•SC-CO2 fracturing experiments with shale were conducted for the first time.•AE tests, DR and CT scans were used to observe fracture morphology.•The fracture initiation and propagation of SC-CO2 ...fracturing was compared with hydraulic fracturing.•Multiple factors influencing propagation were studied.
Supercritical carbon dioxide (SC-CO2) fracturing is a promising technology for developing shale gas because it can effectively solve problems related to shale swelling and lack of water resources. This work conducted simulation experiments on SC-CO2 fracturing in shale for the first time. Compared with hydraulic fracturing, using SC-CO2 as the fracturing fluid reduces the pressure needed to initiate fractures by more than 50%. This reduction is due to the increased percolation and pore pressure effects of using SC-CO2. Acoustic emission tests were used to monitor the progress of fracturing in shale and high-energy CT scanning documented fracture morphology. CT scanning shows that SC-CO2-induced fractures are irregular multiple cracks. These numerous crooked cracks are more likely to induce secondary fractures in shale and to connect with natural fracture and bedding to form complex fracture networks than those formed by hydraulic fracturing. The volume of rock fractured by SC-CO2 is several times that fractured by hydraulic fracturing and the surfaces of the fractures opened by SC-CO2 are more complex and rugged. SC-CO2 fracturing can achieve better fracture networks for reservoir stimulation in shale than in sandstone, and the degree of bedding development has a great influence on the complexity of the SC-CO2 induced fractures. Namely, using SC-CO2 as fracturing fluid can increase fracture conductivity and hence achieve increased shale gas production. This study also determined how fractures propagate under different horizontal stress regimes.
Carbon dioxide (CO2) is an alternative working fluid to water for hydraulic fracturing in shale reservoirs. It offers advantages as a substitute for the use of large quantities of potable water and ...for the concurrent sequestration of CO2, however sorption and swelling effects, and their impact on permeability may be detrimental and are undefined. Hence, it is of great importance to understand the mechanism of supercritical carbon dioxide fracturing in shale and its effect on shale permeability enhancement. We conduct hydraulic fracturing experiments on shale samples using both water (H2O) and supercritical carbon dioxide (Sc-CO2) as fracturing fluids to explore the surface characteristics and permeability evolution of fluid-driven fractures. We use profilometry to measure the roughness and complexity of the resulting fracture surfaces and measure the permeability of the fractures. Results indicate that: (1) Sc-CO2 fracturing creates fractures with larger tortuosity relative to H2O fracturing (macroscale); (2) the topography of Sc-CO2 fracture surfaces is more rough and complex compared to that of H2O fractured surfaces; (3) larger mineral grains are removed and relocated from induced fracture surfaces by Sc-CO2 fracturing – these acting as micro proppants that result in a larger fracture aperture; (4) correspondingly, the permeability of shale fractures increases by ∼5 orders of magnitude with Sc-CO2 fracturing and this enhancement is ∼3 orders of magnitude higher than that by traditional hydraulic fracturing. This observation potentially validates the feasibility of Sc-CO2 as a fracturing fluid for the stimulation of shale reservoirs.
•Sc-CO2 fracturing creates fractures with larger tortuosity.•Sc-CO2 fracturing creates fracture surfaces with a larger roughness and complexity.•Sc-CO2 fracturing increases shale permeability by five orders of magnitude.•Mineral grains are peeled from fracture surface by Sc-CO2 fracturing.
The Lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. A comprehensive understanding of the shale geomechanical and petrophysical properties is crucial for ...the successful exploration and extraction of shale gas. We select four representative locations to acquire Longmaxi formation shale samples for the laboratory experiments, to investigate the geomechanical and petrophysical properties through a series of X-ray diffraction (XRD), scanning electron microscope (SEM), uniaxial compression, triaxial compression, tensile strength, and fracture toughness measurements. Laboratory results indicate that: (1) The quartz is the dominant mineral, and phyllosilicate mineral contents vary largelly from 7.30% to 47.80% in Longmaxi shale, which enables a higher brittleness index and fracbility. SEM results show that the high gas storage potential and well micro-fractures development of Longmaxi shale rocks. (2) The phyllosilicate content is vital in determining the uniaxial compressive strength, triaxial strength and elastic properties due to its weaker mechanical properties than tectosilicate minerals; (3) Fracture toughness of Longmaxi shale are relatively higher than shale formations in the USA, which indicate a higher potential to form fracture networks during hydraulic fracturing operations. (4) The anisotropy affects Longmaxi shale mechanical properities extensively due to the high-density bedding planes, which may further influence the fracture network formation during hydraulic fracturing operations. Our results revealed significant non-linear mechanical response as a consequence of shale fabric and mineralogy, which provides necessary information for the in-situ hydraulic fracturing and wellbore stability application during shale gas development in Longmaxi shale formation.
•Nanopores and high thermal maturities of Longmaxi shale indicate a high gas storage potential.•The micro-fractures are beneficial for hydraulic fracturing stimulations to enhance reservoir permeability.•The geomechanics properties of Longmaxi shale vary largely due to the differences in mineralogical compositions.•The anisotropy cannot be neglected in analyzing Longmaxi shale geomechanical and petrophysical properties.
A method of hydraulic grid slotting and hydraulic fracturing was proposed to enhance the permeability of low permeability coal seam in China. Micro-structural development and strength characteristics ...of coal were analysed to set up the failure criterion of coal containing water and gas, which could describe the destruction rule of coal containing gas under the hydraulic measures more accurately. Based on the theory of transient flow and fluid grid, the numerical calculation model of turbulence formed by high pressure oscillating water jet was used. With the high speed photography test, dynamic evolution and pulsation characteristics of water jet water analysed which laid a foundation for mechanism analysis of rock damage under water jet. Wave equation of oscillating water jet slotting was established and the mechanism of coal damage by the impact stress wave under oscillation jet was revealed. These provide a new method to study the mechanism of porosity and crack damage under high pressure jet. Fracture criterion by jet slotting was established and mechanism of crack development controlled by crack zone between slots was found. The fractures were induced to extend along pre-set direction, instead of being controlled by original stress field. The model of gas migration through coal seams after the hydraulic measures for grid slotting and fracking was established. The key technology and equipment for grid slotting and fracking with high-pressure oscillating jet were developed and applied to coal mines in Chongqing and Henan in China. The results show that the gas permeability of coal seam is enhanced by three orders of magnitude, efficiency of roadway excavation and mining is improved by more than 57% and the cost of gas control is reduced by 50%.
A dense “boss” in the hole bottom by the conventional rotating mechanical drilling in the hard rock requires a more efficient drilling technique with great stress concentration, easy heat dissipation ...and slow bit wear. A new kind of hard rock mechanical drilling technique with the abrasive water jet assistance is developed for this issue, and the abrasive water jet is introduced to erode the “boss” for a “pilot hole” because of the lower rock tensile and shear strength compared with compressive strength. The exposed hole wall of the “pilot hole” provides a large area of free surface which can guide the cracks to the subsequent rock breaking, and promote the cutting edges to overcome the rock tensile strength to complete the tensile and shear failure of the rock around the “pilot hole”, which can reduce the drilling force and achieve the efficient drilling with low bit wear in the hard rock. By designing and manufacturing a hard rock breaking bit and a set of hard rock drilling equipment system with abrasive water jet assistance, the experiments are conducted and compared with the conventional technique. The results show that with the assistance of abrasive water jet, the drilling depth has increased by about 63%, the thrust force and torque have reduced by about 15% and 20% respectively, and the bit wear has been reduced significantly.
► Develops a new kind of hard rock mechanical drilling technique with abrasive water jet assistance. ► Designs and manufactures a hard rock breaking bit and a set of hard rock drilling equipment system. ► Conducts experiments and compares it with the conventional technique. ► Increasing extent of drilling depth and decreased degree of thrust force and torque are provided.
•A new numerical model which can simulate the real flow field characteristics of supercritical CO2 impacting jet was proposed.•Complex coupling was achieved with the modified SIMPLE segmentation ...algorithm.•The reliability of the numerical model was validated in experiment.•The real flow field characteristics of supercritical CO2 impacting jet were described.•The jet flow field can be divided into five areas according to flow characteristics.
A numerical model was developed to study the flow field characteristics of supercritical CO2 (SC-CO2) impacting jet at well bottom. The model took account of jet hydrodynamics, mass transfer and thermodynamic properties of SC-CO2. Formulas of the density, viscosity and thermal conductivity of CO2 were verified for a supercritical state, and they were embedded in a CFD model through the user defined function (UDF). Complex coupling was achieved with the modified SIMPLE segmentation algorithm, and fields of the SC-CO2 impact flow, pressure, temperature and physical parameters were obtained. The reliability of the numerical model was validated in experiments that monitored the pressure and temperature of SC-CO2 impacting jet using sensors. The laboratory experiments show that the model accurately predicts the temperature and pressure fields of SC-CO2 impacting jet. SC-CO2 impacting jet has a more obvious thermal effect on the wall without any phenomena of CO2 freezing and blocking nozzle. The jet flow field can be divided into five areas according to flow characteristics, and the CO2 phase state in flow field is analyzed. The established calculation method and findings reported in the paper will provide guidance for engineering applications.
To investigate the attitude-switching mechanisms of existing jet slotters, which integrate drilling, punching and slotting operations, and to improve its fracture ability, we used the power bond ...diagram theory to analyse the dynamic flow pressure, and force of slotters. A mathematical model was developed for the dynamic characteristics of slotter systems. Furthermore, to study the effect of the main characteristic parameters on the ability of the nozzle to erode sandstone, multi-orthogonal experiments were carried out. And the optimised slots were applied in later practical operations. The research results show that the inlet fluid passed through the time-varying orifice to generate pressure differential thrust, which overcame the spring force, pushed the valve core to open the side nozzle, and closed the rear cavity channel thereby realising the switch of the slotter attitude. An optimal plan was established to balance the diameter, depth, and volume of punching, and a rock-breaking plan was developed for the slotter. Subsequently, the optimised water jet slotter was practically used in coal seam gas drainage. Compared with conventional dense drilling, water jet slotting technology significantly improves the ability, efficiency, and effect of increasing the permeability of the coal seam.
Based on the theory of hydraulic self-reversing and pressurization, a variable cross-section squeeze pulse water jet rock breaking platform was built. When the nozzle diameter is 0.5 mm, the granite ...erosion and crushing experiments under different driving pressures are carried out, and the depth, volume and specific energy consumption of the erosion pit are analyzed. The results show that the device can achieve intermittent injection after pressurizing the input fluid at a rate of 5.8 times. Meanwhile, the pressurization process of fluid is divided into ascending section, stable section and descending section. Compared with the continuous water jet with the same peak output pressure, the depth of destroyed granite is increased by 129 %–288 %, the volume of broken granite is increased by 37 %–121 %, and the specific energy consumption per unit of crushing volume is reduced by 21 %–74 %. The device can use low-pressure fluid to effectively crush granite.
Because the mechanisms that cause hydraulic fracturing to increase coal seam permeability are not clear, the effective application of hydraulic fracturing has limitations. This paper presents ...hydraulic fracturing and gas seepage experiments using raw coal under different stress states. The results showed that fracture propagation in coal was influenced by the properties of natural cleats and their ability to induce complex fractures. Using complex fracture abstraction and a connectivity coefficient
β
, a permeability model of coal subject to hydraulic fracturing with a stimulated reservoir volume (SRV) was established. The SRV effect was optimal when the pressure coefficient (
R
n
) was 1.46. The value of the Biot coefficient
α
remained unchanged at approximately 0.854 as the net pressure coefficient was changed. As
R
n
was increased, the stress sensitive coefficient
C
f
decreased and the increase in permeability caused by SRV was higher. The permeability of coal subjected to SRV was about two to three orders of magnitude higher than before fracturing. Conventional fracturing only increased the permeability to twice that before fracturing. Therefore, the influence and benefits of SRV should be considered when predicting coal-bed methane production from the permeability of coal beds.
The exploitation of deep resources and energy needs to break hard rock. Aiming at the problem of deep hard rock fragmentation, this paper proposes a variable cross-section squeezing pulsed water jet ...technology (SPWJ). SPWJ was generated under pump pressures of 5.2, 6.8, 8.5, 10, 11.9, and 13.8 MPa to carry out erosion experiments. Features such as rock spalling area, erosion depth, volume loss, and decomposition per unit inlet pressure are used to characterize the erosion performance of SPWJ. The results show that SPWJ can effectively crush granite under low input pressure. Granite crushing modes caused by SPWJ are mainly divided into three types: I: drilling type, II: erosion type, and III: cracking type. Compared with continuous water jet (CWJ), SPWJ has better overall erosion ability than CWJ when the erosion pressure is higher than 60 MPa, the dimensionless target distance is greater than 200, and the erosion time is less than 90 s. In addition, the erosion ability of SPWJ is better than that of CWJ under the condition of unit input pressure. The research results provide a reference for further optimizing the performance of SPWJ crushed granite in the future.
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