Abstract
Fracture aperture change under stress has long been considered as one of primary causes of stress sensitivity of fractured gas reservoirs. However, little is known about the evolution of the morphology of fracture apertures on flow property in loading and unloading cycles. This paper reports a stress sensitivity experiment on carbonate core plugs in which Computed Tomography (CT) technology is applied to visualize and quantitatively evaluate morphological changes to the fracture aperture with respect to confining pressure. Fracture models were obtained at selected confining pressures on which pore-scale flow simulations were performed to estimate the equivalent absolute permeability. The results showed that with the increase of confining pressure from 0 to 0.6 MPa, the fracture aperture and equivalent permeability decreased at a greater gradient than their counterparts after 0.6 MPa. This meant that the rock sample is more stress-sensitive at low effective stress than at high effective stress. On the loading path, an exponential fitting was found to fit well between the effective confining pressure and the calculated permeability. On the unloading path, the relationship is found partially reversible, which can evidently be attributed to plastic deformation of the fracture as observed in CT images.
Highlights
Fractured geological formations are ubiquitous throughout the world, and their stress-sensitive behaviors are of primary interest in a number of contexts including: (1) aquifer exploitation for fresh water supply; (2) underground radioactive waste disposal repositories; (3) petroleum reservoir exploitation; (4) geothermal reservoir exploitation and heat storage; (5) mining and mineralization processes; (6) geotechnical applications; and (7) deeper earth systems such as earthquakes and ocean floor hydrothermal venting [1,2]
With an increase in the normal loading, more asperities come into contact, resulting in plastic deformation, and the damages become irreversible during the unloading
Traditional laboratory test experiments do not provide any information on the evolution of fracture apertures, with respect to fracture dips and azimuths in a sample, necessary for predicating permeability along the stress path
Summary
Fractured geological formations are ubiquitous throughout the world, and their stress-sensitive behaviors are of primary interest in a number of contexts including: (1) aquifer exploitation for fresh water supply; (2) underground radioactive waste disposal repositories; (3) petroleum reservoir exploitation; (4) geothermal reservoir exploitation and heat storage; (5) mining and mineralization processes (in situ leaching and location of ore bodies); (6) geotechnical applications (including effects on underground storage reservoirs, tunnels and other structures); and (7) deeper earth systems such as earthquakes and ocean floor hydrothermal venting [1,2]. The in situ evolution of fracture aperture is not yet well understood for better characterizing and modeling of the stress-dependent permeability. Some studies determined aperture structure and fluid flow in a rock fracture by high-resolution numerical modeling on the basis of a flow-through experiment under confining pressure [32,33]. We conducted a stress sensitivity experiment on a low-porosity carbonate sample in which an advanced X-ray CT instrument was used to acquire tomographic snapshots of the deforming sample at a number of applied confining pressures, in a full loading and unloading cycle, by simulating the stress variation of an actual fractured gas reservoir. We successfully presented a numerical basis for the analysis of fractured carbonate reservoirs
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