Reservoir Stress-Path Measured During Depletion and Re-Pressurization in the Valhall Field

Abstract

Abstract Reservoir stress path is most often used to couple the pore pressure change in a reservoir to the resulting change in the minimum horizontal stress in the reservoir. This stress path can directly be measured in a reservoir. The pore pressure is typically measured using standard equipment for doing so during drilling of wells. These pressure measurement devices are either run on wireline or in logging while drilling (LWD) tools. The minimum horizontal stress is typically measured in a hydraulic fracturing operation designed for this purpose. This operation can be performed under a casing shoe (often referred to as an extended leak-off test), through perforations (often referred to as a mini-frac test) or between two packer elements (referred to as a micro-frac test). The stress path will impact many geomechanical aspects in a field development, like wellbore stability, lost circulation (fracture gradient change) and solids production to name three. In the literature there are limited datasets on stress-paths during depletion, and even less on re-pressurization resulting from injection. The Valhall dataset is unique in the industry since it contains hundreds of tests from three different chalk formations. The data has been gathered from mini-frac tests as part of hydraulic propped fracturing stimulation jobs over a 36-year period, and is from an oil and gas industry perspective of very high quality. Water injection has been ongoing in the Tor reservoir the last 13 years and large amounts of data can be used to measure the stress path during re-pressurization. The Valhall data indicate one stress path for depletion and a slightly different stress path during re-pressurization. We compare the measured stress paths with predictions based on core measurements and numerical models and discuss the impact of pore collapse, fault reactivation, arching, and processes during re-pressurization