Experimental Study on the Influence of Slickwater on Shale Permeability

There are two diametrically opposite views of the influence of slickwater on shale permeability among scholars at home and abroad. We used the shale outcrops rock samples from the Lower Silurian Longmaxi Formation in Sichuan Basin. The permeability of these dry samples before and after immersion in different solution systems were tested by pulse attenuation method. The experimental results show that the impregnation of different slickwater components and standard salt solution can promote the increase of the permeability of shale samples. The stress sensitivity of shale samples after liquid immersion is medium weak to weak. The sample stress sensitivity is weak after soaked by the synergist solution and Drag reducing agent solution, and the sensitivity of the sample stress is medium weak after immersed by the standard saline solution, defoamer solution and antiswelling solution; The Ki/K0 of the shale sample after liquid immersion on σi/σ0 is consistent with the exponential stress sensitive evaluation model. With the increase of soaking time, the increase of sample permeability increases first and then decreases

Establishment and Application of Fractal Capillary Tube Bundle Model of Porous Media

In view of the problem of statistical regression constant in the model of capillary tube bundles in the porous media, a capillary bundle percolation model with fractal geometry was reconstructed. The function expressions of the fractal coefficient and Kozeny constant were deduced. The relationship between the macroscopic fractal properties of porous media and the fractal dimension and the micro pore parameters were obtained. Results show: Fractal coefficient is a function of fractal dimension, maximum pore radius and minimum pore radius; The macroscopic physical properties of porous media are a function of the fractal dimension and the radius of the capillary (the maximum capillary radius and the minimum capillary radius). The expression does not contain any empirical or experimental constants. In the fractal capillary percolation model, the relationship between the three kinds of surface volume, skeleton volume and pore volume are the same as the traditional equal diameter straight capillary bundle model. The Kozeny constant can be accurately described by the function expression of the z-h coefficient, which is used for correcting the difference between real and ideal porous media model

Spontaneous Imbibition Characteristics of Slickwater and its Components in Longmaxi Shale

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

Investigation of Slickwater Effect on Permeability of Gas Shale from Longmaxi Formation

Knowing of gas shale permeability reduction is vital for gas production from shale gas reservoirs stimulated by multistage hydraulic fracturing. However, the role of shale matrix permeability reduction is not well addressed for evaluating the contribution on gas well production. In this study, the experiments of slickwater treatment at different injection pressures, injection volumes, and slickwater compositions were conducted by using an experimental apparatus based on the steady-state method. The results show that slickwater flow in the core is seriously affected by microfracture. The cumulative gas volume through the core with microfracture increases nearly linearly with time. Then, we analyzed the effect of injection pressure gradients, injection volume, and slickwater compositions on the permeability reduction and flowback efficiency. The permeability reduction and flowback efficiency both increase slightly at beginning and then nearly linearly in the middle and gradually at last with the increase of injection pressure gradient. Permeability reduction and flowback efficiency both decrease quickly at first and then slightly with the increase of injection volume, indicating only a small proportion of slickwater, which occupied much more small pores, can be displaced and flow back to the entrance piece of core sample. The permeability reduction and flowback efficiency after different chemical solution treatments are in the order from highest to lowest of cleanup additive, slickwater, clay stabilizer, defoamer, and friction reducer, indicating the synergy effect of slickwater compositions on the permeability reduction and flowback efficiency. These results can be helpful for optimizing the slickwater formulation and understanding the damage mechanism of shale formation during hydraulic fracturing

Investigation into Yaw Motion Influence of Horizontal-Axis Wind Turbine on Wake Flow Using LBM-LES

The dynamic yaw motion of the wind turbine will affect the overall aerodynamic performance of the impeller and the corresponding wake flow, but the current research on this issue is inadequate. Thus, it is very necessary to study the complicated near-wake aerodynamic behaviors during the yaw process and the closely related blade aerodynamic characteristics. This work utilized the multi-relaxation time lattice Boltzmann (MRT-LBM) model to investigate the integral aerodynamic performance characteristics of the specified impeller and the dynamic changes in the near wake under a sine yawing process, in which the normalized result is adopted to facilitate data comparison and understanding. Moreover, considering the complexity of the wake flows, the large eddy simulation (LES) and wall-adapting local eddy-viscosity (WALE) model are also used in this investigation. The related results indicate that the degree of stability of tip spiral wake in the dynamic yaw condition is inversely related to the absolute value of the change rate of yaw angular speed. When the wind turbine returns to the position with the yaw angle of 0 (deg) around, the linearized migration of tip vortex is changed, and the speed loss in the wake center is reduced at about the normalized velocity of 0.27, and another transverse expansion appeared. The directional inducing downstream of the impeller sweep surface for tip vortex is clearly reflected on the entering side and the exiting side. Additionally, the features of the static pressure on the blade surface and the overall aerodynamic effects of the impeller are also discussed, respectively

Impact of Cleanup Additive on Methane Desorption on Longmaxi Shale

After multistage hydraulic fracturing, it is essential to understand the effect of fracturing fluids trapped in shale gas reservoirs on the shale-well. To address the effect of cleanup additive as a major composition of slickwater fracking fluids on methane desorption after hydraulic fracturing, we performed a series of experiments, including X-ray to obtain the mineralogical composition of the gas shale sample, low-pressure nitrogen adsorption to obtain pore size distribution, and series of adsorption and desorption tests. We compared the effect of different methane densities of adsorbed phase on methane absolute adsorption and results demonstrate that the methane adsorption is significantly undervalued. The methane adsorption isotherm and desorption isotherms before and after cleanup additive treatment were well fit by Freundlich model. Furthermore, we compared the desorption isotherms, desorption efficiency, free gas content as well as cumulative gas production before and after treatment. These findings indicate that cleanup additive treatment results in the decrease of adsorption capacity and the increase of adsorption intensity. There exists an intersection between two curves of desorption efficiency before and after treatment. Cleanup additive significantly translates adsorbed phase into free phase to improve free gas content, and increases cumulative gas production. These results can be very helpful for reserves evaluation, knowledge of the role of cleanup additive both increasing slickwater flowback and improving gas recovery