Modelling Leakage Risk Along a Fault Using Modified Discrete Elements

Abstract

Capacity estimates for chosen CO2 storage reservoirs are limited by the prognosed maximum pore pressure, itself predicted to cause slippage of mapped faults and thus increase the risk of leakage from the site. In most simulation tools, shear failure criteria are checked for predicted pore pressure increased on inclined lines or surfaces representing faults; if the chosen criterion is met, the simulation stops and the pressure and stress distributions in the entire domain are recorded and assigned as critical values not to exceed. Here, we try a different approach, where instead of a line in 2D, a surface is delimited on both sides of a single fault, representing a fault process zone. An in-house model is used, Modified Discrete Element tool, capable of explicitly creating and propagating fractures. The model is coupled to the commercial flow simulator TOUGH2, thus enabling flow through the created fractures to be evaluated. A simple case is simulated, where a sandstone storage reservoir is bounded by a fault, continuing up through the caprock to the next sandstone layer. In the chosen case, the storage site undergoes depletion through gas production, before CO2 is stored in the reservoir, taking the pore pressure back to its initial level. Results show that during depletion, shear stresses may develop such that fractures run alongside the fault all the way up to the upper aquifer. By assigning a permeability value to the open fractured elements, a leakage rate along the fault can be computed. Thus, geological knowledge about the process zone of faults is translated in our tool into quantitative estimates of leakage rates as a function of stress path, historic and future, corresponding to injection plans.