Contribution of the Water Weakening Effect to Plastic Strain: Case Studies from Danish North Sea Chalk Fields

Abstract

ABSTRACT: The experimental studies highlighting the strength weakening of chalk when water replaces oil in pore space have incited geomodellers to systematically solve for the effect of water saturation on chalk strength at each simulation step to estimate reservoir compaction. The present study demonstrates quantitatively for the first time the important role played by the water weakening effect on the deformation of several producing fields in the Danish North Sea. Although non-negligible, the water weakening effect in the studied fields is caused by the initial water saturation of chalk rather than by its changes through time. Representative of numerous Danish North Sea reservoirs, the outcome of this study suggests that a preliminary assessment of the potential magnitude of the water weakening effect during production should be first performed prior to compaction simulation. If considered negligible, the adjustment of the rock strength according to the saturation conditions at each simulation step can be ignored, thereby reducing the computing time required to perform coupled geomechanical and fluid flow modelling. 1. INTRODUCTION Waterflooding of reservoir is a common stimulation technique to enhance the recovery factor of hydrocarbon fields within chalk intervals of the North Sea. The resulting increase in water saturation in pore space leads to a softening of the mechanical properties of the rock, such as the pore collapse stress, a process referred to as the water weakening effect. This process has been extensively studied in laboratory during the last three decades and the underlying mechanisms are still under debate. Nevertheless, the adsorption of ions and water molecules from the aqueous phase at the surface of calcite particles that involves an additional repulsive force between grains favouring grain -to-grain slippage is commonly pointed out as the main mechanisms. Laboratory studies have shown that the pore collapse stress of oil-saturated outcrop samples is consistently reduced by 50-60% when saturated with water, thereby causing additional strain. Triaxial tests conducted on core samples from the North Sea have also demonstrated the dependency of the pore collapse stress to water saturation.