Abstract
As a type of unconventional oil and gas resources, tight sandstone reservoir has low permeability and porosity properties and thus is commonly necessary to develop through hydraulic fracturing treatment. Due to the coexistence of natural fractures and induced hydraulic fractures, the heterogeneity of reservoir permeability becomes severe and therefore results in complicated fluid seepage mechanism. It is of significance to investigate the oil-water two-phase seepage mechanics before and after the hydraulic fracturing stimulation with the aim of supporting the actual production and development of oilfield. This paper experimentally investigated the influences of fracture system on seepage characteristics of two-phase displacement in sample cores of fractured tight sandstones. In details, the changes of injection rate, cumulative production rate, recovery ratio, and water content were analyzed before and after the hydraulic fracturing treatments. To further analyze the displacement characteristics of the sample core, the displacement indices of four rock samples in different displacement stages were investigated. The sensitivity of sample core displacement indices to many key factors, including injection time, oil production rate, oil recovery factor and injection multiple factor, and moisture (i.e., water content was 95%, 98%, and 99.5%, respectively), before and after the hydraulic fracturing treatments were obtained synthetically. Besides, the relationship between recovery difference and contribution of fracture to permeability was explored at different water contents. The experimental results reveal that the fracture system shortens the water-free production period and hence reduces the recovery rate. The greater the contribution of fractures to permeability, the lower the recovery of water during this period.
Highlights
With the increase of the energy demand, unconventional oil and gas resources have gained more attention and popularity around the world; among them, the tight sandstone reservoir is one of the most important unconventional oil and gas resources
After hydraulic fracturing treatment in the tight reservoir, both the preexisting natural fractures in the reservoir and the hydraulic fractures induced by fracturing treatment increase the permeability heterogeneity of tight sandstone reservoir, resulting in highly complicated oil-water distribution
Since the fractures in the tight sandstone reservoir provide the high-permeable channels, the seepage resistance decreases dramatically at the presence of fractures, which further results in the changes of the water injection speed, cumulative production, recovery ratio, and water content
Summary
With the increase of the energy demand, unconventional oil and gas resources have gained more attention and popularity around the world; among them, the tight sandstone reservoir is one of the most important unconventional oil and gas resources. Since tight sandstone reservoirs generally have low porosity and low permeability properties, hydraulic fracturing treatment has become one of the most feasible and dispensable methods to effectively and economically develop the oil and gas resources from the tight reservoir. After hydraulic fracturing treatment in the tight reservoir, both the preexisting natural fractures in the reservoir and the hydraulic fractures induced by fracturing treatment increase the permeability heterogeneity of tight sandstone reservoir, resulting in highly complicated oil-water distribution. This has a great influence on the two-phase seepage mechanism in the fractured porous media [7]. To characterize the permeability heterogeneity of tight sandstone, Warren and Root [8], Geofluids
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