Permeability Evolution of Shear Failing Chalk Cores under Thermochemical Influence

Summary Development of petroleum reservoirs, including primary depletion of the pore pressure and repressurization during water injection naturally leads to changes in effective stresses of the formations. These changes impose mechanical deformation of the rock mass with subsequent altering of its petrophysical properties. Besides mechanical compaction, chalk reservoirs on the Norwegian Continental Shelf also seem susceptible to mineralogical and textural changes as an effect of the injecting fluid's chemical composition and temperature. Understanding such chemical and thermal effects and how they interplay with the mechanical response to changes in effective stresses could contribute to an improved prediction of permeability development during field life. This article presents results from mechanical testing of chalk cores in triaxial cells allowing systematic combinations of pressure, temperature and injecting fluid, intended to replicate in-situ processes. The sample set consists of water-saturated cores of medium-porosity (32%) outcrop chalk (Niobrara Fm, Kansas). Preliminary results highlight the effect of three different injecting brines (equilibrium sodium chloride NaCl, equilibrium sodium sulphate Na2SO4 and synthetic seawater SSW) at 130°C temperature and low confining pressure (1.2MPa) on the cores’ permeability evolution. Deviatoric loading above yield resulted in a shear failure with a steeply dipping fracture of the core and a simultaneous increase in permeability. This occurred regardless of the brine composition. However, yield and failure stresses were clearly lower in Na2SO4 and SSW test series in comparison to NaCl tests. In addition, the shear failure caused more axial deformation and a higher increase in permeability in these two test series ( Figure 1 ). During creep and unloading, the permeability changes were negligible, such that the end permeability remained higher than the initial values. Further investigations regarding the combined effects of confining pressure, water chemistry, and temperature on the rock permeability are still ongoing. The results will, together with experimental data from actual reservoir rocks, not only enhance the understanding of the impact of typical water-related IOR techniques, but also improve the accuracy of reservoir predictions, and contribute to finding smarter solutions for future IOR.