Impact of Initial Wettability and Injection Brine Chemistry on Mechanical Behaviour of Kansas Chalk

Abstract

The injection of seawater-like brines alters stiffness, strength and time-dependent deformation rates for water-saturated chalks. This study deals with the mechanical effects and oil production upon brine injection through wettability-altered samples. The results from two test programs are presented: (a) ‘Wettability determination program’ and (b) ‘triaxial test program’. Kansas chalk samples were saturated by a mixture of oil and water and aged over time at 90 °C. The wettability index of the altered samples was estimated using chromatographic separation tests by co-injecting sulphate ions that adsorb on the water-wet mineral surfaces and non-affine tracer. A good repeatability was observed. In the triaxial test program, unaged water-wet and aged mixed-wet samples were hydrostatically loaded to 1.5 times yield stress so stiffness and strength could be determined. The samples were kept at the same stress level over time to monitor the volumetric creep. After a stagnant flow period of 15 days, MgCl2 brine and seawater were flushed through the samples so the oil production and ion concentration of the effluent water could be obtained. The combined observations of the bulk volume, oil volume and estimated solid volume (from effluent analyses) enabled us to calculate pore volume and thereby oil saturation with time. The mixed-wet samples were found to be stiffer and stronger than the water-wet samples, and when the stress was kept at 1.5 times yield the creep curves overlapped. During the flow-through period, the changes in ion composition are insensitive to the presence of oil, and ongoing water weakening for mixed-wet samples is the same as in the water-wet samples. Further, we found that oil was only produced during the first 2–3 pore volumes (PVs) injected. Afterwards, no oil was produced even though the chemical reactions took place and pore volume reduced.