1. INTRODUCTION Hydraulic fracturing (hydrofrac) is a successful method used to extract oil and gas from highly carbonate rocks like shale and it has been used by industry since the mid- 50s [1-4]. However, there are many things that are still not clear regarding the mechanisms governing the hydraulic fracturing. To better understand how to improve hydrofrac recovery efficiencies and to lower its costs, LANL recently funded the Laboratory Directed Research and Development (LDRD) project: “Discovery Science of Hydraulic Fracturing: Innovative Working Fluids and Their Interactions with Rocks, Fractures, and Hydrocarbons”. Under the support of this project, the LDRD modeling team is working in collaboration with the experimental team to improve the understanding of fracture initiation and propagation in shale rocks. Fracture permeability in shale is one of the key elements needed to understand production of hydrocarbon after hydraulic fracturing operations and also the process of trapping buoyant fluids in reservoirs, such as CO 2 . Nevertheless, the literature on permeability of fractured shales is limited. Most studies have considered the permeability of artificial fractures (sawn or split samples) or artificially separated natural fractures using triaxial or shear-box devices, while very few studies have been conducted under in-situ conditions with simultaneous fracture and permeability measurements at reservoir conditions. There is an extensive amount of literature on brittle and ductile behavior in shale and the various factors that are be used to predict its mechanical behavior in response to stress. However, much less is known about the relationship between these mechanical properties and the permeability of damaged shale [4]. In order to gain more insight on this type of problem, LANL’s experimental team has designed and conducted triaxial core flooding experiments. As an accompanying study, LANL’s hybrid hydro-mechanical (HM) tool, the Hybrid Optimization Software Suite (HOSS), is being used to simulate the complex fracture and fragmentation processes under a variety of different boundary conditions. ARMA 15-312 FDEM Simulation on a Triaxial Core-Flood Experiment of Shale Lei, Z., Rougier, E., and Knight, E.E. Los Alamos National Laboratory, Los Alamos, NM, USA Munjiza, A. University of London, London, UK Carey, W., and Viswanathan, H. Los Alamos National Laboratory, Los Alamos, NM, USA Copyright 2015 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 49 th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 28 June- 1 July 2015. This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Hydraulic fracturing (hydrofrac) is a very successful method that has been used to extract oil and gas from highly carbonate rocks like shale for a number of decades. However, there are still many aspects related to hydrofrac operations and how they affect the hydrocarbon’s recovery levels that remain poorly understood. At Los Alamos National Laboratory (LANL), the combined finite-discrete element (FDEM) modeling team is working in conjunction with an experimental team to improve the understanding of fracture initiation and propagation in shale rocks. In this paper, in order to address the effects of the fluid pressure, a pseudo fluid solver has been implemented in LANL’s in-house FDEM code. The pseudo fluid solver calculates the pressure in the fluid domain as a function of both the time and the distance from the fluid source. During the hydrofrac simulation, the pressure calculated from the pseudo solver is applied to the original free faces as well as the faces created by fracturing. After that, the sensitivity of the obtained fracture patterns related to uncertainties and/or changes in the boundary conditions is demonstrated using FDEM. Simulation results compared to triaxial core flooding experiments are presented. FDEM numerical simulations were able to replicate the main features of the fracturing processes while showing the importance of fluid penetration into fractures as well as layering in determining final fracture patterns.