Experimental investigation and hybrid numerical analytical hydraulic mechanical simulation of supercritical CO2 flowing through a natural fracture in caprock

Recent work on analogue CO2 storage sites has shown that the single most defining natural factor as to whether the store can be successfully utilised to retain 99% of the injected CO2 for 1000 years is the behaviour of fractures within the low permeability strata acting as caprock. Here we present experimental and numerical investigation of the hydro-mechanical behaviour of a natural fracture in a caprock during the flow of supercritical CO2 through it. The caprock is a naturally fractured dolomitic limestone sample recovered from a depth of ∼1500 m, and is the primary seal to the natural CO2 storage analogue, the Fizzy field, in the Southern North Sea. For the first time the hydro mechanical behaviour of the fracture is examined using unique experimental equipment applying multiple high pressure single phase supercritical CO2 fluid flow experiments at representative in situ reservoir pressures (10-30 MPa, with confining pressures from 35 MPa to 55 MPa) and a temperature of 40 °C. The fracture surfaces are scanned to provide high resolution images both prior to and after the experimental investigation. The results are modelled through the further development of a hybrid numerical analytical approach to fluid flow through a discrete fracture, implemented in the open source code OpenGeoSys. The work indicates that through the statistical approximation of the fracture surface and combination of the application of standard nonlinear flow models and analytical mechanical solutions, the key features of the hydro-mechanical behaviour of the supercritical fluid flow through the sample can be replicated. The results provide insight into erroneous effective stress assumptions at higher fluid pressures and the importance of understanding the coupled process multi-physics behaviour of fractures in a CO2 storage setting. Over-simplistic approximations using the effective stress law lead to a Biot's coefficient greater than 1 being predicted under varying fluid and confining pressures.