I. Introduction Temporary plugging fracturing technology is a key technology for increasing the production of tight oil, and the propagation of multiple - cluster fractures is an important research content in temporary plugging fracturing. The purpose of this study is to deeply understand the propagation mechanism of multiple - cluster fractures during the temporary plugging fracturing process and provide theoretical support for optimizing the design of temporary plugging fracturing.
II. Experimental Research (A) Temporary Plugging Diversion Fracturing Experiments In the laboratory environment, temporary plugging diversion fracturing experiments were carried out using a true triaxial fracturing simulation system to simulate the multi - cluster sand - blasting perforation completion of horizontal wells. The influence of the particle size and concentration of temporary plugging agents, the number of perforations in a single cluster and the number of clusters on the temporary plugging pressure, the multi - fracture diversion mode and the distribution of temporary plugging agents was studied. The experimental results show that using small - particle - size temporary plugging agents in fractures and large - particle - size temporary plugging agents in sections is helpful to increase the temporary plugging pressure and promote the diversion of multiple fractures; the peak temporary plugging pressure increases with the increase of the concentration of temporary plugging agents, and after reaching a peak at a certain concentration, further increasing the concentration has no obvious effect on the increase of the peak temporary plugging pressure; the fracture initiation pressure and the peak temporary plugging pressure show a downward trend with the increase of the number of perforations in a single cluster, a smaller number of perforations in a single cluster is beneficial to increasing the fracture initiation pressure and the peak temporary plugging pressure, and has a more significant effect on controlling the propagation of multiple - cluster fractures; a smaller number of clusters is not conducive to increasing the total number and complexity of artificial fractures, and a larger number of clusters is difficult to be effectively plugged; the temporary plugging agents in fractures are mainly concentrated in complex fracture areas (especially at the intersections of fractures), and the temporary plugging agents in sections are mainly distributed near the perforation holes of the perforation clusters that cause complex fractures. (B) Triaxial Fracturing Research Triaxial fracturing research was carried out to understand the influence of transverse mechanical parameters on the propagation of fractures with multiple coplanar perforations in horizontal wells. The experimental results show that due to the use of coplanar perforations, the coplanar perforation clusters in horizontal wells can successfully initiate two parallel transverse fractures and can still present a guided non - uniform fish - bone - like structural fracture propagation. Under the conditions of a small horizontal principal stress difference and a large pumping rate, transverse fractures and axial fractures will combine to form complex fractures.
III. Numerical Simulation In view of the defects in the fracture propagation model in temporary plugging diversion fracturing, the extended finite - element method was used to establish a mechanical model for the multi - stage temporary plugging fracture propagation in tight oil reservoirs. Numerical simulations were carried out considering key factors such as the horizontal stress difference, the viscosity of fracturing fluid, the injection speed and the initial fracture angle that affect the propagation morphology of diversion fractures. The numerical simulation results show that the main controlling factor affecting fracture propagation is the horizontal stress difference. The smaller the horizontal stress difference, the easier the fracture is to divert and the larger the diversion radius; the influence of the viscosity of fracturing fluid on the fracture diversion radius and diversion angle can be ignored; the larger the injection speed during construction, the easier the diversion and the larger the diversion radius; when the initial fracture angle is greater than 90°, the fracture diversion radius is significantly larger than when the initial fracture angle is less than 90°, indicating that the fracture is more likely to divert.
IV. Productivity Analysis Using the potential theory and the superposition principle, the author respectively derives the linear productivity equations for horizontally fractured wells with multi - stage and multi - cluster fracturing considering fractures with finite and infinite conductivity. The production of each cluster and the total production are obtained by numerically solving the equations. The productivity distribution of fractured horizontal wells and the influence of fracture parameters on it are analyzed with specific examples. When the fracture conductivity is infinite, as all fracture half - lengths decrease, the total productivity decreases significantly. The outer fractures near the endpoints of the horizontal well contribute the most to the total productivity, and the contribution of the edge fractures in the same stage is greater than that of the middle fractures. When the fracture conductivity is finite, the contribution of the outer fractures near the endpoints of the horizontal well to the total productivity decreases, and the contribution of the middle fractures in each stage increases significantly, but the overall distribution characteristics are consistent with the former. The fracture conductivity has a great influence on productivity but is not linearly related. Dividing into three stages with two clusters in each stage can ensure relatively high productivity. In fracturing design, the fracture half - length and fracture conductivity (especially the outer fractures near the endpoints of the horizontal well) should be increased as much as possible, and there is an optimal value for the fracture conductivity.
V. Conclusions In this study, a method combining experiments and numerical simulations was used to deeply explore the propagation mechanism and influencing factors of multiple - cluster fractures in temporary plugging fracturing. The experimental results show that factors such as the particle size and concentration of temporary plugging agents, the number of perforations in a single cluster and the number of clusters have a significant impact on the temporary plugging pressure, the multi - fracture diversion mode and the distribution of temporary plugging agents; the numerical simulation results further reveal the main controlling effects of factors such as the horizontal stress difference, the viscosity of fracturing fluid, the injection speed and the initial fracture angle on fracture propagation. The results of productivity analysis provide an important reference basis for fracturing design. This study provides theoretical support for the optimization and application of temporary plugging fracturing technology and has important practical significance.