High Energy Gas Fracturing Research Articles

Numerical study of the effects of loading parameters on high-energy gas fracture propagation in layered rocks with peridynamics

The objective of this study is to investigate the influence of loading parameters on the propagation pattern of high-energy gas fractures in layered rock formations. To this end, a peridynamic model for brittle rock accounting for material heterogeneity was proposed. The ability of the model to simulate dynamic fractures was validated through laboratory experiments, and the homogeneity coefficient for the critical elongation rate was calibrated. On this basis, a numerical model of high-energy gas fracturing in layered rocks containing interfaces was constructed. Simulations were conducted to analyse high-energy gas fracturing from cylindrical intact boreholes and perforated boreholes under varying loading parameters. The results indicate that as the loading rate increases, the number of radial fractures surrounding the borehole gradually increases, whereas the influence of in-situ stress on fracture propagation diminishes. When the loading rate is fixed, both an increase in the peak pressure and a decrease in the decay rate are conducive to enhancing the propagation length of fractures. The propagation speed of fractures significantly decreases when they reach an interface but recovers after they penetrate it. Fractures tend to penetrate an interface when the angle of approach is closer to a right angle, and the direction of fracture propagation can be controlled through a perforation design. These findings provide valuable insights into the selection and optimization of loading parameters for reservoir stimulation via high-energy gas fracturing.

Research and Application of High-Energy Gas Fracturing Mechanism Based on CT Scanning Technology

Research and Application of High-Energy Gas Fracturing Mechanism Based on CT Scanning Technology

Calculation of fracture number and length formed by methane deflagration fracturing technology

The innovative Methane in-situ deflagration fracturing technology offers several benefits. It can prevent formation pollution and eliminates the need for complex treatment procedures for backflow materials. Additionally, it reduces ground transportation expenses and is poised to become a highly effective and eco-friendly waterless fracturing technology. This paper suggests methods for determining the number and length of fractures in oil and gas well stimulation, considering the main influencing factors. The fracture number is calculated using mass conservation principles, while the fracture length is determined using the crack propagation model. The fracturing experiments with large-sized samples were conducted to verify the technical feasibility of the new fracturing technology and fill the gap in large-scale physical model experimental research for deflagration fracturing technology. Following the experiment, compare experimental data with calculated data, and analyze the factors that affect the fracture number and length. The results indicate that both methods are highly feasible and possess strong predictive accuracy. It is advised that to enhance the fracturing effect on formations with high crustal stress, it is beneficial to appropriately increase the peak pressure, reduce the pressure rate, and increase the fracturing time while ensuring that the wellbore remains undamaged. Moreover, these proposed methods are versatile and adaptable, making them suitable for explosive fracturing within hydraulically fractured formations and high-energy gas fracturing technology.

Experimental Study on High-Energy Gas Fracturing Artificial Coal

The low permeability of coal seams has always been the main bottleneck restricting coalbed gas drainage. To improve the permeability of a coal seam, a high-energy gas fracturing technology is proposed. Firstly, based on the high-energy gas fracturing mechanism and gas production principle of fracturing agent, a fracturing agent applicable to coal reservoirs was developed, and its performance and sensitivity were tested. Then, a high-energy gas-fracturing simulated coal sample test was conducted, and the drilling wall pressure and strain of the simulated coal sample were tested. The results show that high-energy gas fracturing technology is a safe and efficient technical means for improving the permeability of coal reservoirs. The pressure–time curve of the borehole wall under the action of high-energy gas can be divided into three stages, namely, the rapid pressure-rising stage, steady pressure stage, and falling stage; the maximum pressure in the borehole is about several hundred MPa, and the pressure distribution in the borehole is not uniform. Compared with explosives blasting, the stress wave intensity in coal caused by the action of high-energy gases is low, the duration is short, and the peak stress attenuates slowly with increasing distance. Under the action of high-energy gas, no crush zone is generated near the borehole; the number of radial cracks produced is small but long. The extent of the fracture zone depends mainly on the quasi-static splitting wedge effect of the high-energy gas.

Research and method of air burst fracturing

At present, shale gas pressure fracturing volume development technology tends to be mature, shale gas backlog fracturing ends and enters the trial production stage. Due to the improper control of oil nozzle or its causes, the output drops sharply, or the drainage in the production process makes the funnel effect near the wellbore zone, resulting in the closure of the main fracture passage near the wellbore. However, the far-end complex sealing network structure produced by volume fracturing technology is still not fully closed, or secondary fractures occur in the absence of support for the original wellbore fractures or new complex fractures occur in the vicinity of the wellbore. The simple use of high-energy gas fracturing requires low cost, safety and simple and efficient characteristics, and the requirements for surface equipment are not too high. Existing methods mainly include well testing, sand washing by coiled tubing, gas lifting by coiled tubing, wellbore plugging removal, pumping and drainage by supplementary pressure well operator, etc. They can not fundamentally solve the need of closing or reopening new fractures in wellbore, and can not make new complex unclosed fractures near wellbore zone (without secondary fracturing). If the investment of surface equipment for secondary fracturing is similar to that of the first volume fracturing, and the investment is similar, the more complex situation will arise because of filtration and other reasons. At present, low concentration methane in air and wellbore is used to oxidize and exothermic to produce CO2 and H2O, and release energy at the same time. Temperature and pressure are the main means of energy in limited space. High energy gas formed after volume work can help to reopen the original fracture and form a new type of high energy gas fracturing method by means of air ignition and explosion, because the effect of high energy gas fracturing is not affected by stress, new cracks can be produced, and this crack can avoid overlapping with previous cracks. In particular, it can be pointed out that due to the improvement of thermal recovery of heavy oil and other means, the ground operation equipment has the commonality, and reasonable optimization of the construction plan can achieve the established purpose.

Study on the Effect of High-Temperature Heat Treatment on the Microscopic Pore Structure and Mechanical Properties of Tight Sandstone

The investigation of changes in physical properties, mechanical properties, and microscopic pore structure characteristics of tight sandstone after high-temperature heat treatment provides a theoretical basis for plugging removal and stimulation techniques, such as high energy gas fracturing and explosive fracturing. In this study, core samples, taken from tight sandstone reservoirs of the Yanchang Formation in the Ordos Basin, were first heated to different temperatures (25-800°C) and then cooled separately by two distinct cooling methods—synthetic formation water cooling and natural cooling. The variations of wave velocity, permeability, tensile strength, uniaxial compressive strength, and microscopic pore structure of the core samples were analyzed. Experimental results demonstrate that, with the rise of heat treatment temperature, the wave velocity and tensile strength of tight sandstone decrease nonlinearly, yet its permeability increases nonlinearly. The tight sandstone’s peak strength and elastic modulus exhibit a trend of the first climbing and then declining sharply with increasing temperature. After being treated by heat at different temperatures, the number of small pores varies little, but the number of large pores increases obviously. Compared to natural cooling, the values of physical and mechanical properties of core samples treated by synthetic formation water cooling are apparently smaller, whereas the size and number of pores are greater. It can be explained that water cooling brings about a dramatic reduction of tight sandstone’s surface temperature, generating additional thermal stress and intensifying internal damage to the core. For different cooling methods, the higher the core temperature before cooling, the greater the thermal stress and the degree of damage caused during the cooling process. By taking into consideration of changes in physical properties, mechanical properties, and microscopic pore structure characteristics, the threshold temperature of tight sandstone is estimated in the range of 400-600°C.

Simulation on the dynamic variation of downhole gas pressure during deflagration fracturing in screen completed well

Deflagration fracturing technique also named high energy gas fracturing (HEGF) technique was applied for creating several radial fractures nearby the wellbore to improve the conductivity. Because of the good applicability for different reservoirs, deflagration fracturing technique was also tried in a screen completed well. However, the theoretical literatures about deflagration fracturing were mainly focused on the open-hole completed or perforated well. A simulation on the dynamic variation of gas pressure in screen completed well was required to avoid destroying the tool because of the structural difference between the screen pipe and casing tube. A model describing the deflagration fracturing process in screen completed well was built. Except for the traditional equations for a perforated well, an equation for the propellant burning, that for the displacement of pressurized liquid column with downhole pipe string operating, and that for the fluid flow through the screen pipe was introduced. Then, a numerical simulation was carried out to validate. Compared with the real production-increasing ratio, the simulated accuracy approached 90%. The calculated maximum pressure was 50.3 MPa when a simulation was done with the given parameters, in accordance with the safety command of screen completed well. The maximum gas pressure had decreased with the increase of hole density and size of the screen pipe. A suggestion for operating pipe during deflagration fracturing was given. Operation with a downhole pipe string of large size and large wall thickness was beneficial for the safety during fracturing process.

A semi-analytical model for the transient pressure behaviors of a multiple fractured well in a coal seam gas reservoir

Multiple radial fractures can be generated in unconventional gas reservoirs after stimulation treatments such as hydraulic refracturing, high-energy gas fracturing, and explosive fracturing. This study presents a semi-analytical model to simulate the bottom-hole pressure (BHP) performances of a fractured well with multiple radial hydraulic fractures (MRHF) in a stress-sensitive coal seam gas reservoir. The adsorption–desorption and diffusion for fluid in reservoirs are considered. The continuous line-source function associated with discretization method in a composite coalbed gas reservoir is applied in Laplace domain. By means of Stehfest numerical inversion algorithm and Gauss elimination, the transient pressure responses in the well bottom are obtained and discussed.The results demonstrate that, from type curves, the main flow regimes for the proposed model in the coalbed gas reservoir are bilinear flow between adjacent radial hydraulic fractures, linear flow between adjacent radial hydraulic fractures, pseudo radial flow in stimulated region, and radial flow in un-stimulated region. Moreover, an obvious “hump” appears between the bilinear flow regime and the linear flow regime in stimulated region, which represents the interference between adjacent radial hydraulic fractures. The impacts of the ratio of permeability in stimulated region to that in un-stimulated region, the radius of stimulated reservoir volume (SRV), the permeability modulus, and the properties of hydraulic fractures on the transient pressure responses in well bottom are analyzed. The findings obtained in this study can pave the way for the quantitative understanding of the transient performances of multiple fractured vertical wells in unconventional reservoirs.

The transient intrusion effect of high energy fluid on the rock failure around the wellbore during the dynamic stimulation process

Dynamic wellbore stimulation technologies, such as high-energy gas fracturing, electrical pulse fracturing, hydraulic pulsation fracturing, are generally designed to multi-fracture the near-wellbore region through generating high energy pulse with intermediate or high loading rate. During the stimulating process, the dynamic strength and failure mode of saturated rocks around the wellbore can be largely affected by the transient intrusion of high energy fluid. Accordingly, an experimental method was developed to provide a method to apply the tailored pulse loading in the central borehole of the cylindrical rock samples. Three groups (Saturated + Center hole exposed (S-CHE), Saturated + Center hole wall isolated (S-CHI), Dry + Center hole wall isolated (D-CHI)) tests were carried out. The borehole fluid pressure-time curves and the failure modes of rock samples were tracked, analyzed and compared for all of the experiments. Experimental results demonstrate that: The relationship between rock dynamic strength and loading rate is monotonic direct proportionality. The dynamic strength of saturated rock shows a much stronger strain rate dependence than that of the dry rock. At the same loading rate, fluid penetration can largely weaken the dynamic strength of rock samples. The stress pulse generates a complex crushed zone around the wellbore, while the fluid penetration tends to produce radial multi-fractures. These phenomenons were explained using the micromechanics-based macroscopic damage theory, which is capable of obtaining the relationship between the fluid intrusion and the fracture deformation in the rock dynamic failure process.

A Coupled Gas Flow-Mechanical Damage Model and Its Numerical Simulations on High Energy Gas Fracturing

High-energy gas fracturing (HEGF) and gas fracturing (GF) are considered to be efficient to enhance the permeability of unconventional gas reservoir. The existing models for HEGF mainly focus on the dynamic loading of stress wave or static loading of gas pressurization, rather than on the combined actions of them. Studies on the combination of HEGF and GF (HEGF+GF) are also few. In this paper, a damage-based stress wave propagation-static mechanical equilibrium-gas flow coupling model is established. Numerical model and determination of mesomechanical parameters in finite element analysis are described in detail. Numerical simulations on crack evolution under HEGF, GF, and HEGF+GF are carried out, and the impact of in situ stress conditions on crack evolution is discussed further. A total of 11 cracks with length of 2.3-4 m in HEGF, 4 main cracks with length of 6.5–8 m in GF, and 11 radial cracks with length of 2–11.5 m in HEGF+GF are produced. Many radial cracks around the borehole are formed in HEGF and extended further in GF. The crustal stress difference is disadvantageous for crack complexity. This study can provide a reference for the application of HEGF+GF in unconventional gas reservoirs.

Numerical Analysis on Shale Crack Model under Explosive Loading

High Energy Gas Fracturing (HEGF) technology can produce self-supporting crack network in rocks of oil and gas reservoirs under a large number of high temperature and high pressure explosive shock wave caused by explosive detonation. It is expected that the reservoir conductivity will be improved by generating the new crack channels for oil and gas flow, so as to achieve the goal of increasing oil and gas production. However, the numerical study on the crack initiation, extension and propagation of rock under blast loading is still insufficient, which plays a vital role in better understanding the mechanical performance of rocks in the HEGF process. In the present study, taking the typical shale gas demonstration area in Sichuan province as an example, the shale crack model is established, the mechanical properties of shale rock with cracks under explosive loading are studied, the evolution laws of crack propagation are analyzed, and the damage characteristics of crack tip are considered.

Mathematical Modeling of Fluid Flow to Unconventional Oil Wells With Radial Fractures and Its Testing With Field Data

Radial fractures are created in unconventional gas and oil reservoirs in modern well stimulation operations such as hydraulic refracturing (HRF), explosive fracturing (EF), and high energy gas fracturing (HEGF). This paper presents a mathematical model to describe fluid flow from reservoir through radial fractures to wellbore. The model can be applied to analyzing angles between radial fractures. Field case studies were carried out with the model using pressure transient data from three typical HRF wells in a lower-permeability reservoir. The studies show a good correlation between observed well performance and model-interpreted fracture angle. The well with the highest productivity improvement by the HRF corresponds to the interpreted perpendicular fractures, while the well with the lowest productivity improvement corresponds to the interpreted conditions where the second fracture is much shorter than the first one or where there created two merged/parallel fractures. Result of the case studies of a tight sand reservoir supports the analytical model.

An Improved Numerical Model of the High Energy Gas Fracturing (HEGF) Process

An Improved Numerical Model of the High Energy Gas Fracturing (HEGF) Process

Numerical simulation and parametric analysis for designing High Energy Gas Fracturing

This paper presents a new comprehensive approach to model High Energy Gas Fracturing (HEGF) process. This coupled model consists of 6 sub-quantitative modules, including (1) loading build-up due to propellant deflagration, (2) gas-liquid interface displacement caused by killing liquid column movement, (3) stress distribution around the casing wellbore and the perforation holes, (4) fluid penetration through the perforation holes, (5) rate dependent critical pressure of fracture initiation and (6) hydro-mechanical coupled fracture propagation. The solution to the coupled model is obtained combining the analytical method and finite difference method, which solves the mass conservation equation and energy conservation equation with the subsystem pressure and temperature as the main variables. Subsequently, The single pulse and the multi-pulse HEGF processes are simulated. The dynamic changes of the loading built-up of propellant deflagration, the movement of killing liquid column as well as the dynamic propagation of fracture are further studied. Meanwhile, in-depth analysis has been applied on the affect sensitivity of the five key parameters to the single pulse HEGF results as well as the mechanism of multi-pulse HEGF. The results indicate that the final lengths of fractures generated by single pulse HEGF can be extended with the increase of the perforation density, the diameter of perforation holes, the thickness of propellant column and the total propellant quality. However, the effect of the height of killing liquid column is insignificant. It also demonstrates that the multi-pulse HEGF could bring multiplier fractures length by concatenating series propellants with different burning rates. This coupled model could add new dimensions to the understanding of the coupled mechanism of single pulse and multi-pulse HEGF. In addition, the model could be applied as a parameters quantitative design tool to assist the field implementation of these technologies.

爆压酸化技术在海上油田挖潜增产中的应用

爆压酸化技术在海上油田挖潜增产中的应用

Prediction of productivity of high energy gas-fractured oil wells

The use of fresh water as a fracturing fluid has limitations such as limited water availability in arid areas and negative impacts of water on oil and gas production in formations with high clay content. Alternatives to water for well fracturing include tailored energetic materials such as novel explosives. High energy gas fracturing (HEGF) creates radial fractures with fracture-orientations independent of formation stress anisotropy and heterogeneity. This eliminates the requirement of in-situ stress orientation for designing multi-hydraulic water-fracturing horizontal wells. Assuming uniformly distributed radial fractures around wellbore, an analytical well productivity model was derived in this study to predict productivity of HEGF-completed oil wells under pseudo steady state flow condition. Field case studies and sensitivity analyses were performed with the analytical model. Result of field case studies indicates that the analytical model over-predicts productivity of HEGF-completed wells by about 10%. Sensitivity analysis with the analytical model shows that the productivity of HEGF-completed wells reaches a maximum value at an optimal number of radial fractures around the wellbore. The productivity of the HEGF-completed wells increases non-linearly with fracture conductivity. But the benefit of increasing fracture conductivity levels out beyond fracture conductivity 2000 md-inch for the typical case investigated in this study.

Geomechanical modeling - workflow and Applications

This work aimed to present a detailed workflow for building a geomechanical model. For a case study, the workflow is then applied to a horizontal well X. The first step in building a geomechanical model is gathering data regarding well information (tubing, casing, deviation…), geological information (type of fault, permeability, reservoir radius, skin…), logs data (density, resistivity, sonic, caliper…), in-situ test data (leak-off test, formation test,…) and core data (tensile strength test, fracture toughness test, tri-axial test…). The second step is to build the geomechanical model using data analysis so that information about state of stress (vertical and principal horizontal stresses, pore pressure, concentration stress around wellbore) and rock mechanical properties (unconfined compressive strength, tensile strength, fracture toughness, Young modulus, Poisson ratio) can be determined. Moreover, the differences in data analysis for vertical and horizontal wells were also mentioned in this work. Furthermore, it is evident that the more data we get, the more accurately a geomechanical model can be built. However, in reality, not all necessary data can be obtained, so this work also explained how to draw the most information from available data so that we can minimize the number of assumptions and uncertainties. An accurate geomechanical model is very essential for others works such as well bore stability or performance prediction of a well stimulation technique. The case study of this work presented the geomechanical modeling for the well X. The paper then presented the application of geomechanical modeling for the Evaluation of High Energy Gas Fracturing performance as well as for Sand Control analysis.

Liquid nitrogen gasification fracturing technology for shale gas development

Abstract Shale gas, as an important source of non-conventional energy, has only been successfully exploited for commercial use in the United States, Canada and China. The successful commercial exploitation of shale gas depends primarily on the development of horizontal drilling and fracturing technology. Currently, hydraulic fracturing is the main approach used in exploiting shale gas; in addition, there are alternative approaches to high energy gas fracturing and CO 2 , N 2 foam fracturing. However, there are some shortcomings of each of the existing fracturing technologies. For example, hydraulic fracturing can realize large-scale fracturing but cannot avoid the problem of clay hydration swelling. The influence of fracture orientation via ground stress is also larger. Moreover, hydraulic fracturing consumes a large amount of water. As a result, hydraulic fracturing is not applicable to western China. For resource and environmental reasons, the French government has proposed legislation to ban hydraulic fracturing for the exploitation of shale gas ( 2012 ). Based on the survey and study of existing fracturing technologies, the author proposes a new technology: liquid nitrogen gasification fracturing technology. This novel fracturing approach involves liquid nitrogen pressure fracturing, rock contraction fracturing, rock embrittlement fracturing and nitrogen expansion fracturing. The proposed approach can realize large-scale fracturing, does not suffer the problem of clay hydration swelling, has no water resource limitation and does not produce environmental pollution. In short, the proposed approach may better guide shale gas development practices.

Numerical analysis of rock fracturing by gas pressure using the extended finite element method

High energy gas fracturing is a simple approach of applying high pressure gas to stimulate wells by generating several radial cracks without creating any other damages to the wells. In this paper, a numerical algorithm is proposed to quantitatively simulate propagation of these fractures around a pressurized hole as a quasi-static phenomenon. The gas flow through the cracks is assumed as a one-dimensional transient flow, governed by equations of conservation of mass and momentum. The fractured medium is modeled with the extended finite element method, and the stress intensity factor is calculated by the simple, though sufficiently accurate, displacement extrapolation method. To evaluate the proposed algorithm, two field tests are simulated and the unknown parameters are determined through calibration. Sensitivity analyses are performed on the main effective parameters. Considering that the level of uncertainty is very high in these types of engineering problems, the results show a good agreement with the experimental data. They are also consistent with the theory that the final crack length is mainly determined by the gas pressure rather than the initial crack length produced by the stress waves.

Study on “fracturing-sealing” integration technology based on high-energy gas fracturing in single seam with high gas and low air permeability

To improve the gas extraction efficiency of single seam with high gas and low air permeability, we developed the “fracturing-sealing” integration technology, and carried out the engineering experiment in the 3305 Tunliu mine. In the experiment, coal seams can achieve the aim of antireflection effect through the following process: First, project main cracks with the high energy pulse jet. Second, break the coal body by delaying the propellant blasting. Next, destroy the dense structure of the hard coal body, and form loose slit rings around the holes. Finally, seal the boreholes with the “strong–weak–strong” pressurized sealing technology. The results are as follows: The average concentration of gas extraction increases from 8.3% to 39.5%. The average discharge of gas extraction increases from 0.02 to 0.10m3/min. The tunneling speeds up from 49.5 to 130m/month. And the permeability of coal seams improves nearly tenfold. Under the same conditions, the technology is much more efficient in depressurization and antireflection than common methods. In other words, it will provide a more effective way for the gas extraction of single seam with high gas and low air permeability.