ABSTRACT:

Large-scale carbon capture and sequestration (CCS) projects involving annual injections of millions of tons of CO2 are a key infrastructural element needed to substantially reduce greenhouse gas emissions. The large rate and volume of injection will induce pressure and stress gradients within the formation that could activate existing fractures and faults, or drive new fractures through the caprock. Analysis of these systems up until now has employed traditional reservoir modeling with little focus on geomechanics. We will present results of an ongoing investigation to identify conditions that will activate existing fractures/faults or make new fractures within the caprock using the Livermore Distinct Element Code (LDEC). LDEC is a multiphysics code, developed at LLNL, capable of simulating dynamic fracture of rock masses under a range of conditions. As part of a recent project, LDEC has been extended to consider fault activation and dynamic fracture of rock masses due to pressurization of the pore-space. We will present several demonstrations of LDEC functionality and an application of LDEC to a CO2 injection scenario.

1. INTRODUCTION

Large scale carbon capture and sequestration (CCS) raises many diverse geomechanical challenges. A successful CCS scenario typically involves injection of millions of tons of CO2 per year into a porous, permeable formation overlaid with an impermeable caprock. However. the storage integrity could fail due to activation of preexisting faults or creation of new fractures in the caprock. These potential failure modes involve combinations of both continuum and discrete processes. This work discusses recent extensions to the Livermore Distinct Element Code (LDEC), a SPHcoupled discrete/finite element simulation code, that are being used to evaluate the geomechanical sources of risk to successful CCS. There are many geological applications involving materials or systems that manifest both continuous and discontinuous effects beyond just CCS. For example, projectile penetration into a rock or concrete target is a process often controlled by the formation and interaction of produced fragments, which requires continuumdiscontinuum analysis to robustly predict ballistic efficiency. Underground structures in jointed rock subjected to explosive loading can fail due to both rock motion along preexisting interfaces and fracture of the intact rock mass itself. In such applications, it is insufficient to simply predict whether or not the rock mass will fail-instead, the critical issue is how fracture and discontinuous interaction lead to the ultimate fate of rock fragments. LDEC was originally developed by Morris et al. [ 1] as a distinct element (DEM) code to simulate the response of jointed geologic media to dynamic loading. The DEM is naturally suited to simulating such systems because it can explicitly accommodate the blocky nature of natural rock masses. Cundall and Hart [ 2] review a number of numerical techniques that have been developed to simulate the behavior of discontinuous systems using DEMs. LDEC was later extended to include Finite Element-Discrete Element transition [ 3], including an extension to include a nodal cohesive element formulation, which allows the study of fracture problems in the continuum-discontinuum setting with reduced mesh dependence [ 4]. Most recently, a Smooth Particle Hydrodynamics (SPH) capability has been incorporated into LDEC.

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