Outcrop-Scale Hydraulic Fracturing Experiments with a Coagulable Resin and Data Analysis Results

Response characteristics of coal subjected to hydraulic fracturing: An evaluation based on real-time monitoring of borehole strain and acoustic emission

A considerable amount of research has been carried out in the past few decades trying to understand the mechanics of hydraulic fracturing. Conventional modeling methods using a fracture mechanics suggest that a tensile crack is generated in hydraulic fracturing, and elongates in the major-principal-stress direction. Whereas, according to the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment, most AE classified as the shear type mechanisms. Thus, the hydraulic fracturing mechanism has not been sufficiently clarified. In this study, the flow-coupled DEM (distinct element method) simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The DEM can directly represent grain-scale microstructural features of rock without complicated constitutive laws. This suggests that the DEM model may be more appropriate for the analysis of rock fracturing than other numerical analysis techniques. The simulation results were in good agreement with the actual experimental results. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously, and the energy emitted from the tensile crack is small compared with that from the shear crack. Such a small AE is easily buried in a noise and hard to be measured in an experiment, the shear type AE with large energy is dominantly observed in AE measurement experiment. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. At this time, fluid pressure becomes very high. Highly pressurized fluid causes large fracture opening, and generates tensile cracks which emit large energy. Due to this reason, both tensile and shear types of AE are observed during the injection of high viscous fluid in an AE monitoring.

Advancements in the technique for early estimation of the hydraulic fracturing potential of fracturing fluid in different shale formations are needed to successfully stimulate reservoir volume and recover the trap hydrocarbons. The determination of fracture initiation and propagation in shale remains unclear, particularly with regard to the choice of type of fracturing fluid during fracturing operation at high confinement. Hydraulic fracturing with supercritical carbon dioxide (SC CO2) and SC CO2 foam are an encouraging technique to overcome significant use of water in shale gas production. Our study was carried out to explore the fracture propagation and fracture initiation pressure under high confinement stress. The hydraulic fracturing experiments are performed to study fracture propagation in the black shale of three different fields to generalize the scope of work using low, medium and high brittle shale: Eagle Ford, Wolfcamp and Mancos. Three different fracturing fluids were selected for fracturing tests: SC CO2, SC CO2 foam and slick water to investigate the impact of low, medium and high viscous fluids. We evaluated the fracture pressure curve, breakdown pressure (fracture initiation pressure), fracture network variation and the impact of bedding angle, perforation length, injection mediums and formation variations on fracture morphology. During hydraulic fracturing with slick water a consistent rise in the injection pressure is recorded whereas sudden fluctuations are recoded with the injection of SC CO2. With the injection of SC CO2 foam, a sudden decrease in injection pressure appears due to imbibition of SC CO2 foam in the pores matrix. Moreover, fracture initiation pressures in shale rocks varied by changing bedding angle and perforation length. Fracture initiation pressure increases with bedding angles. An appreciable difference in fracture initiation pressure with SC CO2, SC CO2 foam and slick water are 5023psi, 6456psi and 6168psi, respectively at high conferment pressure (3500psi). Comparison of hydraulic fracturing of Eagle Ford, Wolfcamp and Mancos shale with different injection medium shows that SC CO2 foam produced complex fracture networks with high aperture and length for parallel, inclined and perpendicular perforations along the bedding. With all types of shale, SC CO2 foam injection has produced dense fracture network. Thus, fracturing with SC CO2 foam can potentially enhance the stimulated reservoir volume.

Experiments and analysis on the influence of multiple closed cemented natural fractures on hydraulic fracture propagation in a tight sandstone reservoir

Experimental study on the supercritical CO2 fracturing of shale considering anisotropic effects

In this work we study the effect of fluid viscosity on hydraulic fracturing initiation and near-wellbore propagation on block samples of tight shales subjected to representative effective in-situ stress conditions. Combined analysis of acoustic emission, ultrasonic transmission and volumetric deformation indicates that the viscosity of the injected fluid had a strong influence on hydraulic fracturing initiation, fracture propagation and fracture geometry. Injection of high viscosity fluid into the stressed tight shale resulted in fracture initiation at a bore pressure higher than the overburden stress and occurred significantly earlier than the borehole breakdown pressure. After initiation, the hydraulic fracture propagated symmetrically from the borehole in the direction parallel to the maximal horizontal stress, causing significant volumetric deformation of the rock. In the case of injecting a low viscosity fluid into the stressed block, fracture initiation occurred at a borehole pressure significantly lower than it was required with the higher viscosity fluid, and occurred almost simultaneously with the bore pressure breakdown. AE measurements during hydraulic fracturing allowed us to estimate that the average speed of hydraulic fracture propagation was approximately thousand times faster for the low viscosity fluid than for the high viscosity fluid.

Deep shale gas reservoirs in the southern Sichuan Basin feature well-developed geological discontinuities, including bedding planes and natural fractures. The interaction between Hydraulic fractures (HFs) and these weak planes can cause complex pressure responses and microseismic event patterns during multi-fracture propagation. This study introduces a novel experimental method for multi-stage and multi-cluster hydraulic fracturing in a horizontal well. The simultaneous and sequential HF propagation are analyzed by integrating real-time injection pressure monitoring and acoustic emission (AE) event localization. Characteristic pressure and AE response patterns are identified for different HF propagation behaviors. Results show that multi-cluster HFs within a stage may not initiate simultaneously, with multiple pressure peaks and each associated with high-energy AE events. The initiation pressure and AE energy of previously initiated clusters are higher than those of subsequently initiated clusters. For multi-fracture sequential propagation, subsequent HFs may initiate at an angle to the wellbore due to the stress shadow from previous fractures, eventually coalescing with them. Moreover, under poor cementing quality, longitudinal HFs may initiate and the injection pressure decreases slightly after peak pressure, followed by a gradual increase with weak fluctuations. The pressure drops when the HF crosses a natural weak plane, with high-energy AE events generated at the intersection and low-energy events distributing along the weak plane. When the fracturing fluid seeps along natural discontinuities, the pressure curve fluctuates intensely and periodically, with low-energy AE events along the weak plane. The conclusions provide a guidance for evaluating fracture geometries through interpreting operation pressures and microseismic monitoring results.

ABSTRACT: The bedding interfaces with different cohesions usually exist in unconventional reservoirs. During reservoir stimulation, the slips induced by hydraulic fractures are related to the fracturing effects. In this paper, the rock specimens with cemented and uncemented bedding interfaces were selected to conduct direct shearing experiments, and the slipping behaviors were analyzed using the digital image correlation (DIC) method and acoustic emission (AE) technique. The results show that the slip weakening mode exists in cemented bedding interface during slipping, and the corresponding variation of AE b-value is small. The entire uniform slippage will appear in the uncemented bedding interface during slipping, accompanied by the larger variation of AE b-value. The effects of the interlayer cohesion on the slip weakening zone length, AE magnitude, and the difference between static and kinetic friction were discussed. It is found that the opening feature is not significant during slip weakening, but obvious during cohesionless slipping. And a relationship model was established to describe the opening displacement affected by slipping. This study can help to understand the slip mechanisms during fracturing in unconventional reservoirs. 1. INTRODUCTION Unconventional reservoirs, such as shale, have been the key areas in oil and gas production (Gao et al., 2022). Unlike conventional reservoir rocks, unconventional reservoirs are usually characterized by bedding development. Therefore, investigating the slip of bedding plane in unconventional reservoirs during fracturing is one of the focuses in rock fracture mechanics (Zhao & Gray, 2021). Microseismic monitoring results of in-situ fracturing demonstrate that hydraulic fractures can induce interlayer slip (Rutledge et al., 2016; Tan & Engelder, 2016a). The enhancement of hydraulic conductivity by slip (Zhao et al., 2013; Ye & Ghassemi, 2018) is a crucial factor for promoting shale oil and gas production (Urbancic & Baig, 2016; Wang et al., 2019; Zoback et al., 2012). In field hydraulic fracturing, the slip fractures are usually monitored by microseismic moment tensor analysis (Tan & Engelder, 2016b) and micro-resistivity scanning imaging technology (Zoback & Lund, 2019). Laboratory experiments have also demonstrated that hydraulic fractures can induce interlayer slip at the bedding plane (Chen et al., 2023). Some researchers think that the shear slips of interlaminations during fracturing are part of complex fracture networks (Bunger et al., 2013; Huang et al., 2013; Min et al., 2010; Ye & Ghassemi, 2018).

Summary Hydraulic fracture has been a well-known technique for unconventional oil-gas development since the last decades. And laboratory hydraulic fracture experiments have been extensively performed to study the mechanism of the hydraulic fracture. In order to improve our understanding of the hydro-fracture’s development and fluid migration during hydraulic fracturing, we performed a laboratory experiment on a tight sandstone cored from Triassic Yanchang Formation in Ordos Basin, Northwest China. The velocity images and acoustic emission are monitored during the experiment. By comparing them with the stress variations, strain variations and the μCT images of the fractured sample, we analyze the dynamic fracture process and the fluid migration. The experiments show us that the velocity keeps constant before hydraulic fracture initiation and fluid would flow to the fractures after rock fractured. Density maps of AE hypocentre show that the hydraulic fracture surfaces indicated by AE activity have asymmetrical wing shape, growing from the borehole bottom.

Hydraulic fracturing using cyclic pumping is a potential method for effective stimulation of unconventional oil and gas reservoirs. In our study, the laboratory-scale hydraulic fracturing experiments using cyclic pumping were conducted using cuboid concrete specimens with multiple perforation clusters. The results showed that by cyclic pumping more fractures were created from perforation clusters when compared to non-cyclic monotonic pumping. This trend was shown to increase when using high-viscosity fracturing fluids. In our cyclic pumping experiments, the maximum pressure in the first pumping cycle reached the breakdown pressure and at each subsequent cycle the maximum pressure increased. The increased maximum pressure likely acted to generate more fractures initiating from the given perforation clusters even under greater fracture height to spacing ratios and more acoustic emission (AE) events were generated by cyclic pumping compared with those by monotonic pumping. Well compliance was examined and showed a negligible change in the system compressibility among cycles. This suggests that the fractures formed by previous injection cycles were isolated from the wellbore at the start of this cycle and at least their near-well portions are closed. There are a few possibilities contributing to the difficulty in reopening the existing hydraulic fractures in subsequent injection cycles. Detailed discussion is provided based on the varying features of injection pressure. This cyclic injection method could deliver an alternative for multi-stage fracturing treatments in field applications.

Field experiences suggest significant interaction between natural fractures and hydraulic fractures in unconventional reservoir and some tight gas sandstone. Microseismic monitoring is a widely used methodology to understand the real-time evolution of fracture geometry during hydraulic fracturing, which is essential for evaluating the SRV under certain treatment, especially in unconventional reservoir. Howerver, the correlation between the real fracture geometry and microseismic monitoring results is still open to discuss. Therefore, understanding the relationships between the induced fractures and acoustic emission in different laboratory conditions is of high importance for studying the mechanism of hydraulic fracturing and ultimately optimizing fracture complexity. A large polyaxial testing system was built for investigating the factors of inducing complex fractures in sandstones, shale and coal for hydraulic fracturing treatment. This paper presents the experimental results of a study conducted to understand the mechanism of fracture initiation and propagation based on acoustic emission monitoring. Large-scale test was conducted in dry sandstone outcrop of 762×762×914mm to simulate the hydraulic fracturing. True triaxial stresses were applied to the specimen. The acoustic emission events associated with fracture propagation or failure process during injection were monitored to provide further data to characterize the fracture. The hydraulic fractures and oil penetration surface were observed after injection. Results show that the acoustic emission monitoring is in good accordance with the split block surface dyed red. AE localizations clearly demonstrate fracture initiation and propagation dynamics in the sample which can be used to control the fracture. The AE rates increased with opening hydraulically induced fracture and its closure. The real-time fracture geometry can be calculated by integrating the borehole pressure, AE localizations, injection rate, fluid leakage and rock mechanical properties. We are currently investigating various ways for building the fracture propagation model. The work presented here a new hydraulic fracturing testing system integrated with geophysical monitoring and processing technologies that will improve laboratory study of hydraulic fracturing mechanism and provide a trial on new ideas and technology

All Shale Gas reservoirs are not the same. There are no typical Tight Gas reservoirs. These two statements can be found numerous times in the literature on shale gas and tight gas reservoirs. The one common aspect of developing these unconventional resources is that wells in both must be ‘hydraulically fractured’ in order to produce commercial amounts of gas. Operator challenges and objectives to be accomplished during each phase of the Asset Life Cycle (Exploration, Appraisal, Development, Production, and Rejuvenation) of both shale gas and tight gas are similar. Drilling, well design, completion methods and hydraulic fracturing are somewhat similar; but formation evaluation, reservoir analysis, and some of the production techniques are quite different. Much of the experience in shale and tight gas has been developed in the US and in Canada, to a lesser extent; and most of the technologies that have been developed by operators and service companies are transferable to other parts of the world. However, the infrastructure, including equipment and service company availability, governmental regulations, logistics, processing, environmental considerations, and pricing are not the same as in the US. This may impact the rate of the technology transfer as well as the selection of some of the technology. This paper is focused on operations challenges, technologies, and experience associated with shale and tight gas projects. It is likely that environmental concerns and the drive to reduce development costs of tight and shale gas reservoirs will drive new approaches to the development of these reservoirs in China, Latin America, Middle East, North Africa, and other parts of the world.

We evaluate the radiated seismic energy normalized by external work for hydraulic fracturing, beam bending, and uniaxial compression experiments conducted on Opalinus clayshale and Barre granite specimens. Results suggest that normalized radiated seismic energy is highest for the beam bending, followed by uniaxial compression, and, finally, that the hydraulic fracturing experiments radiate the least seismic energy when normalized by external work. We also find that the normalized radiated energy during tests on Opalinus clayshale is 3% to 22% of that in Barre granite across multiple loading mechanisms.

Researchers have recently realized that the natural fractures in shale reservoirs are often cemented or sealed with various minerals. However, the influence of cement characteristics of natural fracture on fracturing network propagation is still not well understood. In this work, laboratory-scaled experiments are proposed to prepare model blocks with discrete orthogonal fractures network with different strength of natural fracture, in order to reveal the influence of cemented natural fractures on the interactions between hydraulic fractures and natural fractures. A series of true triaxial hydraulic fracturing experiments were conducted to investigate the mechanism of hydraulic fracture initiation and propagation in model blocks with natural fractures of different cement strength. The results present different responses of interactions between hydraulic and natural fractures, which can be reflected on the pump pressure profiles and block failure morphology. For model blocks with fluctuated pump pressure curves, the communication degree of hydraulic and natural fractures is good, which is confirmed by a proposed new index of “P-SRV.” The most significant finding is that too high and too low strength properties of cemented natural fracture are adverse to generate complex fracturing network. This work can help us better understand how cemented natural fractures affect the fracturing network propagation subsurface and give us reference to develop more accurate hydraulic fracturing models.

Numerical study of interaction between a hydraulic fracture and a weak plane using the bonded-particle model based on moment tensors