Experimental Study on Microstructure of Temporary Plugging Zone and Relevant Plugging Capacity based on CT Images within Different Particulates Sizes

Abstract

AbstractTemporary plugging fracturing technology is an effective way to control fracture propagation in shale oil and gas development. However, insufficient plugging pressure often causes the failure to promote new fractures. Therefore, it is essential to reduce plugging permeability and increase plugging pressure. The permeability of the plugging zone is closely related to its pore structure, so the quantitative study of microscopic pore structure can provide a basis for reducing permeability. This paper introduces a new method based on micro-CT to characterize the microstructure of temporary plugging zones. Three kinds of particulate temporary plugging agents with sizes of 2-4mm, 1-2mm, and 0.4-0.8mm were used as the research objects. The samples of the temporary plugging zone in natural accumulation and compaction state were prepared by a self-made mold. The total pore structure and parameters (porosity, pore radius, pore throat, and tortuosity) were obtained by three-dimensional (3D) reconstruction of CT images. Finally, the displacement device was used to test the plugging pressure of the temporary plugging zone. Under natural accumulation, the porosity and tortuosity of the temporary plugging zone are independent of particulate size. The pore radius decreases with the decrease of particulate size. In the compaction state, particulates and particulates are cemented with each other, resulting in apparent changes in the pore structure. With the decrease in particulate size, the pore structure changes more obviously. For example, the pore radius of 2-4mm decreases from 889.27um to 736.33um, while the pore radius of 0.4-0.8mm decreases from 370.15um to 63.94um, with a reduction of 82.9%. The plugging pressure test results are consistent with the microscopic pore results. In conclusion, the smaller the particulate, the easier the particulate compression deformation, the smaller the pore radius and porosity, and the denser the plugging zone. Although small particulate-size temporary plugging agents can produce large pressure, forming an effective plugging zone in the fracture is difficult. Therefore, large particulates can bridge the fracture, and small particulates are filled in the pores of the plugging zone formed by large particulates to generate a tight plugging zone. In this paper, we provide a new application of Micro-CT for quantifying the microstructure of the temporary plugging zone. The effect of the properties of the temporary plugging agent on the structure of the plugging zone can be clarified from the microscopic point of view by using this method, which provides a new means for the design and optimization of temporary plugging materials.