Physical plugging of lost circulation fractures at microscopic level

Drilling fluid loss into formation fractures is one of the most common and costly problems encountered during the exploration and development of oil and gas resources. At present, the most extensive solution is to use physical lost circulation materials (LCMs) to form high-strength plugging zone in fractures. However, it is still unclear how the plugging is initiated and formed in the fracture. In this paper, a microscopic visualization experimental device for the formation of plugging zone is used to observe the dynamic plugging performance of spherical materials, flaky materials and fibers in fracture. Experimental results show that the formation of fracture plugging zone can be divided into retention stage and plugging stage. Spherical materials have three main retention modes: single-particle straining, dual-particle bridging and multi-particle bridging. Flaky mate-rials assist the retention and plugging of spherical materials through three modes: embedded, intercepted and supported. Fibers take part in the retention of spherical materials through three ways: forming a net at the entrance of the fracture, forming a net after bridging the particle materials, and transverse filling in the fracture. The results achieved in this work provide a basis for the selection and design of LCMs for lost circulation control in deep fractured tight reservoir.