True Triaxial Experiments Research Articles

Effect of initial minimum principal stress and unloading rate on the spalling and rockburst of marble: a true triaxial experiment investigation

Hard rock often performs as brittle failures, such as cracking, spalling, and rockburst, induced by excavation in deep underground engineering. To understand the effect of the minimum principal stress and unloading rate on the spalling and rockburst of marble, unloading experiments were carried out by using true triaxial equipment combined with a high-speed camera, acoustic emission instrumentation, and scanning electron microscope. The experimental results showed that the failure process of marble specimens was more stable and less inclined to exhibit dynamic ejection with a low strain energy release per unit time under a low unloading rate. At the same time, the initial minimum principal stress controlled the marble’s failure mode by changing its mode from predominantly shear failure to shear-tension failure as the minimum principal stress decreased, similar to the spalling behaviors of surrounding rock observed in the underground tunnel. What’s more, the dynamic rockburst of the marble specimen was simulated under the condition with a high unloading rate, high initial minimum principal, and free face on the specimen. Based on these experimental investigations, a flowchart was also presented to estimate the possible failure mode of marble specimens under different initial minimum principal stress and unloading rate conditions, which would be conducive to the disaster prevention of hard rock in deep underground engineering.

New Mechanical Model of Slotting–Directional Hydraulic Fracturing and Experimental Study for Coalbed Methane Development

When hydraulic fractures do not expand in the direction required by a project, it is difficult to enhance coal seam permeability effectively. Slotting–directional hydraulic fracturing (SDHF) has been proposed as a possible alternative, but there is not enough theoretical mechanism to guide the construction. Based on preliminary study of the directional mechanism of single slotted hole, we established a new slotting–directional hydraulic fracturing (NSDHF) mechanical model by using the complex function theory of elasticity, and the influence of stress interference between adjacent slotted holes and non-uniform pore water pressure was considered. We carried out true triaxial double slotted holes SDHF experiments and used large-scale nondestructive computer tomography to scan the fractured samples to ensure accurate measurement of directional distance. The measured directional distance was used to verify the NSDHF model; the maximum deviation was 5.1%. Taking the experimental data in this paper as example, the stress interference between adjacent slotted holes decreased the fracture directional distance by 20.3%, and the non-uniform pore water pressure increased the fracture directional distance by 47.6%. NSDHF mechanical model realized the quantitative description of the influence of non-uniform pore water pressure on directional distance. The contribution of non-uniform pore water pressure to directional distance accounted for more than 25% of the total directional distance; the effect of non-uniform pore water pressure on fracture direction distance was almost twice the stress interference between adjacent slotted holes. The verified NSDHF model was used to study further the influence of horizontal stress difference, azimuth of slotted hole, slotting size and fluid injection pressure on directional distance with different slotting spacing. The work discussed in this paper will contribute to promoting and apply SDHF on a large scale in low-permeability coal mines.

Research progresses in formation mechanism of complex fracture network for unconventional reservoir

The practice of oilfield development promotes the development theory, and the breakthrough of development theory brings about the revolution of development technology. Using horizontal wells and volume fracturing technology, volume fracturing has been applied in unconventional reservoir, and the formation mechanism of complex fracture network will bring innovation to development technology. At present, the research on the formation mechanism mainly focuses on theoretical research, experimental research, and numerical simulation. Theoretically, the mechanical model of the intersection of HF and NF is established, and some judgment criteria are put forward. Activation of NF by HF is a necessary condition for the formation of complex fracture networks; the activation conditions of NF are verified by true triaxial experiment in terms of horizontal stress difference, approximation angle, and fracturing fluid viscosity; finite element method, finite difference method, boundary element method, and discontinuous displacement method have their own advantages and disadvantages on simulation. Using digital speckle correlation technique, the authors explained the mechanism of shear action on the formation of fracture network, established a macro-meso two-scale non-uniform damage constitutive model, and formed a computing method for dynamic expansion of fractured network in tight reservoir. A field well example is calculated using this method.

Performance Evaluation of Degradable Temporary Plugging Agent in Laboratory Experiment

Abstract Temporary plugging fracturing is an effective way to enhance the fracture complexity and increase the stimulated reservoir volume (SRV) of unconventional reservoirs. The performance of temporary plugging agents (TPA) directly affects the success rate of temporary plugging. Currently, laboratory evaluation of the plugging effects of the TPA is rarely reported, and there are no industrial standards on laboratory evaluation of TPA plugging. In this study, two new experimental methods were used to evaluate a novel particulate TPA. The plugging performance of the TPA to the core end face and the propped fractures was measured through displacement experiments of cores, and the applicability of its basic performance to the temporary plugging fracturing was verified. Furthermore, the large-scale true triaxial simulation experiment of temporary plugging fracturing was carried out to confirm the influence mechanism of different factors on fracture propagation during temporary plugging. Finally, the influence rule of different types of combinations of TPA and placement patterns on the plugging was obtained based on laboratory evaluation of the conductivity. The results show that the novel TPA causes effective temporary plugging on the core end face and the propped fractures and has the strong plugging performance, and the TPA solubility in the carrying fluids decreases with the increase in the TPA concentration. The basic performance of the TPA meets the requirements of temporary plugging fracturing. If the proppants and 20% fibers are placed within the fracture in the mixed pattern, the fracture is initiated along the direction of the horizontal maximum principal stress. The preset fracture reduces the fracture initiation pressure. The fracture complexity is closely related to the placement pattern of TPA and proppants. If the preset fractures are filled by the uniform mixture or the plug of the 20/40 mesh or 20/80 mesh particulate TPA (4%), fibers (1%), and proppants, the fracture initiation pressure significantly increases, and the complex fractures are formed after fracturing. Effective plugging cannot be formed only by mixing the fibers with the proppants, and the uniform mixture of the proppants and 4% particulate TPA and the 6% particulate TPA at the front end of the fracture form a temporary plugging belt, achieving effective plugging. The fibers improve the conductivity under the low closure stress, and it has a certain effect of temporary plugging under the closure stress above 30 MPa. The research results provide the design consideration for creating the complex fracture by temporary plugging.

Study on fracture propagation behavior in ultra-heavy oil reservoirs based on true triaxial experiments

Abstract As the ultra-heavy oil reservoirs developed by steam assisted gravity drainage (SAGD) in the Fengcheng oilfield, Xinjiang have problems such as huge steam usage, long preheating period, low production, and inaccessible reserve in local parts. Based on the rock mechanics and porosity/permeability characteristics of heavy oil reservoir and interlayer, a series of true triaxial experiments and CT tests considering the fracturing fluid injection rate, viscosity, perforation density and location of fracture initiation were conducted to disclose the propagation behavior of micro- and macro-fractures in the reservoirs and mudstone interlayers. These experiments show that fracturing in the heavy oil reservoirs only generates microfractures that cannot break the interlayer. In contrast, when fracturing in the interlayer, the higher the injection rate (greater than 0.6 m3/min), the lower the viscosity, the easier it is to form macro-fractures in the interlayers, and the further the fractures will propagate into the reservoirs. Also, increasing perforation density tends to create complex macro-fracture network in the interbedded reservoirs and mudstone interlayers. The findings of this study can provide scientific guidance for the selection of fracturing layer and the optimization of parameters in the interlayer fracturing of heavy oil reservoirs.

Study of alternating acid fracturing treatment in carbonate formation based on true tri-axial experiment

Acid fracturing is a useful technique to stimulate carbonate reservoirs. Nowadays, acid leak-off becomes a vital problem during the operation of acid fracturing. In order to encounter this issue, alternating acid fracturing has been developed. However, the fracture propagation and the interaction between hydraulic fractures and natural fractures/caves during alternating acid fracturing are ambiguous. Hence, a series of alternating acid fracturing experiments covering coupling mechanics and chemical interaction were conducted for the first time. In this study, slick water, self-generating acid, cross-linked gel, and gelled acid were used as a fracturing fluid whereas the specimens for this experiment are carbonate outcrop which excavated from Dengsi Formation. The results show that: (1) acid fracturing can reduce fracturing pressure effectively as the acid will etch and weaken the natural fractures to let them easily connecting each other; (2) different pad fluid properties will have different impacts on the acid fracturing performance; (3) the interaction of hydraulic fractures and natural filled caves can be divided into five categories, and two of them had been observed in the experiments. Moreover, a new curvature-calculation algorithm has been developed based on the 3-D scanning technology. The results have a great significance for acid fracturing design and physical simulation experiments.

Study on Fracture Morphological Characteristics of Refracturing for Longmaxi Shale Formation

Refracturing technology has become an important means for the regeneration of old wells reconstruction. It is of great significance to understand the formation mechanism of hydraulic fracturing fracture for the design of hydraulic fracturing. In order to accurately evaluate and improve fracturing volume after refracturing, it is necessary to understand the mechanism of refracturing fracture in shale formation. In this paper, a true triaxial refracturing test method was established. A series of large-scale true triaxial fracturing experiments were carried out to characterize the refracturing fracture initiation and propagation. The results show that for shale reservoirs with weak bedding planes and natural fractures, hydraulic fracturing can not only form the main fracturing fracture, which is perpendicular to horizontal minimum principal stress, but it can also open weak bedding plane or natural fractures. The characteristics of fracturing pump curve indicated that the evolution of fracturing fractures, including initiation and propagation and communication of multiple fractures. The violent fluctuation of fracturing pump pressure curve indicates that the sample has undergone multiple fracturing fractures. The result of refracturing shows that initial fracturing fracture channels can be effectively closed by temporary plugging. The refracturing breakdown pressure is generally slightly higher than that of initial fracturing. After temporary plugging, under the influence of stress induced by the initial fracturing fracture, the propagation path of the refracturing fracture is deviated. When the new fracturing fracture communicates with the initial fracturing fracture, the original fracturing fracture can continue to expand and extend, increasing the range of the fracturing modifications. The refracturing test results was shown that for shale reservoir with simple initial fracturing fractures, the complexity fracturing fracture can be increased by refracturing after temporary plugging initial fractures. The effect of refracturing is not obvious for the reservoir with complex initial fracturing fractures. This research results can provide a reliable basis for optimizing refracturing design in shale gas reservoir.

Hydraulic fracture propagation behavior and diversion characteristic in shale formation by temporary plugging fracturing

The exploitation of shale oil and gas has become commercial in the Sichuan basin, China. However, initial fracture closure will result in low production. Therefore, temporary plugging fracturing technology is proposed to create new fractures in alternate directions using a temporary plugging agent that blocks initial fractures, which will lead to a complex fracture morphology. The mechanism of temporary plugging and fracture propagation behavior remains ambiguous, even with many successful operations. Hence, five shale outcrops are used to conduct large-size true tri-axial temporary plugging fracturing simulation experiments to study the issues mentioned above. The effects of horizontal stress difference and temporary plugging agent amount on plugging effectiveness as well as fracture propagation behavior are discussed. The results reveal that the initial fractures are sealed successfully by a water soluble temporary plugging agent. Two plugging positions, fracture heel and tip, result in three main fracture diversion patterns: fracture diversion at the old fracture heel, fracture diversion inside the old fracture, and a new fracture induced at a new position. Based on the fracturing pressure curve, fracturing pressure in the second fracturing is higher than that of the first fracturing, which indicates effective plugging and fracture diversion. A large horizontal stress difference has prevented fracture diversion, and led to a small diversion angle. However, it has improved the induced fracture propagation length. Moreover, the temporary plugging agent amount determines the plugging position and fracture diversion pattern. Fracture diversion inside existing fractures is a result of a small amount sealing at the fracture tip. The other two patterns are created by a large amount of plugging at the fracture heel. Therefore, in order to achieve successful plugging effectiveness, it is necessary to conduct more experiments to optimize operation parameters according to the specific horizontal in-stress state.

True-Triaxial Experimental Study on Mechanical Behaviours and Acoustic Emission Characteristics of Dynamically Induced Rock Failure

Dynamic disturbance is an important factor for the failure of surrounding rocks in underground engineering projects, particularly under high static stress conditions. In this study, three dynamic disturbances are considered: a single-pulse dynamic load with a large stress amplitude (a mild disturbance), a low-frequency cyclic dynamic load with a moderate stress amplitude (a modest disturbance), and a high-frequency cyclic dynamic load with a small stress amplitude (a weak disturbance). Based on the true-triaxial experiments of coupled static and dynamic loads, the mechanical behaviors and acoustic emission characteristics of the dynamically induced rock failure are analyzed. The test results show that the dynamically induced rock failure depends not only on the initial static stress level but also on the dynamic disturbance type. In the rock failure induced by the mild disturbance, the increasing initial static stress produces the dual effect of an initial increase and then a decrease in the failure strength of the rock specimens. In the rock failure induced by the modest disturbance, a sufficiently high initial static stress is the basic condition to trigger rock failure. In the rock failure induced by the weak disturbance, the initial static stress threshold value needed to trigger the rock failure is higher than that needed for the modest disturbance. The rock failure induced by the mild disturbance is mainly contributed by the large input of disturbance energy and the slight impact effect. The rock failure induced by the modest disturbance is mainly contributed by the fatigue damage effect, which reduces the ultimate energy storage capacity of the rocks. The rock failure induced by the weak disturbance is related to not only the fatigue damage effect intensifying the specimen damage, but also the dynamic disturbance effect inducing the release of elastic strain energy. Additionally, the fracture mode for rocks subjected to the mild disturbance is predominantly the shear fracture mode. More tensile cracks are detected for rocks subjected to the modest disturbance than those subjected to the mild disturbance. More shear cracks and fewer tensile cracks are detected for rocks subjected to the weak disturbance than those subjected to the modest disturbance.

Shear-tensile fractures in hydraulic fracturing network of layered shale

Understanding hydraulic fractures generation and geometric properties are significant for optimizing hydraulic fracturing treatment design which improves the ultimate production from shale reservoirs. The propagation of the hydraulic fractures is influenced by bedding planes. Simple opening or tensile crack model is not enough to meet the hydraulic fractures generation process, and the fault slipping model which is regarding as the cause of the natural earthquake is also not compatible with the actual physical meaning of fracking induced fractures. Hydraulic fractures consist of not only the opening but also slippage. Acoustic emission monitoring combined with the true triaxial hydraulic experiment provides direct information about the propagation and the geometric parameters of fractures. Moment tensor of acoustic emission is related to the mechanical mechanics of fractures. In this paper, the moment tensor interpretation is based on the shear-tensile crack model, then the orientation and the opening width of the hydraulic fractures are calculated. Computerized tomography scanning figures of the samples give support to the interpretation process. The distributions of hydraulic fractures orientations are different in the two samples with different normal stress applied on the bedding plane. Higher normal stress on the bedding plane led to a more complex fracture orientation. And the opening widths of the fractures in the far area from the wellbore are smaller than the fractures nearby the wellbore. These two parameters quantify the fractures geometric properties and provide information for adjusting the next step of the fracturing process. The findings of this study can help for better understanding of hydraulic fractures generation in the bedding shale. Shear-tensile crack model is supported by actual physical models has good adaptability in explaining the hydraulic fracture propagation process.

Experimental investigation of fracture initiation position and fluid viscosity effect in multi-layered coal strata

The multi-layered coal strata in Lin-Xing block of China is rich in tight sandstone gas and the coalbed methane (CBM). The feasibility of multi-gas production and the optimal layer for fracturing in multi-layers requires a better understanding of fracture vertical extension mechanism through sandstone coal interbedding. To analyze the problems, six true tri-axial experiments were conducted on the multi-layered specimens of sandstone and coal outcrops. The transition zone was simulated by the cement with certain strength and thickness. In the experiments, the perforation position, fluid viscosity, and lithological difference were discussed. The results showed that the effect of viscosity exhibited a stepped manner as a function of fracture complexity. High-viscosity fluid significantly improved the fracture penetration ability when fracturing in coal layer. The results also show that when indirectly fracturing in sandstone, low-viscosity fluid was under the risk of natural fracture activation near wellbore to cause fracturing failure. When both layers were perforated, complex fracture network was easily formed as multi-layers were fractured. Perforation in both sandstone and coal layers were the best method based on the fracture penetration ability and the overall fracture complexity. T-shaped and step-wise fractures were found in the interface. The transition zone made it easier for fracture vertical extension through layers. The field production data was in good agreement with the experimental results. These results provide a guideline for the optimal perforation position selection and fracturing parameters in coal strata.

Experimental Investigation of Hydraulic Fracturing for Multi-type Unconventional Gas Co-exploitation in Ordos Basin

The accumulated gas production and production cycle would be significantly increased by exploiting the unconventional gas resources of coalbed methane and tight gas jointly in coal measure strata (tight gases–coal interbedding reservoir) in Ordos Basin in China. However, whether hydraulic fractures could connect multiple gas zones effectively in the vertical plane is a key problem. To study hydraulic fracture geometries and vertical propagation rules for coal measure strata, six groups of true triaxial experiments in natural outcrops collected from the eastern margin of Ordos Basin in China were carried out. Results show that unlike conventional symmetric fracture geometry in layered media, the principle of fracture vertical propagation in coal measure strata follows the least resistance and most preferential propagation path. The natural fractures and cleats in coal seam make the overall fracture morphology asymmetric. A moderate stress difference between vertical stress and maximum horizontal stress (4 MPa in this paper) is the key for connection of multiple layers as well as SRV enhancement. Developed natural fractures and bedding planes in interlayer could alter fracture extension path and impede hydraulic fracture crossing layer interface. In the process of hydraulic fracture propagating from interlayer to layer interface and then to the coal seam, the fluid pressure in hydraulic fractures is characterized by an apparent fluctuation of decreasing first and increasing later. Furthermore, the greater the elastic modulus of the interlayer, the greater the pump pressure fluctuations.

Patterns in complex hydraulic fractures observed by true-triaxial experiments and implications for proppant placement and stimulated reservoir volumes

Rocks are host to complex fracture networks that are difficult to locate in situ, and yet characterization of these fractures is crucial to predicting the effects of hydraulic stimulation. We analyze three-dimensional hydraulic fracture patterns among varied laboratory experiments to identify recurring geometries. Building on the constitutive tensile and shear fracture modes, we observe examples of offset fracture branching, traversing fracture coalescence, and smooth fracture reorientation as relatively simple structures within complex fracture networks. The evolution of fracture branching, also referred to as stranding, is revealed to be a fundamentally three-dimensional process, in which continued propagation can result in traversing fracture coalescence. Fracture branching, therefore, can create an illusion of unconnected, staggered, and offset hydraulic fracture growth when viewed from a single cross section; meanwhile, these fractures are likely connected through a common fracture surface elsewhere. The fractures are also investigated at a smaller scale, where similar fracture patterns are observed. In the field, these complex patterns are likely to hinder proppant settling, reduce open fracture permeability, create larger fracture surface areas, and lead to increased stimulated reservoir volumes. A balance of stimulation methods to prevent this complexity in some areas and exploit it in others could be key to improving recovery from oil and gas resources, improving geothermal energy efficiency, and optimizing disposal of waste water or CO2 via injection wells. Representation of these complex structures is needed for accurate modeling predictions for reservoir management.

Observing the effects of transition zone properties on fracture vertical propagation behavior for coal measure strata

The transition zone is a typical interface with enough thickness and high heterogeneity between the sandstone and coal and has a great effect on the fracture propagation in coal measure strata. In the study, the true tri-axial experiments were conducted with the combination of sandstone, coal and limestone outcrops. The effect of cementing strength and thickness of the transition zone was mainly studied. Three types of hydraulic fracture initiation positions were used to simulate three fracturing situations in the field. The results showed that the effect of the transition zone on the fracture vertical extension could be concluded: the fracture was easy to cross or slip with low cementing strength and turned inside the transition zone with high cementing strength; the fracture tended to branch or be blocked by the thick transition zone. The horizontal flow through face cleats and natural fracture activation in coal contributed to a more developed fracture network. The characters of pumping curve pressure “increasing – fluctuation – dropping”, and the “fluctuation pressure more than initiation pressure” indicated the interface crossing. The results of experiments agreed with the previous numerical simulation and helped to optimize the fracturing design in layered coal strata.

A generalized plasticity model for the stress-strain and creep behavior of rockfill materials

The generalized plasticity constitutive equations that simulate, in a unified manner, the stress-strain response and the creep behavior of rockfill materials are derived using the concept of elastoplasticity. A single yield surface is assumed to capture the onset of plastic strains with, however, two separate potential functions for the stress-induced plastic strains and the creep strains, respectively. The involved tensors and scalars are then specified directly, following the generalized plasticity method, to substantiate the constitutive equations. The model thus obtained is verified using triaxial compression experiments, true triaxial experiments and triaxial creep experiments. The effectiveness of the model is also demonstrated by a successful application in studying the behavior of a high concrete face rockfill dam (CFRD). It is found that for a high CFRD with a long construction period, neglecting the creep of rockfill materials during construction results in an underestimation of the deformation of the dam. The deformation and stress of the concrete slabs may also be considerably underestimated.

Mechanical and Coal Burst Breeding Mechanism of Coal Samples under True Triaxial Loading and Unloading

A true triaxial loading and unloading experiment entailing “y‐direction stress loading, z‐direction stress unloading, and x‐direction displacement fixing” of coal samples was conducted. Through analysis of the stress characteristics, fracture characteristics, and energy evolution in coal samples, the mechanical and coal burst breeding mechanisms of coal samples under true triaxial loading and unloading were revealed. The experiment found that the yield stress and peak stress of coal samples were not only affected by the initial loading and unloading of lateral stress but also had a negative exponential relationship with the ratio of the unloading rate and the loading rate (RURLR), thereby establishing the stress equation of coal samples under a true triaxial loading and unloading. There was a yield turning point in the stress‐time curve of coal samples, and the difference in triaxial stress and acoustic emission before, and after, yield was significant. It was found that a high unloading rate and high initial stress are precursors to coal sample bursting. During loading and unloading, the high‐energy area expanded, but its location was always fixed to within a certain area. The energy in this area was rapidly released to form a burst source when the sample was subjected to high‐speed unloading. The nonbursting coal samples and the burst coal samples showed characteristic slabbing and bursting behaviours, respectively: the former corresponding to the acoustic emission energy value being two orders of magnitude lower than the latter. The research results can provide a reference for the study of mechanical behaviours and coal burst criteria in the rock surrounding a coal roadway excavation.

Damage plasticity model for passively confined concrete

This paper presents a modified concrete damage plasticity model (CDPM) for passively confined concrete within the concrete damage plasticity theory frame in ABAQUS. The modified CDPM can be used to simulate concrete under non-uniform passive confinement, for example, Fiber-reinforced polymer (FRP)-confined square concrete columns. The modification of CDPM includes a flow rule and a strain hardening/softening criterion in which dilation angle and yield stress are important parameters. Based on the true-triaxial experiment results of passively confined concrete, the dilation angle and yield stress were determined considering different confinement stiffness and non-uniform confinement stiffness ratio. Finally, the modified CDPM were incorporated in the ABAQUS model. The prediction of the finite element model of FRP-confined square concrete columns shows good prediction accuracy.

Effect of Intermediate Principal Stress on the Strength, Deformation, and Permeability of Sandstone

Although the mechanical behaviors and flow aspects of sandstone have been previously investigated, studies of the effect of the intermediate principal stress (σ2) on the strength, deformation, and permeability of sandstone are lacking. In this work, the mechanical behaviors and permeability of sandstone under true triaxial stress conditions were investigated using a newly developed true triaxial geophysical apparatus. The experimental results showed that with increasing σ2, the peak strength, octahedral effective normal stress, and octahedral effective shear stress of the sandstone increased, and the rate of increase decreased. This is because a larger intermediate principal stress coefficient b has an inhibitory effect on rock strength. In our study, as the ratio of σ2/σ3 increased, the specimen entered compressive strain in the σ2 direction during the first stress drop. The stress and strain path deviations occur during rock failure. The amount of deviation increased as the σ2 increased before the peak stress. This phenomenon indicates that elastic mechanics are not suitable for understanding this sandstone rock during its failure. The permeability evolution of the sandstone under true triaxial stress conditions was measured and analyzed to investigate the effect of σ2. During the complete true triaxial stress-strain experiments, the variation we found in gas seepage velocity could be divided into two stages. Before the first pressure drop, the gas seepage velocity was mainly affected by volume strain. After the first pressure drop, the seepage velocity was affected by the deviator strain, which can change the seepage channels.

Time-dependent behaviour and modeling of Jinping marble under true triaxial compression

The time-dependent deformation, creep rate, and long-term strength of Jinping marble under different 3-D stress states were studied by using true triaxial creep experiment. A 3-D visco-elastic-plastic creep model, which can describe the accelerated creep， was developed according to the true triaxial time-dependent deformation curve. The results show that the time-dependent behaviour of Jinping marble is affected by the stress level. When the stress is less than the long-term strength, not obvious time-dependent behaviour of the rock sample is found, and the strain remains unchanged after primary creep. When the stress exceeds the long-term strength, the time-dependent behaviour of the rock sample gradually increases with increasing σ1 and a clear steady state creep stage is seen in the strain-time curve. With constant σ1 and σ3, the time-dependent deformation and creep rate in the three main stress directions gradually decrease with increasing σ2. Compared with conventional triaxial test data, a large intermediate principal stress increases the time-dependent dilatation in the σ3 direction. In addition, a 3-D visco-elastic-plastic creep model considering damage is proposed to describe the creep behaviour of hard rock under different 3-D stress states. The model results fit the experimental results, and the selection of model parameters is closely related to the intermediate principal stress. The model results also verify the applicability of the 3-D element model.

An innovative technology of directional propagation of hydraulic fracture guided by radial holes in fossil hydrogen energy development

Conventional hydraulic fracturing fails to develop low permeability reservoirs of fossil hydrogen energy that are not located in the direction of maximum principal in-situ stress. A new technology of fracture propagation guided by radial holes is proposed, which can realize directional propagation of hydraulic fracture along radial holes in fossil hydrogen energy development. In order to verify this new technology, a model of radial holes combined with hydraulic fracturing is established by the ABAQUS extended finite element method. Simulation results show that radial holes play a guiding role in fractures propagation. The influence extent of seven factors on the directional propagation of hydraulic fracture is listed as follows (from strong to weak): azimuth of radial holes > horizontal in-situ stress difference of fossil hydrogen reservoir > injection rate of fracturing fluid > Young's modulus of rock > permeability of fossil hydrogen reservoir > Poisson ratio of rock > viscosity of fracturing fluid. True tri-axial experiment is carried out to verify the accuracy of numerical simulation, and the result is consistent with numerical model, which indicates that numerical simulation is reliable.