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
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.
Shales play a major role in petroleum exploration and production because they are commonly considered to be both source rocks and seals.Their ability to exhibit good sealing characteristics arises from their small, water-wet, pores.These small pore throats are responsible for generating high capillary pressures, which excludes hydrocarbons.Shales are also underground seals for CO2 sequestration.One of the critical aspects of CO2 sequestration is the ability of the shale to stop the flux of CO2 through it.This ability is complicated by the partial solubility of CO2 in the water.The capillary pressure is given by:
(1)
where s is the interfacial tension between the hydrocarbon phase and the water, ? is the contact angle and r is the shale pore throat radius.
In order for hydrocarbons to enter a shale, the differential pressure between the hydrocarbon column and the water must exceed the minimum capillary entry"threshold" pressure of the shale.By definition, the minimum capillary entry pressure is the capillary pressure at which the non-wetting phase, usually oil, starts to displace the wetting phase, usually brine, contained in the largest pore throat within a water-wet formation.It can be seen from equation (1) that the capillary entry pressure can be significant, especially for shales with very small pore throats (permeability).Hale et. al. (1993) shows that the hydraulic permeability of shales is extremely low (10–7 to 10–12 Darcies) and also that oil will not enter shale pores until the differential pressure exceeds the minimum capillary entry pressure.