Search

Stimulated reservoir volume (SRV) is a key parameter affecting the post-fracturing production of ultra-low permeability reservoirs. Accurate prediction of the SRV shape and size is of vital significance for pre-fracturing stimulation design and post-fracturing effect evaluation. In this paper, an equivalent mathematical model for simulating two-dimensional SRV growth is proposed by integrating the fracture width equation, tensor permeability conversion, cubic law and calculation method of fracture porosity with mass conservation equation of injected fluid in natural fracture system. The finite difference method is used to derive the numerical discrete equation, and the fluid pressure and average fracture width in each grid block are obtained by solving the coupled fluid conservation equation and fracture width equation. The model is validated by comparing the results obtained from this work with others from literature. A parametric sensitivity study was performed to analyze the effects of some factors incorporating approaching angle, natural fracture spacing, horizontal principal stress difference, pumping rate and fracturing fluid viscosity on the SRV (including size, aspect ratio, and arithmetic mean fracture width in SRV region). The results show that SRV increases with the decrease of approaching angle, principal stress difference, injection rate, viscosity of fracturing fluid and fracture density. The width of branch fracture decreases with the increasing approaching angle and principal stress difference but increases with the increasing pumping rate and fluid viscosity. [ABSTRACT FROM AUTHOR]

Influenced by natural fractures, fracture networks may be created when hydraulic fractures propagate in naturally fractured reservoirs. Simulating fracture network propagation is crucial to fracture design. In this paper, a two‐dimensional mathematical model for the direct simulation of fracture network propagation that considers fluid filtration and reservoir fluid flow is developed, and a simulation method is proposed based on the coupled flow idea of natural fractures and matrices in dual‐porosity media. The model uses dynamic coordinates to represent the extended hydraulic fracture network and static coordinates to describe natural fractures that have not been activated. The activation of natural fractures by a hydraulic fracture is determined according to the hydraulic fracture propagation scenario after an intersection with natural fractures. The geometric shape and size of the network fracture are obtained by changing the size of the simulated area and solving the mass balance, fracture width, and reservoir fluid flow equations numerically in a coupled manner. After validating the new model, a parameter sensitivity analysis is conducted to study the effects of injected fluid volume, fracture height, fracture spacing, elastic modulus, horizontal principal stress difference, pumping rate, and fracturing fluid viscosity on the fracture network shape, fracture network size, average fracture width, and fracture‐reservoir contact area. The numerical results indicate that the model can be directly used to simulate the propagation of fracture networks. The volume of injected fluid is the most critical factor affecting the fracture network size. Increasing the fluid viscosity and injection rate can increase the width of the fracture network and the average width of a secondary fracture. However, the total contact area of the reservoir and fracture will decrease, and the reservoir‐fracture contact area and fracture network size will increase with a decrease in the horizontal stress difference and increase in elastic modulus. Decreasing a fracture's height can significantly increase the size of the fracture network and reservoir‐fracture contact area. [ABSTRACT FROM AUTHOR]

Complex fracture systems including natural fractures and hydraulic fractures exist in shale gas reservoir with fractured horizontal well development. The flow of shale gas is a multi-scale flow process from microscopic nanometer pores to macroscopic large fractures. Due to the complexity of seepage mechanism and fracture parameters, it is difficult to realize fine numerical simulation for fractured horizontal wells in shale gas reservoirs. Mechanisms of adsorption–desorption on the surface of shale pores, slippage and Knudsen diffusion in the nanometer pores, Darcy and non-Darcy seepage in the matrix block and fractures are considered comprehensively in this paper. Through fine description of the complex fracture systems after horizontal well fracturing in shale gas reservoir, the problems of conventional corner point grids which are inflexible, directional, difficult to geometrically discretize arbitrarily oriented fractures are overcome. Discrete fracture network model based on unstructured perpendicular bisection grids is built in the numerical simulation. The results indicate that the discrete fracture network model can accurately describe fracture parameters including length, azimuth and density, and that the influences of fracture parameters on development effect of fractured horizontal well can be finely simulated. Cumulative production rate of shale gas is positively related to fracture half-length, fracture segments and fracture conductivity. When total fracture length is constant, fracturing effect is better if single fracture half-length or penetration ratio is relatively large and fracturing segments are moderate. Research results provide theoretical support for optimal design of fractured horizontal well in shale gas reservoir. [ABSTRACT FROM AUTHOR]

A large portion of slickwater is trapped in gas shale formations after the hydraulic fracking process. To address the effect of slickwater retention on methane desorption, the adsorption and desorption isotherms for methane before and after slickwater treatment were measured based on the volumetric method using shale samples from the Longmaxi formation, China. We found the adsorption and desorption data for the methane before and after treatment fit well with the Freundlich model. We compared the effect of slickwater volume on desorption isotherms, desorption efficiency, adsorbed methane content and cumulative gas production. Slickwater treatment significantly reduces methane adsorption. The Freundlich constant k reduction decreases nearly linearly with the slickwater volume, and constant n increases nearly linearly first and then becomes slower with slickwater volume. Test results also reflected that slickwater treatment could improve methane desorption efficiency. The curve of desorption efficiency after treatment is significantly lower than before. Under 30 MPa pressure, the desorption efficiency increases by 50.93% when slickwater volume increases to 10.40 ml from 2.02 ml. A closed loop exists between two curves of adsorbed gas content before and after treatment, and its area increases with slickwater volume. Slickwater treatment improves cumulative gas production. The cumulative production of methane before treatment is smaller than after. 8.16 ml of slickwater is the best option for 130 g of shale powders to obtain the most cumulative gas production in the lab-scale condition. These results can be used to optimize the slickwater volume during hydraulic fracturing. • Methane adsorption/ desorption isotherm can be well fit by Freundlich modell. • Slickwater promote methane desorption from gas shale powders. • Slickwater volume significantly affects the desorption isotherm model and desorption efficiency. • Slickwater volume has an obvious impact on adsorbed methane content and cumulative gas production. [ABSTRACT FROM AUTHOR]

Particle transport in a vertical nonplanar fracture is a typical but poorly understood element in hydraulic fracturing. Particle-fluid flow in a nonplanar fracture with a narrowing width is investigated experimentally by a laboratory-scale slot, and the relevant transport mechanisms are compared with that in the planar slot. The effects of fluid flow rate, particle density, particle size, and particle volume fraction on particle distribution are investigated. The results indicate that the narrowing width complicates the slurry flow and lowers the bed coverage area. The vortex flow appears at the contraction of the cross-section as the bed grows to a threshold height and resuspends more particles further into the slot. An irregular bed with a descending stepped surface is formed due to non-uniform placement. Smaller size sands injected at a high flow rate and a low particle volume fraction build up a smaller bed. Larger and denser particles would reverse the trend. A rational model expressed by four dimensionless numbers is developed by the linear regression method to predict the coverage percentage of the particle bed. The model and experimental results provide directions to quantitatively evaluate the particle transport and distribution in a nonplanar fracture. [Display omitted] • The particle transport in a nonplanar fracture with a narrowing width is investigated by the experiment. • The dimensionless parameters are used to describe the particle distribution in nonplanar fractures. • The irregular bed with a descending stepped surface is smaller than the regular bed in the planar slot. • A correlation is developed for predicting the coverage percentage of the particle bed. [ABSTRACT FROM AUTHOR]

During and after multi-stage hydraulic fracturing, the spontaneous imbibition of water-base fracturing fluid in shale formation is considered as the main mechanism responsible for the retention of large amounts of fracturing fluid. Slickwater is widely applied in Fuling shale gas field, the largest shale gas field in China. To characterize the spontaneous imbibition of slickwater and its four major components, experiments were carried out by using modified imbibition cells. Results show that all of the imbibition curves can be divided into three stages: linear imbibition stage, transition stage and stable imbibition stage. Their imbibition capacities are in the order of cleanup additive, clay stabilizer, slickwater, defoamer and friction reducer from highest to lowest. We have found that the imbibed volume is larger than the initial pore volume. Nuclear Magnetic Resonance (NMR) tests, core images and porosity comparison before and after imbibition all indicate the secondary microfractures were created during the imbibition process. The calculated volume of secondary microfractures which is the difference between the pore volume before and after the imbibition is very close to the imbibed volume, indicating the imbibition mainly through the microfractures. We also found that the imbibition rate of slickwater and its components in the order from highest to lowest is cleanup additive (0.087 cc/d), slickwater (0.058 cc/d), clay stabilizer (0.035 cc/d), defoamer (0.022 cc/d) and friction reducer (0.014 cc/d), indicating the synergy effect of slickwater components on the imbibition process and the role to optimize the cleanup additives during slickwater design. • Spontaneous imbibition tests were performed with modified imbibition cells. • The imbibition characteristics of slickwater and its major components were investigated. • All of the imbibition curves can be divided into three stages in this study. • The secondary microfractures were created during the imbibition process. • The imbibition is mainly through the microfractures. [ABSTRACT FROM AUTHOR]