Experimental Study on True Triaxial Pressure-Bearing Plugging in Micro-Fractured Formation
Authors: Jiang Kaining , Hou Bing , Qiu Xiaojiang , Cao Wenqi , Wei Yimeng , Cui Zhuang , Elsworth Derek 
Published: September 2022

ABSTRACT:
The pressure-bearing capacity of the micro-fractured formation is low, and the leakage phenomenon is serious under the action of high-density drilling fluid. The pressure-bearing plugging can improve the pressure-bearing capacity of the formation that is vulnerable to leakage. In this paper, the sandstone outcropping was made into 295mm×297mm×300mm samples to conduct true triaxial pressure-bearing plugging physical experiments. The result shows:(1) Large granular lost circulation materials were accessible to accumulate around the open hole to form a dense mud cake, which greatly improved the pressure-bearing capacity of the formation. (2) Under the action of high pressure, the small granular lost circulation materials were rapidly pumped into the micro-fractured formation, forming a plugging layer in the fracture. (3) It was found that when increasing the plugging slurry injection rate, the plugging zone was more prone to instability, and the final pressure-bearing value of the formation was reduced. This study explored the plugging effect of lost circulation materials with different particle sizes and ratios on the fractures and optimized the corresponding construction parameters by carrying out true triaxial pressure-bearing plugging experiments in micro-fractured sandstone formations under different working conditions, in order to provide guidance for pressure-bearing plugging site construction of micro-fractured sandstone formations.

1. INTRODUCTION
Loss circulation refers to a phenomenon in which the drilling fluid in the wellbore is lost to the formation under the action of the pressure difference during the drilling operation[1]. Economic losses to the global oil industry due to lost circulation run into millions of dollars each year. Pressure-bearing plugging technology[2,3,4] is a method for plugging lost formations and fractures when lost circulation occurs during the drilling process, which is significant to drilling engineering. The pressure-bearing plugging technology is to use different shapes and sizes of lost circulation materials to mix with drilling fluids in different concentrations by bridging, stacking and filling to form a plugged zone. The plugged zone can block fluid pressure transmission and fluid medium passing through. It reduce leakage, meet the needs of control and cementing by improving the pressure bearing capacity and bottom-hole pressure of low-pressure and weak formations[5,6]. Because of the low pressure-bearing ability of the micro-fracture formation and the serious leakage phenomenon under the action of high-density drilling fluid[7]. This paper carried out pressure-bearing plugging experiments based on the actual micro-fracture formation stress field, researched the effect of plugging slurry injection speed on the pressure-bearing capacity of the sandstone formation and different particle size ratio on the plugging effect of induced cracks by analyzing the pumping pressure curve.