Abstract
The microscopic interaction mechanism between working fluids and shale reservoirs is the key basic issue for the efficient development of shale gas. The initial water saturation of clay-rich shale is low, and the water absorption through strong chemical osmosis is an important factor for the wellbore instability of the drilling fluid filtration loss and the low flowback rate of hydraulic fracturing. Membrane efficiency is a key parameter in evaluating the mechanical-chemical coupling of shale-fluid interaction. Because microcracks develop in reservoir shale, pressure transfer experiments are no longer capable of obtaining membrane efficiency value. In this paper, the characteristics of shale water saturation are considered. The model calculating membrane efficiency is obtained, and the shale membrane efficiency of the reservoir studied, based on the triple-layer model of clay mineral-water interface electrochemistry. Membrane efficiency of unsaturated shale depends on the excess charge density of the surface of the solid in different water saturations. The analysis of factors influencing shale membrane efficiency in unsaturated reservoirs shows that the shale membrane efficiency decreases with the increase of water saturation under unsaturated conditions. The partition coefficient of counterion in the Stern layer, cation exchange capacity, and solute concentration in pore fluid will affect the membrane efficiency of unsaturated shale. The membrane efficiency of the reservoir section shale in Fuling area is calculated and analyzed, and the water-absorbing capacity by chemical osmosis of the reservoir interval shale is evaluated based on the membrane efficiency model of unsaturated shale.
Highlights
Horizontal wells with long horizontal sections and staged fracturing technology are the key technologies for the successful development of shale gas
Aiming at the electrokinetic problem of clay-rich porous media, Leroy and Revil [24] proposed a triple-layer model (TLM) that describes the electrochemical properties of clay minerals
Based on the triple-layer model of clay-water interfaces established by Leroy and Revil [24], a computational model of unsaturated shale membrane efficiency is established in this paper by using the coupling constitutive equations of heterogeneous porous media
Summary
Horizontal wells with long horizontal sections and staged fracturing technology are the key technologies for the successful development of shale gas. Aiming at the electrokinetic problem of clay-rich porous media, Leroy and Revil [24] proposed a triple-layer model (TLM) that describes the electrochemical properties of clay minerals By using this model, the charged triple-layer structure of Stern and diffuse layers in clay-water interfaces was analyzed: the calculated results of ζ potential and low-frequency (few kHz) surface conductivity show that the structure has good applicability to the 1 : 1 type (kaolinite) and 2 : 1 type (montmorillonite) of clay minerals. Based on the triple-layer model of clay-water interfaces established by Leroy and Revil [24], a computational model of unsaturated shale membrane efficiency is established in this paper by using the coupling constitutive equations of heterogeneous porous media. The distribution law of the membrane efficiency under initial watersaturated condition is obtained, and the water-absorbing capacity by chemical osmosis of per unit pore of the shale in the reservoir section is evaluated
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