Stress Estimation at the Otway CO2 Storage Site, Australia

Abstract

Abstract We present an estimation of the full stress state between 0.5 and 2.1 km depth at the Otway CO2 storage pilot site, Australia, where the Cooperative Research Centre for Greenhouse Gas Technologies is conducting a large-scale demonstration project. This estimation is the first step of a geomechanical study on seal integrity. One principal stress is assumed vertical and of magnitude equal to the weight of the rock above, calculated from the density log data. The vertical stress gradient is on average 22.01 MPa/km. Extended leak-off test data, a borehole wall electrical image and dipole sonic log data in the CO2 injector CRC-1 are used to constrain principal horizontal stress orientation and magnitudes. Consistency of the stress model is then checked against the occurrence of breakouts using a mechanical earth model built along CRC-1 well. We conclude that the maximum horizontal stress direction is oriented N141 +/- 9oE. To first order, principal horizontal stress magnitudes both follow a linear trend with depth. Results indicate minimum horizontal and maximum horizontal stress gradients on average equal to 15.98 and 18.13 MPa/km, respectively, corresponding to a normal stress regime. Introduction Warming of the climate system in relation with the radiative forcing of anthropogenic, long-lived greenhouse gases in the atmosphere is beyond debate. In this process, carbon dioxide (CO2) alone contributes to 60 percent of the total greenhouse gases. Carbon capture and storage (CCS) is recognized as a means to reduce CO2 releases in the atmosphere significantly. CCS is the process whereby CO2 emitted by massive stationary point sources, such as fossil fuel-fired power plants, is captured, transported, and permanently stored underground. Along with capacity and injectivity, containment is agreed to be a primary function in geological storage performance. It must be evidenced that CO2 will, and indeed does, remain within the intended repository. In particular, seal integrity must not be impaired by the mechanical effects associated with storage operations,. Forecasting such effects requires a 3-dimensional geomechanical model of the site describing in-situ rock stresses, fluid pressures and the poro-mechanical and strength properties of the formations. This mechanical earth model is coupled to a reservoir model to compute initial conditions at static equilibrium and to further simulate the stress path, rock deformations and potential rock failure associated with CO2 injection,. The initial state of stress is amongst the key parameters controlling the stress perturbation a rock mass can sustain and yet remain stable. This distance to failure envelope is particularly relevant when it comes to analyzing the stability of long term underground storage or disposal facilities for determining upper bounds to acceptable fluid pressure changes in both the injectivity at the well(s) and the storage capacity of a repository. Insofar as the stability of faults or the compressive strength of rocks, generally, depend on all of the three principal in-situ stresses, it is necessary to estimate the full stress tensor as a function of depth. Yet no single measurement technique can solve for all the 6 independent components of the stress tensor at a point, and so procedures for in-situ stress estimation at depth must combine complementary datasets.