Abstract
Abstract The productivity of shale oil reservoirs is mainly determined by the hydraulic fractured reservoir volume. The branch fractures with low sand laying concentration are the main channels connecting the shale matrix and the main fractures. Maintaining the branch fracture conductivity is significant to the production of shale oil. In this study, a series of shale branch fracture conductivity and soaking experiments were conducted using a core flooding device and a small reactor, and the influences of different factors on the fracture conductivity were evaluated. The results show that when the sand laying concentration in fractures is less than 3 kg/m2, the branch fractures present a significant stress sensitivity. Particularly when the sand laying concentration is less than 1 kg/m2, and the closure pressure is greater than 15 MPa, there will be a risk of proppant embedded and fracture closed. The antiswelling agent has an inhibitory effect on the shale swelling. When the concentration of the antiswelling agent in the gel-breaking fluid is 2%, the swelling factor of shale powder is only 1.84%. Comparatively, the 0.5% of the antiswelling agent has a poor effect which can cause the shale rock to crack when the effective stress decreases. It can cause the fracture conductivity to decline by 70-90% when the gel-breaking fluid flows back associated with the shale oil. The probability of core breaking and proppant embedding will increase. The frequent shut-in and rapid open-flow for production can accelerate the damage of fracture conductivity. It is necessary to optimize the fracturing fluid and the open flow scheme to prevent the rapid decline of production in shale oil reservoirs.
Highlights
Shale oil is an important unconventional oil and gas resource, which occurs in the nanoscale pore and fracture system of shale oil reservoirs in the form of adsorbed and free state [1,2,3]
According to the analysis, (1) when ceramsite was selected as the proppant, as the back pressure of the core holder was gradually reduced, the effective closure pressure acting on the fractures gradually increased from 5 MPa/ conductivity (md·mm) Pressure (MPa) to 25 MPa, the conductivity of fractures obtained by calculation correspondingly dropped from 828.60 md·mm to 133.00 md·mm, and the apparent permeability dropped from 478.96 md to 76.88 md, down to 16% of the original; (2) when quartz sand was used as the proppant, similar laws can be obtained
The effective closure pressure increased from 5 MPa to 25 MPa, and the conductivity of fractures dropped from 687.14 md·mm to 102.81 md·mm, and the apparent permeability dropped from 319.60 md to 47.82 md, a total down to 15%; (3) in general, the permeability of branch fractures with low sand laying concentration in shale shows significant stress sensitivity
Summary
Shale oil is an important unconventional oil and gas resource, which occurs in the nanoscale pore and fracture system of shale oil reservoirs in the form of adsorbed and free state [1,2,3]. China is still in the early stage of shale oil exploration and development, which is 30-40 years later than the United States [5, 6]. The technology of fracturing shale oil reservoirs in the United States is not completely suitable for China, and the technology suitable for Chinese geological characteristics needs to be tackled and developed [5,6,7,8,9]. China Geological Survey, CNPC, SINOPEC, and Yanchang Oilfield have increased their exploration and development efforts of shale oil, which have made important discoveries and breakthroughs in multiple series of strata in different basins. In this study, aiming at the branch fractures with low sand laying concentration in shale oil reservoirs, a core flooding device and a small reactor were used to carry out fracture conductivity tests. Suggestions for the optimization of fracturing fluid and open flow schemes for production were proposed
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