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Radial drilling-fracturing, the combination of the hydraulic fracturing and radial borehole, is a technology that can guide the hydraulic fractures to directionally propagate and efficiently develop low permeability reservoir. In this paper, an analytical model of two radial boreholes (a basic research unit) is established to predict fracture initiation pressure (FIP) from one particular radial borehole and the interference between radial boreholes when the hydraulic fracturing is guided by multi-radial boreholes. The model is based on the stress superposition principle and the maximum tensile stress criterion. The effects of in situ stress, wellbore pressure, and fracturing fluid percolation are considered. Then, sensitivity analysis is performed by examining the impact of the intersection angle between radial boreholes, the depth difference between radial boreholes, the radius of radial boreholes, Biot coefficient, and the number of radial boreholes. The results show that FIP declines with the increase of radial boreholes number and the decrease of intersection angle and depth difference between radial boreholes. Meanwhile, the increase of radial borehole number and the reduction of intersection angle and depth difference strengthen the interference between radial boreholes, which conduce to the formation of the fracture network connecting radial boreholes. Besides, FIP declines with the increase of radial borehole radius and the decrease of Biot coefficient. Large radius and low Biot coefficient can enlarge the influence range of additional stress field produced by radial boreholes, enhance the mutual interference between radial boreholes, and guide fracture growth between radial boreholes. In hydraulic fracturing design, in order to reduce FIP and strengthen the interference between radial boreholes, the optimization design can be carried out by lowering intersection angle, increasing radius and number of boreholes, and reducing the depth difference between boreholes when the conditions permit. The research clarifies the interference between radial boreholes and provides the theoretical basis for optimizing radial boreholes layout in hydraulic fracturing guided by multi-radial boreholes. [ABSTRACT FROM AUTHOR]

A regenerable polyvinyl alcohol/organic boron fracturing fluid system with 1.6 wt% polyvinyl alcohol (PVOH) and 1.2 wt% organic boron (OBT) was studied, and its main regeneration mechanism is the reversible cross-linking reaction between B(OH)4- and hydroxyl groups of PVOH as the change of pH. Results of rheology evaluations show that both the apparent viscosity and the thermal stability of the fracturing fluid decreased with the regeneration number of times increasing. In addition, the apparent viscosity of the fluid which was without regeneration was more sensitive to the shear action compared with that of the fluid with regeneration once or twice. When the fracturing fluid was without regeneration, the elasticity was dominating due to the three-dimensional network structure of the formed gel; the viscosity gradually occupied the advantage when the fracturing fluid was regenerated once or twice. The settling velocity of proppant was accelerated by both the regeneration process and the increasing temperature, but it was decelerated when the proppant ratio increased. Results of core damage tests indicate that less permeability damage was caused by the PVOH/OBT fracturing fluid compared with that caused by the guar gum fracturing fluid after gel breaking. [ABSTRACT FROM AUTHOR]

Artificial fracture parameter optimization with dimensionless proppant index method is an analytical approach widely used, but the effect of long-term artificial fracture conductivity performance change with time on the well productivity have not been taken into account. The optimization results by dimensionless proppant index method are not the best one in the view of practical production of stimulation wells. In this paper, combined with the experimental results of long-term proppant conductivity, transferring the fracture width and permeability in dimensionless proppant index method into the functions of time, the dimensionless cumulative productivity index has been presented though the integral of dimensionless proppant index with respect to time. And this result will serve as the optimal fracture parameter. Because of variable time being introduced into the formula and considering the change of proppant conductivity performance with time, the calculating method is more suitable in practice and more reasonable comparing to dimensionless proppant index method. The numerical solutions show that greater fracture width and higher concentration of proppant should be reached for low and extra-low permeability reservoirs with long-term conductivity taken into account in order to get more cumulative production. The views of lower demand artificial fracture conductivity and longer fracture for low and extra-low permeability reservoirs should be changed. This method can prolong the validity period of post-treatment production increase for single well and is of significance to evaluate operation effect. It is suggested that the influence of proppant concentration on long-term conductivity should be developed pertinently for certain areas, which could provides reliable reference for the application of dimensionless cumulative production index method. [ABSTRACT FROM AUTHOR]

For a better understanding of the proppants distribution in complicated hydraulic fractures, a numerical model was established to investigate the proppants transport in tortuous fractures after the shear displacement. First, a method combining the normal distribution random function and the random growth method was proposed to construct three-dimensional tortuous fractures. Because of the shear deformation, the tortuous fracture was divided into several branch channels, which was much more complicated than the parallel and smooth fracture. And then, proppants motion in tortuous fractures was simulated by the analysis of forces on proppants, considering the interaction between particles, between proppants and the slurry, and between proppants and the fracture surface. The accuracy of this model was verified by the comparison between the simulated result and the experimental observation. From the simulation under different conditions, it can be concluded that increasing the pumping rate and fracturing fluid viscosity and decreasing the proppant density are more effective ways to improve the proppant distribution compared with the increase of proppant concentration. However, problems of proppants distribution still remained by adjusting a single factor, which can be solved by the combination of multi-foctors. And the slurry pumping rate and proppant concentration needed to be optimized to avoid the sparse distribution of proppants in the fracture. This study is helpful to understand the mechanism of proppants transport in the complex fracture. • The normal distribution random function can be used to construct tortuous fractures. • Flow path of proppants is complex in tortuous fractures after shear displacement. • Changes of single factors cannot achieve the expected placement of proppants. • Proppant concentration and slurry velocity need to be optimized in proppant transport. [ABSTRACT FROM AUTHOR]

Fracture conductivity after fracturing fluid loss is significant for the optimization of pumping schedule and the productivity modeling. A new model was established to evaluate the fracture conductivity under closure pressure, considering the proppant deformation and the mechanical properties of the shale. The deformation of proppants and formation rock and the embedment of proppants were calculated by the contact mechanics. The average pore radius in the fracture was calculated. After matching the simulated embedment with experimental data, the mechanical properties of shale after fluid loss were obtained. Based on that, a mathematical model combined with Kozeny-Carman correlation was established. The accuracy of this model was also verified by experimental data. The simulation shows that the change of the mechanical properties of shale leads to more nonlinear variation of the fracture conductivity. Improving the fracture aperture is a better way to improve the fracture conductivity compared with raising the fracture permeability. • Mechanical properties of the shale after fracturing fluid loss change nonlinearly. • The larger the proppant, the more obvious the nonlinear change of its embedment. • Fracture porosity is less effected by proppant size comparing with proppant layers. • Increasing the fracture width is a better way to improve its conductivity. [ABSTRACT FROM AUTHOR]