No abstract

The conventional method to predict hydraulic fracture height depends on linear elastic mechanics, and the typical Gulrajani–Nolte chart fails to reflect fracture height when the net pressure in the fracture is too high. Based on fluid–solid coupling equations and rock fracture mechanics, a new chart is obtained by the ABAQUS extended finite-element method. Compared with the Gulrajani–Nolte chart, this new chart shows that longitudinal propagation of hydraulic fracture is still finite when the net pressure in the fracture is higher than in situ stress difference between reservoir and restraining barrier. The barrier has a significant shielding effect on the longitudinal propagation of hydraulic fracture, and there is a threshold for an injection rate of fracturing fluid to ensure hydraulic fracture propagates in the barrier. Fracture height decreases with the increase of in situ stress difference. When the ratio of net pressure to in situ stress difference is less than 0.56, the propagation of hydraulic fracture is completely restricted in the reservoir. Hydraulic fracturing parameters in Well Shen52 and Well Shen55 are optimized by using the new chart. Array acoustic wave logging shows that the actual fracture height is at an average error within 14.3% of the theoretical value, which proves the accuracy of the new chart for field application.

Based on the competitive adsorption of CH 4 and CO 2 , molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) are used to study the mechanism of increasing shale gas recovery by carbon dioxide injection. The influence of different factors on the diffusion capacity of CH 4 in unrestricted space and restricted space is analyzed, and the diffusion capacities of CH 4 and CO 2 in shale nanopores are compared. Under different conditions, the difference in the sorption amount and sorption heat of CH 4 and CO 2 is studied, and the displacement simulation of CH 4 and CO 2 is conducted. The obtained results show that the diffusion capacity of CH 4 in restricted space is much smaller than that in unrestricted space, and the difference between them is related to temperature, pressure, and pore size. When pressure exceeds 10 MPa, the difference gradually decreases. The diffusion capacity of CO 2 is weaker than that of CH 4 under the same conditions, which contributes to the retention of CO 2 . There is a competitive sorption relationship between CH 4 and CO 2 . The sorption amount and sorption heat of CH 4 and CO 2 are affected by the combination of pressure, temperature, density ratio, pore size, and mineral type. The adsorption capacity of CO 2 is much higher than that of CH 4 . When sorption conditions are more favorable, the adsorption difference between CH 4 and CO 2 will become larger. In shale nanopores, CO 2 can replace CH 4 that is adsorbed on the pore surface to improve shale gas recovery.

The simulation of hydraulic fracturing by the conventional ABAQUS cohesive finite element method requires a preset fracture propagation path, which restricts its application to the hydraulic fracturing simulation of a naturally fractured reservoir under full coupling. Based on the further development of a cohesive finite element, a new dual-attribute element of pore fluid/stress element and cohesive element (PFS-Cohesive) method for a rock matrix is put forward to realize the simulation of an artificial fracture propagating along the arbitrary path. The effect of a single spontaneous fracture, two intersected natural fractures, and multiple intersected spontaneous fractures on the expansion of an artificial fracture is analyzed by this method. Numerical simulation results show that the in situ stress, approaching angle between the artificial fracture and natural fracture, and natural fracture cementation strength have a significant influence on the propagation morphology of the fracture. When two intersected natural fractures exist, the second one will inhibit the propagation of artificial fractures along the small angle of the first natural fractures. Under different in situ stress differences, the length as well as aperture of the hydraulic fracture in a rock matrix increases with the development of cementation superiority of natural fractures. And with the increasing of in situ horizontal stress differences, the length of the artificial fracture in a rock matrix decreases, while the aperture increases. The numerical simulation result of the influence of a single natural fracture on the propagation of an artificial fracture is in agreement with that of the experiment, which proves the accuracy of the PFS-Cohesive FEM for simulating hydraulic fracturing in shale gas reservoirs.

Hydraulic fracturing is an important technology for the development of unconventional oil and gas. The complexity and filling of fractures are important factors for determining the fracturing effect. In order to control the fracture shape and improve the filling of the fracture, it is necessary to inject granular materials such as a temporary plugging agent (TPA) and special performance proppant into the fracture. Understanding the transport law of these granular materials in the fracture can guide us to optimize the fracturing scheme and improve the fracturing effect. In this paper, we studied the transport characteristics of granular TPA, floating agent, new coated quartz sand, micron proppant, ultra-low-density (ULD) proppant, and conventional quartz sand based on the large visual granular transport equipment. The results show that when the TPA of different sizes is injected successively to plug fractures, 20 mPa•s fracturing fluid is suggested for carrying 16/20 mesh TPA in the early stage, and slickwater is suggested for carrying 40/60 mesh TPA in the later stage. The methods of stopping the pump many times with low floating agent concentration or stopping the pump once with high concentration are two ways to realize that the floating agent is laid in the upper part of the fracture to form an artificial barrier. When the injection rate is large, we can only choose the latter. In slickwater, the newly coated quartz sand produces bubbles on its surface and adsorbs each other, which enhances its transport capacity and reduces the amount of sand used. Micron proppant and ULD proppant have the characteristics of less settlement in the major fracture and more settlement in the branched fracture, so they are suitable for filling branched fractures.