Fractured Shale Gas Wells Research Articles

A new type of automatic ball injector for multistage fracturing

Ball-drop fracturing is a kind of technology that is commonly used for multistage fracturing of shale gas horizontal wells. At present, the ball injector cannot satisfy the operation demands of large bore and small spacing fracturing of shale gas wells due to its disadvantages, such as a small size of fracturing ball, a limited number of balls, complicated structure and poor applicability. In this paper, a new type of automatic ball injector with rotating plate and large bore was designed. There is a ball storage at the upper part of the ball injector and a ball injecting mechanism inside the pedestal at the lower part. And the ball injecting mechanism is controlled by the drive motor to inject the balls one by one. Then, collision analysis was carried out on its ball injecting process by using finite element method (FEM). Finally, simulation and prototype tests were conducted by using the arbitrary Lagrangian–Eulerian (ALE) mesh adaption method and the coupled smoothed particle hydrodynamics method (FEM–SPH). And the following research results were obtained. First, in the process of ball collision, the maximum equivalent stress, equivalent plastic strain and displacement are generated at the edge of the rotating plate. The maximum stress is about 600 MPa, which is less than the yield strength of 42CrMo (930 MPa), so the strength requirement is satisfied with a certain safety margin. Second, the prototype successfully passes the continuous ball injecting test of 12-stage fracturing process with the ball diameter of 64–108 mm and the ball contrast of 4 mm. In conclusion, this automatic ball injector with rotating plate and large bore is advantageous with simple operation and wide applicability and can effectively address the needs of multistage fracturing of shale gas horizontal wells.

Numerical simulation of cement-to-formation interface debonding during hydraulic fracturing of shale gas wells

The hydraulic fracturing process involves high pump pressure and large displacement, which increase the risk of debonding on the interface of the cement sheath and rock formation. Therefore, in this study, a three-dimensional numerical calculation model of casing-cement sheath-formation assembly was established, and the failure mechanism of the bonding surface caused by fracturing due to fluid migration was investigated. The effects of the cement sheath’s Young’s modulus and Poisson’s ratio, bonding strength, perforation azimuth angle, and non-uniform in-situ stress on the anti-debonding ability of the cement-to-formation interface were analyzed. Finally, the following conclusions were drawn: The calculation results show that the failure of the bonding surface is greatly affected by the bonding strength, and its anti-debonding ability significantly increases linearly with increasing bonding strength. Furthermore, while increasing the Young’s modulus improves the anti-debonding ability, the Poisson’s ratio of the cement sheath and the perforation azimuth angle have little effect. Under non-uniform in-situ stress, the anti-debonding ability decreases with the increase of the difference between the horizontal and vertical in-situ stress. Thus, the non-uniform in-situ stress accelerates the failure of the bonding surface.

Fracture Characterization Using Flowback Water Transients from Hydraulically Fractured Shale Gas Wells.

Fracture characterization is necessary to evaluate fracturing operations and forecast well performance. However, it is challenging to quantitatively characterize the complex fracture network in shale gas reservoirs because of the unknown density and reactivation of natural fractures. The flowback water transients can provide useful information about the complexity of the fracture network after the fracturing operations. In this paper, a mathematical model for modeling fracturing fluid flowback of hydraulically fractured shale gas wells is established. This proposed model characterizes the flow of water and gas in a hydraulic fracture-induced natural fracture–shale matrix system. Hydraulic, capillary, and osmotic convections; gas adsorption; and natural fracture closure are considered in this model. Flowback simulation of a hydraulically fractured shale gas well is conducted using the developed numerical simulator, and the water/gas transients between hydraulic fractures, natural fractures, and matrix are obtained. Finally, two field cases from the Longmaxi Formation, Southern Sichuan Basin, China, are used for comparison of the flowback data with the model results. The good match of the two water transients provides a group of fracture network parameters, that is, the effective length and conductivity of main hydraulic fractures and the density of induced natural fractures. The proposed model for describing the flowback process and its meaningful relationship with the fracture–network complexity provides an alternative approach for post-stimulation evaluation.

A new numerical method for evaluating the variation of casing inner diameter after strike‐slip fault sliding during multistage fracturing in shale gas wells

AbstractCasing shear deformation has become a prominent problem in the process of completion in shale gas wells. It was believed that the slide of strike‐slip fault induced by multistage fracturing was the main reason. This paper presented a new numerical investigation for evaluating the casing inner diameter after strike‐slip fault sliding based on the microseismic data. A 3D finite element model, considering the mechanical anisotropy of shale and heat‐fluid‐solid coupling effect during multistage fracturing, was developed to calculate the variation of casing inner diameter after fault sliding under different engineering and geological conditions. The calculation result was verified by comparison with the measurement result of the multi‐finger caliper survey. Sensitivity analysis was carried out and the results showed that decreasing the sliding distance, maintaining high pressure, increasing the casing thickness, and increasing the elasticity modulus and decreasing the Poisson ratio of cement sheath were beneficial to protect the casing integrity. Finally, the engineering verification demonstrated that the numerical method has an accuracy up to 85.9%. Numerical model in this study was expected to provide a better understanding of casing shear deformation and an evaluation method of casing inner diameter after fault sliding during multistage fracturing in shale gas wells.

A new investigation on casing shear deformation during multistage fracturing in shale gas wells based on microseism data and calliper surveys

Casing shear deformation has become a serious problem in the development of shale gas fields, which is believed to relate to fault slipping caused by multistage fracturing. This paper presents a new investigation aiming at understanding the reduction of casing inner diameter after fault slipping under different conditions, according to microseism data and calliper surveys. A mathematical model, which was verified by measurement results, was established to build a relationship between microseism moment magnitude and slip distance. A 3D numerical model, which was verified by physical model experiments, was developed to quantify the relationships between slip distance and the reduction of casing inner diameter. And then the relationships among microseism moment magnitude, slip distance and the reduction of casing inner diameter were established. The influence of casing internal pressure, slip distance, mechanical parameters of cement sheath, and casing thickness on the reduction of casing inner diameter was analyzed. The mathematical calculation results showed that with the increase of the degree of moment magnitude, the radius and slip distance increase, and with the increase of stress drop, the radius decreases and the slip distance increases. The numerical analysis results showed maintaining high pressure, increasing the elasticity modulus and decreasing the Passion ratio of cement sheath and increasing the casing thickness were beneficial to decrease the reduction of casing inner diameter. Keeping the designed horizontal segment of well trajectory away from the fracture-developed area, or be parallel to natural fracture can decrease the slip distance of fault, which can decrease the reduction of casing inner diameter. Finally, some effective measures to reduce or prevent casing shear deformation were recommended.

Cement sheath sealing integrity evaluation under cyclic loading using large-scale sealing evaluation equipment for complex subsurface settings

Cement sheaths should provide zonal isolation and structural support through the entire service lifetime of wells. However, even if high-quality primary cementing was achieved, variations in pressure, for example, those due to staged fracturing in shale gas wells, are likely to lead to sealing integrity failure of the cement sheath. This work presents large-scale sealing evaluation equipment to assess the failure mechanisms of cement sheaths under cyclic loading. The equipment includes a borehole system, pressure system, and control and data acquisition system for automatic loading and unloading, gas channeling monitoring and pressure change and gas output recording. The pressure limit of the equipment is 120 MPa. A finite element model of the borehole system and sealing equipment was built to study the stress and strain under internal pressure. The failure mechanisms of the sealing integrity of the cement sheath were obtained by combining the test results with the finite element calculations. At a high internal pressure, plastic deformation was produced during loading due to the residual strain that arose after unloading of the cement sheath. Therefore, incompatible deformation occurred, and the tensile stress increased at the interfaces of the cement sheath. Under repeated loading and unloading, the plastic deformation increased, and the residual strain and interface tensile stress also increased. When the interface tensile stress exceeded the bonding strength, a microannulus formed between the cement sheath and casing. Based on these results, guidelines for the assessment of the long-term sealing integrity of a shale gas well cement sheath and for the selection of mechanical properties of the cement systems are given to mitigate the risk of zonal isolation failure.

Mechanisms and Influence of Casing Shear Deformation near the Casing Shoe, Based on MFC Surveys during Multistage Fracturing in Shale Gas Wells in Canada

Casing shear deformation has become a serious problem in the development of shale gas fields, which is believed to be related to fault slipping caused by multistage fracturing, and the evaluation of the reduction of a casing’s inner diameter is key. Although many fault slipping models have been published, most of them have not taken the fluid-solid-heat coupling effect into account, and none of the models could be used to calculate the reduction of a casing’s inner diameter. In this paper, a new 3D finite element model was developed to simulate the progress of fault slipping, taking the fluid-solid-heat coupling effect during fracturing into account. For the purpose of increasing calculation accuracy, the elastoplastic constitutive relations of materials were considered, and the solid-shell elements technique was used. The reduction of the casing’s inner diameter along the axis was calculated and the calculation results were compared with the measurement results of multi-finger caliper (MFC) surveys. A sensitivity analysis was conducted, and the influences of slip distance, casing internal pressure, thickness of production and intermediate casing, and the mechanical parameters of cement sheath on the reduction of a casing’s inner diameter in the deformed segment were analyzed. The numerical analysis results showed that decreasing the slip distance, maintaining high pressure, decreasing the Poisson ratio of cement sheath, and increasing casing thickness were beneficial to protect the integrity of the casing. The numerical simulation results were verified by comparison to the shape of MFC measurement results, and had an accuracy up to 90.17%. Results from this study are expected to provide a better understanding of casing shear deformation, and a prediction method of a casing’s inner diameter after fault slipping in multistage fracturing wells.

New Analytical Method to Evaluate Casing Integrity during Hydraulic Fracturing of Shale Gas Wells

An accurate analysis of casing stress distribution and its variation regularities present several challenges during hydraulic fracturing of shale gas wells. In this paper, a new analytical mechanical‐thermal coupling method was provided to evaluate casing stress. For this new method, the casing, cement sheath, and formation (CCF) system was divided into three parts such as initial stress field, wellbore disturbance field, and thermal stress field to simulate the processes of drilling, casing, cementing, and fracturing. The analytical results reached a good agreement with a numerical approach and were in‐line with the actual boundary condition of shale gas wells. Based on this new model, the parametric sensitivity analyses of casing stress such as mechanical and geometry properties, operation parameters, and geostress were conducted during multifracturing. Conclusions were drawn from the comparison between new and existing models. The results indicated that the existing model underestimated casing stress under the conditions of the geostress heterogeneity index at the range of 0.5–2.25, the fracturing pressure larger than 25 MPa, and a formation with large elastic modulus or small Poisson’s ratio. The casing stress increased dramatically with the increase of in situ stress nonuniformity degree. The stress decreased first and then increased with the increase of fracturing pressure. Thicker casing, higher fluid temperature, and cement sheath with small modulus, large Poisson’s ratio, and thinner wall were effective to decrease the casing stress. This new method was able to accurately predict casing stress, which can become an alternative approach of casing integrity evaluation for shale gas wells.

Casing Shear Deformation Based on Micro Seismic Data

Casing shear deformation, which is caused by fault slipping, is the main morphology of casing deformation occurred during multistage fracturing. In order to determine the relationship between fault slipping and casing shear deformation, the micro seismic data collected from engineering field which can reflect the reality of the formations was analyzed. A new numerical model was developed, and the influential factors on casing shear deformation were studied, including the fault slip distance of lower and upper interface, fault dip angle, the thickness of cement sheath and casing. The results of research shows that: (1) Fault is easily activated by fracturing, which was the main reason of casing shear deformation; (2) The greater the fault dip angel, the slip distance, the greater the casing shear deformation; (3) Increasing the wall thickness of casing or cement sheath is beneficial to decrease the degree of the shear deformation; (4) Two ways can be used to avoid and control the casing shear deformation, one is keeping the designed horizontal segment of well trajectory keep away from fracture-developed area, or be parallel to natural fracture, the other is using stage cementing technology. Research results can provide important reference for design and control of casing integrity during multistage fracturing in shale gas wells.

Analytical Method to Evaluate Casing Stress during Multi-Fracturing for Shale Gas Wells

An analysis model of casing stress distribution and its variation regularities presents several challenges during hydraulic fracturing of shale gas wells. In this paper, an analytical mechanical - thermal coupling method was provided to evaluate casing stress. In the model, the casing, cement sheath, and formation (CCF) system was divided into four stress field induced by uniform stress, deviator stress, shear stress, and thermal stress,. Based on this analytical model, the parametric sensitivity analyses of casing stress such as mechanical properties, operation parameters, and geo-stress were conducted during multi-fracturing. The results indicated the casing stress increased first, then decreased with the increase of cement sheath modulus. However, it always decreased with the increase of cement sheath Poisson's ratio and the injection fluid temperature. The casing stress increased dramatically with the increase of δ. However, it decreased first, then increased with the increase of fracturing pressure. Higher fluid temperature, cement with small modulus and large Poisson’s ratio were effective to decrease the casing stress. In conclusion, the analytical model can accurately predict casing stress and become an alternative method of casing integrity evaluation for shale gas wells. It is a useful and efficient method for a preliminary design, being capable of simulation the actual situations in order to assess the casing stresses and integrity.

Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42- concentrations dropped. These observations are attributed to iron and SO42- reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42- that could distinguish newly leaked methane from older methane sources in aquifers.

An analytical model for multiple fractured shale gas wells considering fracture networks and dynamic gas properties

Multi-stage fractured horizontal wells play an important role in developing shale gas reservoirs by significantly improving productivity. By considering fracture networks, gas desorption, stress-sensitive fracture permeability, and pressure-dependent gas PVT properties, an analytical model is developed for shale gas wells. Fracture networks are handled based on transient linear flow, gas desorption is handled by defining a new total compressibility, stress-dependent hydraulic fracture permeability is handled by variable substitution, and pseudo-pressure and pseudo-time are used to handle pressure-dependent PVT properties. After obtaining the solution of the linearized model, a material balance method and successive substitution iteration procedure are proposed to convert the pseudo-time into real time and calculate the production contribution from gas desorption. The results show that induced fractures also have a great impact on the production of the well. Production contribution from free gas and adsorbed gas could be quantified using the proposed material balance principle and iterative method. The rank of parameters that influence the ultimate recovery is the following: half-length of hydraulic fracture, induced fracture length/hydraulic fracture spacing, hydraulic fracture spacing, conductivity of induced fractures, conductivity of hydraulic fracture, and induced fracture spacing.

Numerical simulation of casing deformation during volume fracturing of horizontal shale gas wells

Casing deformation problems occur frequently during volume fracturing of shale gas wells. It is crucial to assess the casing deformation near heel (the beginning of the horizontal wellbore trajectory) where natural faults exist among 84.6% of the deformation positions in some shale plays. Based on the source mechanism, the fault slip distance was derived from microseismic inversion. Then, three-dimensional physical and finite element models were established using a stage finite element model procedure to investigate the influences of fault slippage on casing stress and displacement. Other factors such as pump rate, fracturing pressure, formation anisotropy, formation elastic modulus, cement sheath elastic modulus, casing size, and cement sheath thickness were taken into consideration. The results show that (1) fault slippage is an important mechanism of casing deformation near heel. The casing has a higher risk of deformation under the conditions of fault slippage combined with high external stress and internal pressure. (2) Under different slip distances, the casing Mises stress dramatically increases with an increase in the cement sheath and formation elastic moduli and the ratio of the casing diameter to the thickness Dout/t. (3) The high pump rate, high inner casing pressure, large formation anisotropy, and small cement sheath thickness can aggrandize casing stress. The calculated casing displacements using this model obtained good agreement when compared to the deformations of the actual wells. Countermeasures such as avoiding natural fault areas, using cement with an elastic modulus smaller than 10 GPa, using a higher grade and smaller diameter casing with a value of Dout/t smaller than 12.7 can be effective to prevent casing deformation during multi-fracturing operation.

Draft Genome Sequences of Two Chemosynthetic Arcobacter Strains Isolated from Hydraulically Fractured Wells in Marcellus and Utica Shales.

ABSTRACTGenome sequences were obtained for two isolates of the genus Arcobacter from saline fluids produced from hydraulically fractured shale gas wells in the Marcellus and Utica formations. These genomes provide insight into microbial sulfur cycles occurring in a high-salt deep terrestrial shale environment.

On methane emissions from shale gas development

Environmental and economic impacts of methane escaping from the natural gas supply chain remain uncertain. Flowback emissions from hydraulically fractured natural gas wells are a key component of emissions from unconventional gas wells. While reduced emission completions in the United States are required by regulation, Canada’s proposed regulation will only be implemented in 2020 with the two highest producing provinces under exemption. To understand potential benefits of regulations, we use predictive modelling of well-level production data of 1633 hydraulically fractured shale gas wells in five plays to estimate pre-production emissions. The mean estimate for flowback emissions (2346 ± 95% confidence interval of 91 Mg CO2e/completion) fall within the 95% confidence limits of measured potential emissions (2566 ± 777 Mg CO2e/completion). Our results indicate that in 2015, the average emissions per shale gas well undergoing flowback was 2347 Mg CO2e/completion in the U.S. and 1859 Mg CO2e/completion in Canada. Mean potential profits from controlling methane emissions using reduced emission completions were US$17,200/well in the U.S. and US$11,200/well in Canada.

Study on the sealing integrity of cement sheath during volume fracturing of shale gas of horizontal well

The integrity of the cement sheath is very important for reducing the band pressure of the empty cement sheath and improving the safety of the shaft (well hole). The injection of high pressure fluid may lead to the expansion of the fracture of the interface of the well cementation during the volume fracturing of shale gas of horizontal wells. The sealing property of cement sheath is affected. Therefore, the finite element calculation model of fracture expansion in the cement interface is established on the stepped finite element method, which is combined with the CZM model. This model is used to calculate the field examples. The fracture expansion of the cement interface is affected by the degree of interfacial cementation, formation and the properties of cement sheath, the influence of which is studied. The results of the study show that the initial stress of the cement sheath and the change of the formation pressure on the overlying formation do not have any significant influence on the length of the fracture expansion of the cement interface. The greater difference of the horizontal stress while, the smaller the length of the crack growth. The fracture expansion length of the cement interface decreases with the increase of the elastic modulus of the formation. The amplitude of the change is increased first and then reduced. The elastic modulus of the cement sheath and the degree of consolidation of the cement interface are beneficial to reduce the length of crack propagation of the interface. All results have certain implications for the evaluation and prediction of the integrity of the cement sheath during the volume fracturing of shale gas wells.

An Analysis for the Influences of Fracture Network System on Multi-Stage Fractured Horizontal Well Productivity in Shale Gas Reservoirs

This paper presents two representative models to analyze the flow dynamic of multi-scale porous medium in hydraulic fractured horizontal shale gas wells. In this work, considering the characteristic mechanisms (multi-scale porous space, desorption and diffusion), flow equations in shale are established. After that, two representative models (discrete fracture model and dual-porosity model) are tailored to our issues. Solved by the control-volume finite element method (CVFEM), influences of fracture network system on productivity in shale reservoirs are analyzed in detail. Based on the analysis, the effects can be summarized as follow: at the beginning of production, high conductivity fracture network means more free gas could be produced; at the later part of production, high conductive fracture network can form a large low pressure region, which can not only stimulate the desorption of adsorbed gas, but also reduce the flow resistance to the well. Finally, the sensitivities of characteristic parameters in shale are discussed.

Effects of Cyclic Loading on the Mechanical Properties of Mature Bedding Shale

We investigated the mechanical properties of mature bedding shale under cyclic loading conditions, with an application to the design of hydraulic fracturing in shale gas wells. Laboratory experiments were conducted on shale samples under two principal loading orientations. Testing results showed that accumulated fatigue damage occurs in a three‐stage process. Analysis of fatigue damage at different maximum stress levels shows that fatigue life increases as a power‐law function with maximum stress decreasing. And the maximum stress significantly affects the fatigue life. Further, the elastic part of shale rock deformation was recovered in the unloading process, whereas the irreversible deformation remained. The irreversible deformation, growth trend, and accumulation of the total fatigue were directly related to the fatigue damage. This process can be divided into 3 stages: an initial damage stage, a constant velocity damage stage, and an accelerated damage stage, which accounted for about one‐third of the fatigue damage. Shale rock is a nonhomogeneous material, and the bedding is well developed. Its fatigue life differs greatly in two principal loading orientations, even under the same loading conditions. All of these drawn conclusions are of great importance for design of hydraulic fracturing in shale gas wells.

Investigation on nonlinear multi-scale effects of unsteady flow in hydraulic fractured horizontal shale gas wells

A unified mathematical model is established to simulate the nonlinear unsteady percolation of shale gas with the consideration of the nonlinear multi-scale effects such as slippage, diffusion, and desorption. The continuous inhomogeneous models of equivalent porosity and permeability are proposed for the whole shale gas reservoir including the hydraulic fracture, the micro-fracture, and the matrix regions. The corresponding semi-analytical method is developed by transforming the nonlinear partial differential governing equation into the integral equation and the numerical discretization. The nonlinear multi-scale effects of slippage and diffusion and the pressure dependent effect of desorption on the shale gas production are investigated.

A new numerical investigation of cement sheath integrity during multistage hydraulic fracturing shale gas wells

Volume fracturing of shale gas wells has been observed to lead to the failure of cement sheath and to sustained casing pressure (SCP). This paper presents a new numerical investigation aimed at understanding the failure mechanism of the cement sheath during volume fracturing. A wellbore temperature model was established to obtain the required input parameters for the dynamic temperature boundary. The numerical model considers the coupling of casing, cement sheath, and formation rock to calculate the radial and tangential stresses in the cement sheath. In addition to the cement mechanical properties, the influencing factors considered include the casing pressure, fracturing fluid displacement, and initial temperature. The cement sheath integrity was evaluated using the Mohr-Coulomb failure criterion. The results show that the temperature of cement sheath changes drastically during fracturing. The radial and tangential stresses in the cement sheath change continuously with time. Lowering the internal wellbore pressure can effectively reduce the radial and tangential stresses in the cement sheath. Reducing the fracturing fluid displacement can significantly lower the radial and tangential stresses in the cement sheath. Increasing the initial fracturing fluid temperature will cause the radial stress of cement sheath to increase and the tangential stress to decrease. The use of cements with low Young's modulus values can significantly reduce the radial and tangential stresses in the cement sheath. The use of cements with low Poisson's ratio values can lower the tangential stress. The results obtained using the model were verified by field data that demonstrated no occurrence of casing pressure when using a cement with a low elastic modulus, as suggested by the model.