Abstract
With the continuous development of the petroleum resources, unconventional oil reservoirs such as shale oil and tight oil have gradually become a main development direction of oil reservoirs in various countries. The reserves of shale oil in China are huge, reaching 1.42 × 1011 t; therefore, China has a great development potential and prospects for exploitation. However, in the process of developing shale oil reservoirs, we encountered many problems, such as un-replenishment of formation pressure and low flowback rate. At this stage, the development technology of shale oil reservoirs cannot effectively solve these problems. The proposition of shut-in technology can effectively improve these problems in theory, but the current shut-in technology of shale reservoirs after fracturing in China is still in its infancy. There is no in-depth understanding of the mechanism of shut-in wells. In addition, the factors affecting the change of oil-water distribution during shut-in after fracturing are complex, mainly including reservoir permeability, capillary force, fracture stress sensitivity, and reservoir damage. This paper investigates the mechanism of shut-in in shale reservoirs after fracturing and summarizes the mechanism of the shut-in process. Then, a single well shut-in numerical simulation model is established for the three complex characteristics of spontaneous imbibition, fracture stress sensitivity, and reservoir damage, and the oil-water distribution and change laws of shut-in shale reservoirs after fracturing are analyzed. Finally, the numerical model is used to study the influence of reservoir permeability, capillary force, fracture stress sensitivity, and reservoir damage on oil-water replacement, pressure increase, and daily fluid production during shut-in. The research results show that the influence of reservoir permeability and capillary force is more obvious, and the influence of fracture stress sensitivity and reservoir damage is relatively small.
Highlights
The effective development of shale oil usually relies on horizontal wells and multi-stage hydraulic fracturing technology, which usually requires the injection of a large amount of fracturing fluid into the reservoir to form a complex fracture network, so that the oil and gas production of shale reservoirs can achieve economic benefits
Wang (Wang et al, 2012) discovered the change of fluid seepage phenomenon under the action of osmotic pressure and capillary force, and studied the relationship between microfractures caused by hydraulic fracturing and imbibition in shale oil reservoirs
In the process of shut-in in shale reservoirs after fracturing, under the action of higher bottom hole pressure, fracturing fluid with a large amount of energy enters the near-fracture shale matrix, and the formation energy is improved through imbibition and oilwater replacement is carried out
Summary
The effective development of shale oil usually relies on horizontal wells and multi-stage hydraulic fracturing technology, which usually requires the injection of a large amount of fracturing fluid into the reservoir to form a complex fracture network, so that the oil and gas production of shale reservoirs can achieve economic benefits. Wang (Wang et al, 2012) discovered the change of fluid seepage phenomenon under the action of osmotic pressure and capillary force, and studied the relationship between microfractures caused by hydraulic fracturing and imbibition in shale oil reservoirs. Water distribution state at different shut-in times, and explore the influence rules of different capillary forces, different fracture stress sensitivity coefficients, and different reservoir damage levels on productivity, and determine the hidden laws and main control factors of the shut-in effect. In the process of shut-in in shale reservoirs after fracturing, under the action of higher bottom hole pressure, fracturing fluid with a large amount of energy enters the near-fracture shale matrix, and the formation energy is improved through imbibition and oilwater replacement is carried out. Number of grids Grid size, m3 Model size, m3 Porosity, % Permeability of inner zone, mD Permeability of outer zone, mD Original formation pressure, MPa Mode of production So, % Sw, %
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