Estimating the Reservoir Hydrocarbon Capacity Through Measurement of the Minimum Capillary Entry Pressure of Shale Caprocks

Abstract

Abstract Shales play a major role in petroleum exploration and production because they occur both as source rocks and cap-rocks. Their small pore throat size means that very high capillary pressures are required to establish any hydrocarbon saturation in the shales. The minimum capillary entry pressure (trap capacity) defines the maximum height of a hydrocarbon column that can be trapped by a shale. In this paper, pressure transmission tests were used to experimentally measure capillary entry pressures of various non-wetting fluids (oil-based mud, crude oil. Decane and Nitrogen gas) through different shales. These capillary entry pressures are needed for the estimation of a shale's seal capacity (h). Results show that measured capillary entry pressure (seal capacity) of shales are correlated with other shale properties such as CEC and permeability. The effects of fluid type and interfacial tension on capillary entry pressure of shales were also investigated. Results show that the presence of a hydrocarbon phase (decane) in the shale reduces the shale capillary entry pressure (seal capacity) and significantly increases hydrocarbon flux. Also, the presence of surfactants in the hydrocarbon phase significantly reduces the capillary entry pressure and, therefore, the shale's seal capacity. Measured capillary entry pressures were also used to estimate the pore throat radius of shales. Since pressure transmission tests are so difficult to run, a quick and easy rig-site electrochemical test on shale cuttings to characterize the capillary entry pressure of drilled shale formations is suggested. Introduction One of the major issues that face oil companies and governments is the exact estimation of their hydrocarbon reserves. Volumetric methods and materials balance methods are used extensively to estimate reservoir hydrocarbon volume. These methods depend on using the reservoir properties such as porosity, resistivity, permeability and saturations to estimate its hydrocarbon volume. In order for hydrocarbon accumulations to exist, there must be a source rock to produce the hydrocarbon, a reservoir rock to host the hydrocarbon and a good cap rock to trap it and stop its migration upwards. According to Vavra et al (1992), a seal is generally defined as a sediment, rock or immobile fluid with high capillary entry pressure (also known as capillary breakthrough or capillary entrance pressure) that acts to stop the flow of hydrocarbon. Salts, anhydrites, silty shales and clay mineral-rich shales are some of the common seal lithologies.