Application of a Coupled Thermo-Hydro-Mechanical-Chemical Simulation to a North Sea Hydrocarbon Chalk Reservoir

Abstract

ABSTRACT: Water weakening effect in the chalk reservoir causes additional compaction due to the interaction between the rock and injected water. To consider the impact of the coupled interactions on the fluid transport (e.g., production) and reservoir deformation due to the seawater injection into the chalk reservoirs, in this study, a wrapper is developed in Matlab that combines Eclipse 100 reservoir simulator and Visage geomechanics simulator to capture the induced alteration of mechanical and petrophysical properties of chalk. Here, we utilize the history-matched reservoir model of the Halfdan sector model to investigate the impact of the temperature-dependent fluid-rock interactions induced by sulfate adsorption on the surface of calcite grains on the deformation behavior of the reservoir during waterflooding. Our sector-scale simulation results show that while considering the geomechanics model has a considerable impact on calculated reservoir pressure and recovery, the impact on history matched data due to water weakening is not significant when yield stress and bulk modulus are expressed as functions of temperature and sulfate concentration for the Halfdan model. We argue that the minor contribution of the water weakening effect at in situ conditions is due to 1) the relatively high initial water saturation in the water flooded section of the reservoir and 2) the low initial temperature (70 °C) of Halfdan reservoir, especially towards the northern part of the Danish North Sea. 1. INTRODUCTION High porosity and low permeability chalk reservoirs respond significantly to the interaction of Thermal, Hydrological, Mechanical, and Chemical (THMC) effects that causes changes in rock porosity and permeability that ultimately influence the performance of hydrocarbon production (Minde & Hiorth, 2019). Generally, as the reservoir pressure is reduced during production, the effective stress increases, causing the reservoir to compact, a mechanism that can potentially enhance production (Barkved et al., 2003). However, not only does pore pressure reduction causes subsurface deformation and subsequently seabed subsidence, but water injection may also lead to additional compaction in the North Sea fields (Doornhof et al., 2006). For example, cold seawater has been injected to improve oil recovery in the North Sea hydrocarbon chalk fields for many years. This has caused a softening of the rock strength (i.e., yield stress) and stiffness (i.e., bulk modulus), referred to as the water weakening effect. Megawati et al. (2013) and Royne et al. (2015) that rock softening results from the adsorption of water molecules and key ions such as magnesium and sulfate from the aqueous phase to the surface of calcite grains. The water weakening effect is also dependent on the temperature. Both an increase in temperature and sulfate concentration favor a high repulsive force between calcite grains, thereby a more pronounced weakening of chalk strength and stiffness. Besides, the amount of ion adsorbed on the chalk surface is dependent on pH, salinity, surface properties, and the presence of other ions. Therefore, the injection of seawater with different chemical compositions and temperatures compared to those of the reservoir fluid can significantly impact the behavior of the reservoir intricately.