Vertical Fracture Propagation Research Articles

Effects of elastic anisotropic models on the prediction of horizontal stresses and hydraulic fracture geometry

Abstract Stress barriers play a key role in the propagation of hydraulic fractures. They are local maxima in the stress field that constrain vertical fracture propagation. The development of stress barriers is influenced by rock mechanical properties, pore pressure and tectonic stresses. However, stress prediction models are highly sensitive to available geophysical measurements and assumptions made on rock constitutive models. We compare stress estimations performed with elastic isotropic, anisotropic and viscoplastic models using Thomsen's notation ( ɛ , δ , γ ) to quantify anisotropy and its effects on hydraulic fracture geometry. Using a single depth for principal stress calibration, we compare stress distributions and simulate hydraulic fracture geometries along simple vertical and horizontal well sections. Prediction errors stemming from isotropic models along anisotropic intervals increase when tectonic stresses increase. Errors generated by either over- or underestimation of δ increase for tectonically passive environments, while errors generated by either over- or underestimation of ɛ increase for tectonically active environments. Additional corrections, but also uncertainties, can be introduced by considering viscoelastic rock behaviour. Because of stress shadowing and fracture interaction, the risk of stress barrier underestimation is higher when estimating hydraulic fracture geometries along the various stages of horizontal wells.

Influence of Engineering Parameters on Fracture Vertical Propagation in Deep Shale Reservoir: A Numerical Study Based on FEM.

The geometry of hydraulic fractures in deep shale facies is significantly affected by the longitudinal inhomogeneity of rock physical properties and stresses. Numerous studies have been conducted on the influence of the longitudinal inhomogeneity of rocks on fracture morphology. However, there is still a lack of research that simultaneously considers the reservoir dip, bedding plane interface, and longitudinal inhomogeneity of the reservoir. To fill this gap, a three-dimensional (3D) numerical model of multireservoir hydraulic fracturing, which takes into account the bedding plane interface, was developed using the finite element method (FEM). The Drucker-Prager elastic-plasticity criterion was incorporated to accurately represent the plasticity of deep shale. The research revealed the influence of the formation dip angle on fracture morphology. Additionally, the perforation layer position and pump rate were optimized based on the actual geological parameters in North Jiangsu shale reservoir. The study findings indicate that reservoir fractures with a formation dip are easily detected by the interface. However, it is not necessarily true that the larger the formation dip, the easier it is for fluids to enter the interface. Fracturing from high-strength and stress reservoirs to lower reservoirs promotes the propagation of fracture height and the connectivity of multiple reservoirs. On the other hand, fractures initiated from low-strength and stress reservoirs tend to be confined to adjacent reservoirs more easily. The pump rate significantly affects the vertical propagation of fractures. At high interface strength, fractures with pump rate below 2.4 m3/min can only propagate at the perforation layer. The limited fracture height in shale reservoirs is likely due to substantial energy consumption by the fracturing fluid at the bedding plane interface. These studies offer theoretical guidance for understanding the vertical propagation of fractures in a deep multilayer reservoir.

Mechanism of hydraulic fracture vertical propagation in deep shale formation based on elastic–plastic model

Deep shale reservoirs exhibit strong plasticity, and the simulation results from previous fracturing simulations based on elastic models may not adequately account for the effects of plastic. Therefore, the research objective is to develop a 3D finite element method (FEM) model of hydraulic fracture vertical propagation in multilayer reservoirs. The model simultaneously considering Drucker-Prager elastic–plastic rock model and bedding plane. To investigate the impact of various parameters on the vertical propagation pattern of fracture, it is crucial to consider factors such as friction angle, yield strength, interlayer strength, and vertical stress. This study quantitatively characterizes the mechanical parameters such as ground stress, matrix strength, and interface strength on hydraulic fracture morphology. Additionally, a new integrated control diagram is established to provide a comprehensive understanding of these effects. The results indicate that rocks with high friction angles and low yield strength are more resistant to cracking, and hydraulic fractures tend to have narrower fractures. Fracturing from high-strength and high-stress zones to lower zones promotes vertical fracture propagation. The high vertical stress plays a crucial role in preventing the fracturing fluid from entering the interface. Moreover, the strength of the formation interface greatly influences the vertical extension of hydraulic fractures. The bedding plane interface with a high cementation level can enhance the possibility of fracture vertical connection with multiple reservoirs. Conversely, breaking through low strength cemented interfaces in the formation can pose challenges for hydraulic fractures. The present simulation conditions determine that the fracture's ability to cross the layer depends on the ratio of interface strength to matrix strength, which should be between 0.25 and 0.5. These findings offer valuable theoretical guidance on the vertical propagation of fractures in deep multilayer fracturing.

Mechanical characteristics and reservoir stimulation mechanisms of the Gulong shale oil reservoirs, the northern Songliao Basin

Shale oil of the Qingshankou Formation of the Gulong Sag, the northern Songliao Basin, represents the first attempt at large-scale development of pure-shale-type shale oil in China. By integrating the multi-scale refined reservoir characterization with macro-micro-scale mechanical testing, it is clarified that the Gulong shale is characterized by high clay mineral content, high rock plasticity, highly-developed bedding, and prominent mechanical anisotropy. A three-dimensional (3D) fracture propagation model of hydraulic fracturing was built for the Gulong shale, which fully captures the hydraulic fracture distribution pattern affected by the high bedding density, in-situ stress, and fracturing treatment parameters. Our research showed that due to influences of bedding, hydraulic fracturing in the Gulong shale forms a complex fracture morphology featuring the main fracture with multiple perpendicular branches that have different lengths (like the outdoor directional TV antenna); however, the vertical propagation of fractures is inhibited, and the fracture height is commonly less than 10 m. The limited stimulated reservoir volume (SRV) is the main problem facing the fracturing stimulation of the Gulong shale oil. Bedding density has vital effects on fracture morphology, so case-specific fracturing designs shall be developed for shale intervals with different bedding development degrees. For reservoirs with well-developed bedding, it is suggested to properly increase the perforation cluster spacing and raise the volume and proportions of viscous fluids of the pad, so as to effectively promote vertical fracture propagation and improve reservoir stimulation performance. This study integrates multi-scale fine reservoir characterization and macro-micro-scale mechanical testing, as well as the construction and numerical simulation of hydraulic fracturing models for high-density layered shale reservoirs, providing a new approach and methodological framework for the fracturing research of high-density layered shale reservoirs.

Case Study on Dynamic Identification of Overburden Fracture and Strong Mine Pressure Mechanism of Isolated Working Face Based on Microseismic Clustering

Strong mine pressure has a great impact upon the safety production of coal mines. Microseismic information provides a more advanced technical means for overburden fracture dynamic identification and mine pressure mechanism research, since it contains rich information on rock fracture sources. In this study, the isolated LW8102 working face in Tongxin Mine was investigated in order to propose a spatio-temporal microseismic event data analysis method based on the k-means clustering algorithm. This algorithm can handle dynamic identification of overburden fractures constrained by spatiotemporally discrete distributions of microseismic events. This provided the dynamic extension process and the fracture distribution pattern of the overburden: eight fracture extensions were formed in the overburden. In each extension, vertical fractures connected the low and high rock layers in the LW8102 and LW8103 goafs, and through fractures connected the LW8102 and LW8103 goafs in their high, middle, and low levels. Some extensions had fractures that were connected to form a closed loop structure. In the vertical fracture, there was a tendency for one or two layers of the stratum to fail first, and then extend to one or both sides. The process of through and vertical fracture propagation followed a certain temporal sequence, reflected primarily in two forms: firstly, as the vertical fracture extended to a certain layer, it provided the initial rupture space for through fracture spreading; secondly, the through fracture first broke, and then extended to the vertical fracture until it intersected with the vertical fracture or provided the initial rupture space for the expansion of the vertical fracture. By matching the overburden fracture to the mine pressure that responded to the support resistance, we analyzed the mechanism of mine pressure at the working face. Through fracture at the high level was found to be the primary cause of the occurrence of mining pressure. It was precisely placed that the formation of multiple adjacent high through fractures 110 m from the floor, triggering simultaneous instability motion of the lower multi-layer level rock; this was the main reason for the phenomenon of strong mine pressure at the working face. Meanwhile, high through fracture at 80 m from the floor was the main reason for the phenomenon of large mine pressure at the working face.

The influence of multi-metal-veins on fractures propagation investigated by the experiment and simulation

Fracture propagation is affected by multi-metal-veins formed by geological diagenesis in shale during the hydraulic fracturing. However, the influence of multi-metal-veins on fractures propagation remains unclear. To solve the problem, based on the semi-circle bending (SCB) test and the extended finite element (XFEM) theory, the interaction between multi-metal-veins and fractures is investigated. The experimental results reveal that the fractures usually deflect at the upper or lower interfaces between metal veins and rocks (e.g. the specimen S-2), which is different from the propagation behavior of fractures in calcite veins. Meanwhile, the fracture toughness of the specimen S-1 is 24.40% higher than that of the specimen S-2, indicating that the increasing of total thickness of multiple metal veins increases the resistance to the fracture vertical propagation. The simulation results show that the increasing of the number, total thickness of veins, the modulus difference between veins and rock, the approach angle and the notch angle all increase the resistance of the fracture passing through metal veins. The maximum deviation distance (Dmax) of the fracture decreases with the number of veins, while thickness combination types of metal veins do not affect Dmax. The reduction of the notch angle leads to the more tortuous fracture propagation path. Finally, we propose a new comprehensive fracture network pattern. Fracture networks are divided into two categories, including orthogonal fracture networks and sub-orthogonal fracture networks, and then divided into six sub-categories further. The research results will provide reference for hydraulic fracturing of shale reservoirs containing multi-metal-veins.

Vertical fracture propagation behavior upon supercritical carbon dioxide fracturing of multiple layers

There are abundant unconventional oil and gas resources in the sandstone–shale interbed of the Yanchang Formation in Ordos Basin. However, it is difficult to predict the growth of supercritical carbon dioxide (Sc-CO2) fracture height in sandstone–shale interbed reservoirs due to the developed interface and strong heterogeneity. To enhance the productivity of sandstone–shale interbed reservoirs, it is necessary to clarify the Sc-CO2 fracture evolution mechanism. In this study, a global embedded zero-thickness cohesive element method is proposed for numerical simulation of Sc-CO2 fracturing of sandstone–shale interbeds, and the effects of stress difference, interface strength, and perforation position on fracture vertical propagation pattern are investigated. The results lead us to three main conclusions. (1) The fracturing model is established using the global embedded cohesive element method combined with the nonlinear constitutive equation, which solves the problem that the cohesive element cannot effectively simulate the random propagation of Sc-CO2 fractures. (2) Under the combined influence of in situ stresses and natural weak surfaces, there are three typical fracture extension patterns: I-shaped, cross-shaped, and single-shaped. In addition, there are six interaction modes between the main fracture and the interlayer interface. Branch fractures and microfractures are easily formed when the main fracture penetrates the sandstone–shale interface. (3) The vertical stress difference is greater than 4 MPa, and the dimensionless comprehensive interface strength is between 0.3 and 0.4, so the simultaneous perforation in shale and sandstone is a favorable factor for the full expansion of fracture height. The results provide theoretical guidance for selecting the Sc-CO2 fracturing horizon of sandstone–shale interbed reservoirs.

Hydraulic Fracture Vertical Propagation Mechanism in Interlayered Brittle Shale Formations: An Experimental Investigation

Hydraulic Fracture Vertical Propagation Mechanism in Interlayered Brittle Shale Formations: An Experimental Investigation

Mechanism and Numerical Simulation of Vertical Fracture Propagation in Composite Coal Rock

Vertical fracture propagation mechanism is important to understand the effect of hydraulic fracturing on the roof of outburst coal seams. In this paper, the differences in physical parameters and in situ stress between an outburst coal seam and its roof strata were compared, and influencing factors of roof-fracturing fractures connecting coal seams were vertically analyzed. The impact of different fracturing strata and injection rates on fracture propagation was studied by numerical models. Results show that the horizontal principal stress of an outburst coal seam is less than that of roof strata, and the fracture length of roof fracturing is larger than that of an outburst coal seam. Roof-fracturing fracture of an outburst coal seam has the material conditions to communicate downward with the coal seam. The downward propagation height of roof-fracturing fracture is positively correlated with the minimum horizontal stress difference between an outburst coal seam and its roof strata. Soft coal fracturing cannot form a long fracture dominated by tensile failure, so the coal seam can be communicated by transforming the roof strata of soft coal to form vertical fractures. The injection rate affects the failure location, fracture width, and fracture-propagation path of the numerical model. Construction parameters should be reasonably designed in accordance with the physical parameters of coal and rock, construction displacement, and in situ stress. These research results can provide a theoretical basis and data support for the design, and optimization of construction parameters of roof fracturing in outburst coal seams.

Laboratory investigation on hydraulic fracture propagation in sandstone-mudstone-shale layers

During the stimulating unconventional reservoirs, the vertical propagation of hydraulic fractures is crucial for enlarging the stimulated reservoir volume, especially in multi-layers of sandstone, mudstone and shale (sand-mud-shale). To investigate the effects of lithological interface and fracturing fluid viscosity on the fracture propagation vertically in the multi-layers, hydraulic fracturing experiments in laboratory were performed on the outcrop samples of 30 cm × 30 cm × 30 cm collected from Yanchang Formation in Ordos Basin. The results show that hydraulic fractures are multi-branched and zig-zagged when they initiate in shale, simple when they commence in sandstone or mudstone. Hydraulic fractures created with low-viscosity fracturing fluid can only cross sandstone from mudstone, but those induced by high-viscosity fracturing fluid can cross the sand-mud-shale layers. Furthermore, the high-viscosity fracturing fluid reduces the fractures complexity in shale, facilitating vertical fracture propagation. The injection pressure fluctuates slightly as the hydraulic fracture extends from shale to sandstone or mudstone, otherwise it fluctuates significantly. From the laboratory investigation, a hydraulic fracturing scheme for Chang 7 Member was proposed, with its feasibility proved in field tests.

The study of hydraulic fracture height growth in coal measure shale strata with complex geologic characteristics

The coal measure shale strata (CMSS) is rich in coalbed methane and shale gas. However, different thicknesses and high heterogeneity between the coal and shale typically result in difficultly predicting hydraulic fracture height growth. The feasibility of multi-gas production and the optimal layer for fracturing in CMSS requires a better understanding of fracture vertical propagation mechanism. In this paper, a three-dimensional model of a hydraulic fracture crossing CMSS layer was built according to geological characteristics, based on the rock seepage-stress-damaged coupled finite element method. The influence of geological factors and construction factors on the hydraulic fracture expansion was investigated. The results showed that (1) The physical property and geological conditions presented a huge difference between different lithological rock with various depth; (2) The increase of displacement and fracturing fluid viscosity, the inhibition effect on vertical fracture extension became weak. The augment of displacement was more helpful to the vertical propagation of fracture; (3) Under low permeability difference and small horizontal minimal principal stress difference, fracture length propagated enough distance and fracture height extension gap connected more the production layer. Therefore, the favorable conditions for forming balanced fracture geometry were as follows: permeability difference from 0–3 and stress difference from 1–2 MPa. (4) The 11th layer was chosen as the best fracturing layer in Shanxi Formation. The fracture height was about 26.13 m and the thickness of pay layers accounted for 42.32% of the height. The research results provide theoretical guidance for the optimal fracturing layer and fracturing construction parameters in CMSS.

Lattice Numerical Simulations of Hydraulic Fracture Propagation and Their Geometry Evolution in Transversely Isotropic Formations

Accurate prediction of the fracture geometry before the operation of a hydraulic fracture (HF) job is important for the treatment design. Simplified planar fracture models, which may be applicable to predict the fracture geometry in homogeneous and continuous formations, fail in case of fractured reservoirs and laminated formations such as shales. To gain a better understanding of the fracture propagation mechanism in laminated formations and their vertical geometry to be specific, a series of numerical models were run using XSite, a lattice-based simulator. The results were studied to understand the impact of the mechanical properties of caprock and injection parameters on HF propagation. The tensile and shear stimulated areas were used to determine the ability of HF to propagate vertically and horizontally. The results indicated that larger caprock Young’s modulus increases the stimulated area (SA) in both vertical and horizontal directions, whereas it reduces the fracture aperture. Also, larger vertical stress anisotropy and tensile strength of caprock and natural interfaces inhibit the horizontal fracture propagation with an inconsiderable effect in vertical propagation, which collectively reduces the total SA. It was also observed that an increased fluid injection rate suppresses vertical fracture propagation with an insignificant effect on horizontal propagation. The dimensionless parameters defined in this study were used to characterize the transition of HF propagation behavior between horizontal and vertical HFs.

Numerical simulation study on fracture penetration behavior in a reservoir with multiple coal seams

Coal seams are widely deposited in marine-continent transitional facies shale reservoirs. For this type of reservoir, horizontal wells and hydraulic fracturing technology are the main ways to exploit the oil and gas resources. However, multiple layers with uneven thickness and significant differences in physical and mechanical properties among them lead to insufficient vertical fracture propagation, limiting the application of horizontal well and hydraulic fracturing technology. Therefore, to reveal the influence of in situ stress conditions and injection parameters on the vertical propagation of fractures, numerical models with multiple coal seams based on logging lithology were established and the behavior of fractures crossing coal seams under different conditions was studied based on the extended finite element method. In addition, the fracture width and pressure at the injection point were also studied. The simulation results showed that the uneven thickness of the coal seam limited the vertical penetration of the fractures compared with that of a homogeneous formation as the fractures tend to pass through the thinner coal seam. Due to the physical properties difference and stress shadow effect, multiple clusters of fractures were prone to deflection in the coal seam, which made it difficult for hydraulic fractures to grow in height. However, the stress shadow effect in the process of multiple fractures propagation also has a positive side. The stress shadow becomes stronger as the difference in fluid distribution increases, leading to a difference in the growth rate of the fracture up and down, and ultimately improving the overall penetration effect of the fractures. Based on the work presented in this paper, the perforation spacing can be optimized to either avoid the stress shadow effect or effectively utilize its positive influence. Meanwhile, the results can guide the placement of horizontal well and the selection of injection rate.

Key Technology of Volumetric Fracturing in Vertical Wells of Hugely Thick Shale Oil Reservoirs in the Fengcheng Formation of the Mahu Sag

Shale oil reservoirs in the Fengcheng Formation of the Mahu Sag are characterized by a large buried depth of sandbodies, huge thickness, overall oil-bearing properties, dense matrix, and enrichment in metal ions. As a result, fracturing is prone to be insufficient in vertical extension and unable to cover the reservoir, with low fracture complexity, high sanding risks, and an incompatibility of conventional guar gum fracturing fluid. Based on a detailed description of reservoir mechanical properties and an evaluation of the main controlling factors of fracture networks, a calculation model of fracability index was built to fully exploit the exploration potential of the shale oil in this area. In light of the vertical fracture propagation, the perforation clusters and cluster spacing were determined to develop a fine vertical layered approach. In accordance with the development of natural fractures, the optimized volumetric fracturing technology was upgraded, and the net pressure in fracture was increased by construction with high pumping rate. Multi-scale filling was realized with the combination of proppants of different sizes, and a gradual pumping rate increase was employed to ensure safe sand adding. Further, a series of temperature-resistant and salt-resistant polymer fracturing fluids were developed. As a result, the key technology was developed for volumetric fracturing in vertical wells of hugely thick shale oil reservoirs in the Fengcheng Formation of the Mahu Sag. This technology has been successfully applied in Well MY1 in the shale oil reservoirs of the Fengcheng Formation, with a high-yield oil flow of 50 m3 per day after stimulation treatment. This started a new and promising prospect of unconventional reservoir exploration in the Mahu Sag, which is of great significance in guiding the subsequent shale oil development in this area.

Propagation of Vertical Fractures through Planetary Ice Shells: The Role of Basal Fractures at the Ice–Ocean Interface and Proximal Cracks

Abstract The presence of smooth, young surfaces indicates that regions of Enceladus and Europa have been resurfaced through recent or ongoing activity related to the eruption of liquid water from subsurface reservoirs. For interior material to erupt or flow out onto the surfaces of these satellites, fractures would have to vertically penetrate the ice shell to the depth of a subsurface reservoir or ocean. Here we use linear elastic fracture mechanics to show that accounting for fracture interactions makes it much more difficult for fractures to penetrate the entire ice shell than previous estimates. We found that fractures that originate from the surface are unlikely to penetrate the entire shell thickness, even for the upper range of tectonic stresses estimated for each moon. Tensile fractures that initiate from the bottom of the icy shell—as observed in terrestrial ice shelves—propagate further into the icy shell than surface crevasses but still do not penetrate the entire ice thickness. However, full ice shell thickness fracture is possible if shear failure connects the surface with deep-penetrating basal fractures in thinner ice shell thicknesses and under certain stress conditions. This suggests that the combination of tensile and shear failure may be important and necessary for the formation of a connection from the surface to the ocean below.

Hydraulic fracture vertical propagation behavior in transversely isotropic layered shale formation with transition zone using XFEM-based CZM method

The transition zone was defined as an interface with certain thickness, strong heterogeneity and anisotropy in the vertical profile for layered formations, which had great effect on hydraulic fracture (HF) height propagation. In this paper, a numerical model for transversely isotropic layered shale with transition zone was established by utilizing the extended finite element method (XFEM) based on cohesive zone model (CZM). The effects of in-situ stress, dip angle, anisotropy and tensile strength of transition zone, and anisotropy of shale matrix, and injection rate on fracture vertical propagation behavior were investigated. Dimensionless fracture offset distance in transition zone (DFODT) and dimensionless fracture height (DFH) were defined to quantitatively evaluated the results of hydraulic fracture vertical propagation. Numerical results showed that the transition zone could induce the HF to turn and distort within and between layers, significantly improving the degree of fracture tortuosity. Also, the greater the DFODT, the smaller the DFH. The anisotropy of transition zone and coefficient of effective vertical stress difference had the greatest influence on DFODT; anisotropy of shale rock and injection rate had the greatest influence on DFH; while tensile strength of transition zone had a relatively weaker influence on both DFODT and DFH.

Theory and Application of Pseudo-Reservoir Hydraulic Stimulation for Coalbed Methane Indirect Extraction in Horizontal Well: Part 2—Application

This paper provides details of the application of pseudo-reservoir stimulation technique to a U-shaped coalbed methane (CBM) well (i.e., Well ZU-01) in Jiaozuo mining area. Multistage hydrojet fracturing and channel fracturing were adopted to promote fracture network forming and fracture vertical propagation in pseudo-reservoir. The results of the pseudo-reservoir stimulation indicate that influenced by the reservoir lithology and coal structure between the horizontal well and coal seam, the fractures at the first stimulation stage only propagated in the pseudo-reservoir. However, the stimulations at the second, third and fourth stages achieved integrated stimulation of pseudo-reservoir and coal seam. The gas production rate of Well ZU-01 was only 1000 m3/d in the stable production stage. However, through negative-pressure extraction in the mine roadway, the joint gas extraction rate from the underground roadway and Well ZU-01 reached 6000 m3/d. After 450 days of extraction, the cumulative gas production was 15.55 × 105 m3, and the CBM extraction rate reached approximately 50%, indicating that good pseudo-reservoir stimulation and gas pre-extraction effects were achieved. The results of the adsorption/desorption and wettability tests show that water blocking damage caused by the slickwater fracturing fluid severely hindered the gas desorption and production processes, which was the main reason for the low gas production of the well. However, the water blocking damage was mitigated through negative-pressure extraction, resulting in a significant increase in CBM production.

Unraveling the complex deformation pattern at Yellowstone plateau through seismicity and fracture analysis

The intense unrest of the Yellowstone volcanic plateau is manifested through diffuse seismic activity, earthquake swarms, and episodes of complex surficial deformation that have been related to magmatic fluid transfer in the upper crust over the past several decades. While past studies have focused on modeling contemporary geophysical data, there has not been a fully-integrated evaluation of seismicity, fault kinematics, and stress field. Here we analyze a catalog of 10,201 relocated earthquakes recorded between 2010 and 2016 and determine 224 well-constrained double-couple focal mechanisms. The majority of the focal mechanisms (65%) are associated with the 2010 Madison Plateau seismic swarm. The focal mechanisms are predominantly strike-slip with subordinate normal faulting mechanisms. Possible causes of this predominance and of the concurrence of both kinematics are here discussed, in order to unravel the influence of magmatic processes such as past sill intrusions. The earthquake catalog has been analyzed in terms of location, time, and kinematics according to the phases of surficial deformation documented by GPS data in order to identify systematic patterns of deformation and has been compared to the 1988–2009 seismicity. The continuous downwarping of the overburden from 2010 to 2016 was accomodated by structural adjustment of the shallow crust through strike-slip motions on a multitude of scattered small fault planes. Furthermore, the predominance of strike-slip faulting during seismic swarms occurs when the fluid overpressure induces horizontal propagation of vertical fractures with strike-slip motions, followed by horizontal fluid flow.

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.

Investigations into Mining-Induced Stress–Fracture–Seepage Field Coupling in a Complex Hydrogeology Environment: A Case Study in the Bulianta Colliery

The effect of mining-induced multi-physics coupling, characterized by stress–fracture–seepage field coupling (SFCC), is critical to safe mining and environmental protection. We investigated the spatiotemporal characteristics of the SFCC at the #31401 panel of the Bulianta Colliery. Changes in overburden movement, groundwater level, and groundwater inrush were monitored by means of borehole and working face observations, water leakage, and real-time video imaging. The results indicate that the distribution of mining-induced fractures was dominated by the key stratum, distributed as a “ladder shape” 30–75 m above the coal seam. The fracture penetrated into the lower key stratum and developed into the confined aquifer with a height of 140.5–154.0 m, while being truncated by the higher key stratum. Horizontal and vertical fractures developed sequentially in the confined aquifer-disturbed zone. Initiation, propagation, and interconnection of the seepage channel occurred in the fracture field, mediated by the redistributed stress. An abnormal discharge of 120–300 m3/h along the direction of mining commenced at the transition of the lower key stratum to the higher key stratum, and concentrated below the Bulian Gully. A fracture height model was developed based on the vertical fracture propagation, and mining parameters were modified, including reducing the mining height and increasing the advance rate and support resistance of the longwall mining face to effectively reduce the SFCC effect.