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 per- meability 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 in- duced 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). https://doi.org/10.1016/j.jconhyd.2020.103663 Received 5 March 2020; Received in revised form 30 April 2020; Accepted 1 June 2020 ⁎ Corresponding author. E-mail address: Jilin.Zhang@AramcoAmericas.com (J.J. Zhang). Journal of Contaminant Hydrology 233 (2020) 103663 Available online 05 June 2020 0169-7722/ © 2020 Elsevier B.V. All rights reserved. T