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For most shale gas wells, frequent shut-ins and nozzle size changing often occur, leading to significant discontinuities in production data. This study proposes a new production data analysis (PDA) method to address the abrupt change issue based on the virtual equivalent time. With the virtual equivalent time, the qualities of type curve matching and production data analysis can be improved when there are abrupt changes in production data. The mathematical model with a variable flow rate provides new definitions of normalized pseudopressure and pseudotime with consideration of the pressure dependent permeability for multi-fractured horizontal wells in shale gas reservoirs. Subsequently, the virtual equivalent time is calculated based on the average formation pressure to consider the abrupt changes of the production data, which also can reduce the time of superposition principle calculations. Duhamel's principle, Laplace transform and inversion, and Newman's method are employed to solve the PDA model, which is validated by analytical and numerical solutions. Then sensitivity analysis are performed on the dimensionless constrained axial modulus, which shows that the larger the dimensionless constrained axial modulus, the lower and later the curves. Finally, a field case study is carried out using the proposed method. The results show that by using the virtual equivalent time, the matching qualities are good, and the issue caused by abrupt changes is satisfactorily addressed. Therefore, it has great potential for estimating the formation parameters and predicting the well production performance more effectively and practically. [ABSTRACT FROM AUTHOR]

Rock deformation is an important mechanism in shale reservoirs and occurs simultaneously with gas flow during the production period. It may not only result in production variation, but also have an impact on reservoir flow behaviors. Accurate simulation of gas flow in deformable reservoirs is challenging considering dual porosity structure of shale reservoirs. Besides, the complex fracture network topology and interactions further complicates the simulation of the fracture system. In this work, we implement a discrete fracture model (DFM), in which the reservoir is divided into the matrix and fracture system. The DFMs use flexible Delaunay triangulation to represent individual fractures, hence they could handle complex fracture network. Based on the comprehensive study of the deformation of matrix and fracture, a flow model that considers rock deformation is presented to predict well production and reservoir dynamics. Numerical simulations are conducted to investigate the influence of rock deformation on a reservoir with complex fracture network. Results under no deformation (ND) condition and integrated deformation (ID) condition show that rock deformation has an obvious impact on the gas production and the pressure distribution. The percentage of production loss increases with reservoir pressure, while decreases as matrix permeability or fracture conductivity increases. Comparisons of production loss caused by the individual deformation depict that the natural fracture deformation (NFD) is dominant, the hydraulic fracture deformation (HFD) is minor, while the matrix deformation (MD) is negligible. Less percentage of production loss caused by the ID is observed than the sum caused by the NFD, the HFD, and the MD. [ABSTRACT FROM AUTHOR]

Combined perforation and well test system is commonly used in oil and gas fields, especially in reservoirs with low energy and transmissibility. For the conventional inversion method in a combined perforation and well test system, only the pressure data are used to perform the type curve matching, with the variation in temperature being neglected. This may complicate the matching procedure and result in errors in the inversion results. Thus, we proposed a new inversion method in this study, where the gas volume and flow rate terms substituted the pressure and pressure derivative terms to perform type curve matching. This was because the variation in gas volume coupled the variations in pressure and temperature. To make the type curve matching feasible, on the one hand, we need the type curves that depict dimensionless gas volume and flow rate; on the other hand, the field data of pressure and temperature should also be transformed into gas volume and flow rate data, which are the core contents of this paper. A corresponding mathematical model is established and the analytical solutions of gas volume and flow rate are derived with the Laplace transformation. Based on the solutions, the flow regimes analysis and sensitivity analysis on controlling parameters are conducted. Next, a workflow that can convert the pressure and temperature data into gas volume and flow rate data is proposed. Last, the application on a well from Puguang gas field is presented to show the feasibility of our method. Results show that our method successfully interpret the field data comparing to the conventional method that only uses the pressure data. Moreover, comparisons between the results of our method and other field data calibrate the reliability and accuracy of our method. • A novel parametric inversion method for combined perforation and well test system is proposed. • Gas volume and flow rate substituting the pressure and derivative to perform the type curve matching. • The analytical solutions of gas volume and flow rate are derived. • The workflow that transforms the pressure data and temperature data to gas volume and flow rate is proposed. [ABSTRACT FROM AUTHOR]

The propagation of hydraulic fracture network in naturally fractured shale reservoirs remains a complicated and challenging issue. To reveal the propagation principle of the complex fracture network, a numerical fracturing model is presented on the basis of the extended finite element method (XFEM) in this work. The intersection criterion containing five intersection patterns is studied. True triaxial fracturing experiment is conducted to verify the fracture geometry obtained from our model. After that, hydraulic fracturing in naturally fractured formations is studied to investigate the influences of natural fracture azimuth on fracture network geometry. The numerical results show that, hydraulic fracture is inclined to open the obtuse angle side of natural fracture in uniform azimuth model. In random azimuth condition, occurring of various intersection patterns enables complex fracture network to generate easier. And hydraulic fracture is harder to propagate if it is closer to the middle domain due to the squeezing effect. Meanwhile, the fracture network can penetrate deeper shale layer under large in-situ stress difference. In addition, the stimulation effect (e.g., Stimulated Reservoir Volume) in random azimuth condition is much stronger than that in uniform azimuth condition. The findings in this work are of benefit to provide us a better understanding of complex hydraulic fracture network and develop high-efficiency fracturing technology. • A novel numerical hydraulic fracture network model based on the XFEM is presented. • An intersection criterion containing five intersection patterns is introduced. • Higher chance to induce fracture network in randomly naturally fractured reservoir. • The in-situ stress difference has strong influence on fracture network trajectory. [ABSTRACT FROM AUTHOR]

The production performance of horizontal well with complex fracture networks remains a challenging issue. To more accurately evaluate the stimulation effect of fracture network on the reservoirs, an equivalent method is presented based on the extended finite element method (XFEM) and the Peaceman well model. We discretize the governing equations with finite volume method and solve the linearized equations with fully implicit scheme. The reliability of the equivalent method is validated against the available numerical method in the literature. After that, two cases of different fracture network structures are investigated. Results show that the equivalent method is feasible to assess the effect of fracture network on horizontal well production. And more broadly distributed fracture network has better enhancing effect. Meanwhile, fracture tips have little stimulation effect on the production. The findings are of benefit to provide us a simplified way to investigate the flow in reservoirs containing complex fracture network. • A simplified way to assess the stimulation effect of fracture network is provided. • The fracture network model is presented based on the XFEM. • The fracture segment is changed into an equivalent well model to simplify the flow. • The constant flux condition is adopted to assign the flow rate. • Crack aperture is a significant influencing factor for the stimulation effect. [ABSTRACT FROM AUTHOR]

Nano-scale pore structure and fractal characteristics are significant information to understand shale gas reservoirs. Therefore, seventeen marine-continental transitional shale samples of Leping Formation in Qingjiang Basin, southeast China were investigated to study the potential relationship between pore structure and fractal characteristics. In this work, a dual-pore-shape method was firstly applied to construct pore structure in the shapes of cylinder and slit, which is closer to reality. Meanwhile, non-local density function theory method and Barrett-Joyner-Halenda method were employed for comparison as well. Fractal dimensions obtained from nitrogen isotherms via Frenkel-Halsey-Hill method were divided into two parts, one characterizes larger pores and the other characterizes smaller pores. Geochemical index, pore structure information, and fractal characteristics showed different correlated behaviors with each other. Total organic carbon content strongly affects pore structure of transitional shale, but clay and quartz content have no clear influence. Correlations between fractal dimensions and geochemical parameters were affected by the degree of isotropy. Pore parameters have conspicuous relationships with the fractal dimension of larger pores but not with that of smaller pores. It is found that slit pores play a significant role in pore space in shale matrix. Moreover, the connections between different shapes of pores and fractal dimensions are displayed. Pore volume of cylindrical pores positively correlates with fractal dimension of larger pores, whereas pore volume of slit pores negatively correlates with fractal dimension of smaller pores. The comparison of all pore parameters obtained by different methods shows that dual-pore-shape method could better construct real pore structure and obtain the pore volume distributions more accurately. • Dual-pore-shape method was firstly applied to characterize pore structure of shale. • Besides cylindrical pores, slit pores play an important role in nano-pore space. • Smaller pores have stronger correlation between volumes and fractal dimensions. • Fractal dimensions of larger and smaller pores have connections with pore shape. • Pore volumes and specific surface areas were compared by using three methods. [ABSTRACT FROM AUTHOR]