Hydraulic stimulation strategies in enhanced geothermal systems (EGS): a review

Jia, Yunzhong; Tsang, Chin‐Fu; Hammar, Axel; Niemi, Auli

doi:10.1007/s40948-022-00516-w

Cited by 18 publications

(8 citation statements)

References 234 publications

(225 reference statements)

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“…As a result, the development and opening of complex fracture networks, created by the coalescence of newly formed wing cracks with pre-existing fractures, enhance the permeability of the geothermal reservoir. This mechanism of hydraulic stimulation combines (i) fracture opening by shear-dilation of pre-existing fractures and (ii) propagation of fractures, and it is referred to as mixed-mechanism stimulation [11,17,20]. It should be noted that this term is not to be confused with the term mixed-mode failure, which refers to the mode of fracture propagation.…”

Section: Introductionmentioning

confidence: 99%

Modelling of mixed-mechanism stimulation for the enhancement of geothermal reservoirs

Dang-Trung,

Keilegavlen,

Berre

2024

Phil. Trans. R. Soc. A.

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Hydraulic stimulation is a critical process for increasing the permeability of fractured geothermal reservoirs. This technique relies on coupled hydromechanical processes induced through pressurized fluid injection into the rock formation. The injection of fluids causes poromechanical stress changes that can lead to fracture slip and shear dilation, as well as tensile fracture opening and propagation, so-called mixed-mechanism stimulation. The effective permeability of the rock is particularly enhanced when new fractures connect with pre-existing fractures. While hydraulic stimulation can significantly improve the productivity of fractured geothermal reservoirs, the process is also related to induced seismicity. Hence, understanding the coupled physics is central, for both reservoir engineering and seismic risk mitigation. This article presents a modelling approach for simulating the deformation, propagation and coalescence of fractures in porous media under the influence of anisotropic stress and fluid injection. It uses a coupled hydromechanical model for poroelastic, fractured media. Fractures are governed by contact mechanics and a fracture propagation model. For numerical solutions, we employ a two-level approach, combining a finite volume method for poroelasticity with a finite element method for fracture propagation. The study investigates the impact of injection rate, matrix permeability and stress anisotropy on stimulation outcomes. This article is part of the theme issue ‘Induced seismicity in coupled subsurface systems’.

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Section: Introductionmentioning

confidence: 99%

Modelling of mixed-mechanism stimulation for the enhancement of geothermal reservoirs

Dang-Trung,

Keilegavlen,

Berre

2024

Phil. Trans. R. Soc. A.

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“…; (2) Engineering geological conditions: mainly including in-situ stress state, natural fractures, proportion of thin interbeds, formation temperature and heterogeneity; (3) Rock and fluid physical factors: mainly including elastic modulus, Poisson's ratio, rock bedding, fracturing fluid viscosity and pumping rate, etc. [4][5][6][7]. Stress state, bedding, and natural fractures are the most significant geological factors on the artificial fracture expansion [8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Study on Fracture Propagation Rules of Shale Refracturing Based on CT Technology

Zhang,

Wang,

Xiao

et al. 2024

Processes

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Reactivating oil and gas wells, increasing oil and gas production, and improving recovery provide more opportunities for energy supply especially in the extraction of unconventional oil and gas reservoirs. Due to changes caused by well completion and production in pore pressure around oil and gas wells, subsequently leading to changes in ground stress, and the presence of natural and induced fractures in the reservoir, the process of refracturing is highly complex. This complexity is particularly pronounced in shale oil reservoirs with developed weak layer structures. Through true triaxial hydraulic fracturing experiments on Jimsar shale and utilizing micro-CT to characterize fractures, this study investigates the mechanisms and patterns of refracturing. The research indicates: (1) natural fractures and the stress states in the rock are the primary influencing factors in the fracture propagation. Because natural fractures are widely developed in Jimsar shale, natural fractures are the main influencing factors of hydraulic fracturing, especially in refracturing, the existing fractures have a greater impact on the propagation of secondary fracturing fractures. (2) Successful sealing of existing fractures using temporary blocking agents is crucial for initiating new fractures in refracturing. Traditional methods of plugging the seam at the root of existing fractures are ineffective, whereas extensive injection of blocking agents, forming large “sheet-like” blocking bodies in old fractures, yields better sealing effects, promoting the initiation of new fractures. (3) Moderately increasing the pumping rate and viscosity of fracturing fluid is advantageous in forming “sheet-like” temporary blocking bodies, enhancing the complexity of the network of new fractures in refracturing. (4) When there is a high horizontal stress difference, after sealing old fractures, the secondary hydraulic fractures initiate parallel to and extend from the old fractures. In cases of low horizontal stress difference, the complexity of secondary hydraulic fractures increases. When the horizontal stress changes direction, the secondary hydraulic fractures also change direction. It is recommended to use high-viscosity fracturing fluid and moderately increase the pumping rate, injecting blocking agents to seal old fractures, thereby enhancing the complexity of the network of refracturing. These findings provide important technical guidance for improving the efficiency of shale oil reservoir development.

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“…Complex fracture networks are formed by the reactivation of natural fractures and creation of new fractures [4]. In addition, hydraulic fracturing is applicable to the development of an enhanced geothermal system (EGS) [5,6].…”

Section: Introductionmentioning

confidence: 99%

Assessment of Saturation Effect on Hydraulic Fracturing in Sandstone and Thermally Treated Granite

et al. 2023

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In this study, a set of laboratory experiments was carried out to study the parameters of hydraulic fractures induced in the dry and mineral-oil-saturated rocks and compare them with the geomechanical characteristics of tested samples. We chose sandstone and thermally treated granite as the materials for research. There are very few known studies related to the mechanical and acoustic properties of oil-saturated rocks, and even fewer studies describing, in detail, the parameters of hydraulic fractures generated in oil-saturated rocks. The hydraulic fracture parameters were determined using a set of independent sensors installed to measure the axial deformation of the sample (which is directly related to the aperture of created hydraulic fracture), fluid pressure, fluid volume injected into hydraulic fracture, and localization of acoustic emission (AE) events, generated during the propagation of hydraulic fractures. Our study focuses on the investigation of the influence of rock properties, altered by mineral oil saturation and thermal treatment, on such parameters of hydraulic fracturing as breakdown pressure (BP), fracture aperture, and the resulting roughness of the hydraulic fracture surface. In addition, we studied the influence of injected fluid viscosity on the parameters of created hydraulic fractures. It was revealed that the saturation state caused a reduction in the values of mechanical parameters such as Young’s modulus, compressive strength, and cohesion, and had a similar reducing influence on the breakdown pressure. The values of HF parameters, such as fracture width and the volume of fracturing agent injected into the HF, are higher in the tests for both saturated sandstone and saturated TT granite. However, we found out that thermal treatment of granite samples led to a much more significant reduction in the values of mechanical and acoustic parameters than the mineral-oil saturation procedure because it created a dense network of thermally induced cracks. The results obtained in our laboratory studies can be taken into account in the modeling of hydraulic fracturing in the field.

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Hydraulic stimulation strategies in enhanced geothermal systems (EGS): a review

Cited by 18 publications

References 234 publications

Modelling of mixed-mechanism stimulation for the enhancement of geothermal reservoirs

Modelling of mixed-mechanism stimulation for the enhancement of geothermal reservoirs

Study on Fracture Propagation Rules of Shale Refracturing Based on CT Technology

Assessment of Saturation Effect on Hydraulic Fracturing in Sandstone and Thermally Treated Granite

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