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In unconventional reservoir hydraulic fracturing, using a pump schedule with proppant concentration constant (PCC) instead of the proppant concentration stepwise increasing (PCSI) is considered an innovative approach to reduce operation costs and risks. However, the large-scale application of PCC lacks sufficient theoretical support. This study investigates the impact of two pump schedules on proppant transport behavior through visualized proppant transport experiments and the model that couples fracture propagation with proppant transport. The experiments demonstrate that PCC facilitates proppant transport and reduces flow fluctuations in complex fractures, while PCSI enhances the filling of near-well fractures. The proppant concentration (the ratio of proppant volume to fracturing fluid volume) reaches 20 % or when a combination of 40/70-mesh and 20/40-mesh quartz sands is used for injection, the pump schedule influence becomes negligible. An increase in fracture complexity, proppant size, and proppant concentration all lead to a reduced influence of the pump schedule on the dune shape. Numerical simulation reveal that the fracture area and average fracture conductivity is similar for different pump schedules, but PCC yields a slightly larger propped fracture area (ranging from 1 % to 4 %) and results in a more uniform conductivity. PCC could achieve similar fracture filling when the proppant concentration is more than 15 % or there is no significant demand for near-well conductivity. Overall, both two pump schedules have a relatively insignificant impact on the transport process of proppants and the final sand dune shape. By comparing pressure curves during hydraulic fracturing, it was evident that PCC resulted in fewer fluctuations. Analyzing production data from 93 wells with PCSI and 59 wells with PCC, it was observed that PCC enhances production and reduces proppant flowback post-fracturing. This research provides a theoretical foundation for the application of PCC pump schedule in hydraulic fracturing. • Compared the effects of different pump schedule on proppant transport. • Proppant concentration constant and stepwise injection have little difference. • Demonstrated the feasibility of proppant concentration constant injection. • Field tests show proppant concentration constant injection performs equally or better. [ABSTRACT FROM AUTHOR]

Under the influence of tectonic stress and faults, there are many natural fractures (NFs) in hot dry rock (HDR) reservoirs. However, the effect of NFs on the propagation of hydraulic fractures (HFs) has often not been considered in previous studies. A coupled thermo-hydro-mechanical-damage (THMD) model by the finite element method is constructed to investigate hydraulic fracturing and production performance in a fractured geothermal reservoir. The results show that thermal stress and injection pressure jointly promote rock initiation and propagation. HF can propagate tortuously and deviate from the maximum horizontal stress direction under the influence of NFs. HF and open NFs constitute the main channel of heat transfer, which determines the heat extraction performance. The fluid viscosity (μ) and injection flow rate (q in) are the main factors affecting the fracturing and production performance. A higher μ and q in can significantly increase HF length and activate the NFs along the propagation path, so the fracture area and production temperature have a distant ascension. The horizontal stress difference (Δ σ) and NF number (n) are secondary factors affecting the fracturing and production performances. A higher or lower Δ σ and n are not conducive to forming a long HF and enhancing the production temperature. A larger fracture area or higher fracture complexity does not necessarily lead to better production performance. For the development mode of HDR using two injection wells and one production well, it is beneficial to enhance the production temperature to give priority to ensuring the uniform propagation of HF. The results can provide a theoretical basis for the optimal design of an enhanced geothermal system in a fractured geothermal reservoir. • A coupled THMD model for the fracturing and production is constructed and verified • HF propagation under the influence of NFs and the production performance are studied • The main controlling factors affecting the fracturing and production are determined • The uniform propagation of HF is beneficial to improve the production performance [ABSTRACT FROM AUTHOR]

Hydraulic fracturing (HF) has been proved to be a promising technology to achieve economic production of hydrate. However, the research on HF of hydrate reservoirs is at its initial stage with limited information being available. In particular, the mechanism of fracture propagation in hydrate reservoirs is not well understood and requires further investigations. In this study, a new coupled thermo-hydrologic-mechanical-damage (THMD) model for HF simulations in hydrate reservoirs is proposed. Damage mechanics theory is used as the criterion of hydraulic fracture initiation and propagation, and the variation of hydrate properties due to hydrate phase transformation is considered in this THMD coupled model. The influence of hydrate saturation, reservoir permeability, fracturing fluid viscosity and fluid injection rate on HF were analyzed, and the mechanism of fracture initiation and propagation during HF in hydrate reservoir was revealed for the first time. The results showed that fracturing fluid destroys phase equilibria and causes hydrate dissociation, which in turn, releases the pore spaces occupied by hydrate, resulting in the increase of reservoir permeability near fracture surface. Besides, the hydrate dissociation reduces its cementation on sediment particles, causing the decrease of cohesion near the fracture surface. The fracture initiates and propagates perpendicularly to the direction of minimum principle field stress. Hydrate dissociation leads to heterogeneity of reservoir physico-mechanical properties, resulting in a irregular fracture surface morphology. Enhancing the fracturing fluid viscosity and injection rate promotes to improve the reservoir pore pressure and inhibit methane hydrate dissociation, which is beneficial to increase the fracture length. The hydrate reservoir with high methane hydrate saturation and low permeability is conductive to form long fractures during HF. • A new thermo-hydrologic-mechanical-damage (THMD) coupling model is proposed. • The dynamic property of reservoir caused by hydrate decomposition and damage is considered. • The mechanism of fracture initiation and propagation during hydraulic fracturing is revealed. [ABSTRACT FROM AUTHOR]

To realize the effective extraction of high-temperature thermal energy in enhanced geothermal system (EGS), it is necessary to explore how to efficiently construct fracture network in hot dry rock (HDR). Considering the cryogenic induced thermal stress, we investigate the cracking mechanism when using supercritical CO 2 (SCCO 2) and hydraulic fracturing to stimulate HDR. Firstly, the established three-dimensional THMD coupling model is verified by conducting high temperature granite fracturing experiments. Then, characteristics of the heat and mass transfer and fracture propagation when using SCCO 2 and H 2 O in HDR fracturing are explored. Finally, the cracking mechanism of hydraulic and SCCO 2 fracturing in HDR under thermal stress is revealed. The results indicate that the cryogenic induced thermal stress can reduce the cracking pressure and tend to form branch fractures under the cooperation of injection pressure. Adopting SCCO 2 in HDR fracturing more micro fractures can be generated in near-well zone. Hydraulic fracturing has better cooling efficiency and preferably capability of extending fractures. For HDR fracturing, the scope of fracture network tends to increase first and then decrease with the raise of injection mass flux. The lower viscosity and higher specific heat capacity of fracturing fluid can promote the formation of fracture network. • True-triaxial fracturing tests on granites are conducted under different temperature. • Three-dimensional THMD coupling model is established to simulate hot rock fracturing. • Damage evolution of hydraulic and SCCO 2 fracturing in HDR stimulation is investigated. • Effect of injection mass flux on fracture morphology in HDR fracturing is researched. • Lower viscosity and higher heat capacity of fracturing fluid can promote the formation of EGS. [ABSTRACT FROM AUTHOR]

Thermal fluid injection is proven to be one of the effective methods to dissociate hydrate. The injection of thermal fluid can destroy the hydrate phase equilibrium, resulting in hydrate decomposition and hydrate saturation decline, which further cause reservoir damage. Previous works separated the study of hydrate saturation variation caused by thermal fluid injection and the reservoir damage caused by hydrate saturation variation. In this paper, a novel thermo-hydrologic-mechanical (THM) coupling model is proposed to simulate the thermal fluid injection in hydrate reservoir, and its effects on reservoir damage were analyzed. The results showed that thermal fluid injection causes the decomposition of hydrate, resulting in the decrease of hydrate saturation. The solid phase hydrates decompose into methane and water, which increases the pore pressure of hydrate reservoir and further affects the in-situ stress field. The change of effective stress induces pore deformation, and the hydrate decomposition also releases the space occupied by hydrate in reservoir pores, both affecting the reservoir permeability. Hydrate phase transition weakens the cementation of hydrate on rock particles and reduces the reservoir rock cohesion. In addition, the elastic modulus of hydrate decreases due to hydrate decomposition. The temperature and injection rate of thermal fluid are the main external factors affecting the hydrate phase transition. With the increase of thermal fluid temperature, hydrate saturation decreases due to hydrate decomposition, resulting in variation of reservoir parameters. Enhancing fluid injection rate can increase the reservoir pore pressure, which inhibits the hydrate decomposition. • A new thermo-hydrologic-mechanical (THM) coupling model is proposed. • The dynamic property of reservoir caused by thermal fluid injection is considered. • The mechanism of hydrate phase transition during thermal fluid injection is analyzed. • Increasing injection rate and reducing fluid temperature can reduce reservoir damage. [ABSTRACT FROM AUTHOR]

The existence of solid hydrate in porous media always greatly reduces the relative permeability, which limits the development of hydrate reservoirs. Therefore, to recover the gas, it is necessary to carry out reservoir reconstruction measures that induce fractures around the production wells to increase the gas production of hydrate reservoirs. At present, there is limited quantitative research on the promotion of gas production by induced fractures in hydrate reservoirs. In this paper, numerical models of Class Ⅲ hydrate reservoirs with induced fractures are established. By comparing the gas production with and without induced fractures, the effectiveness of induced fractures in promoting the depressurization production of natural gas hydrates is verified. An index of improved efficiency is established and calculated to measure the promotion effect of induced fractures during the fracture promotion stage. The multiple regression calculation formulas of improved efficiency with reservoir geological parameters are obtained by integrating nonlinear regression and linear regression. The calculation model is helpful to quickly and directly measure the rationality and economy of the induced fracture depressurization method to promote hydrate development. • Numerical models of Class Ⅲ hydrate reservoirs with induced fractures are established. • An index of Improved Efficiency is established to measure the promotion effect of induced fractures. • The improved efficiency of induced fractures under different geological parameters is calculated and compared. • The multiple calculation formulas of improved efficiency with geological parameters are obtained by stepwise regression. [ABSTRACT FROM AUTHOR]