Thief-zone plugging mechanism and application of calcite particles in fractured formations

Abstract

Lost circulation in fractured strata during drilling incurs additional costs and leads to difficulties in promoting drilling safety and efficiency. Plugging-while-drilling is a feasible means to address lost circulation in fractured formations. The particle size distribution (PSD) of plugging particles is determined empirically; therefore, it is often not matched with the fracture sealing requirement. This study investigates the lost circulation mechanism of fractured strata and identifies a calcite particle-based material as the preferred plugging agent. The plugging mechanism and the design of PSD and particle concentration are demonstrated. Based on the concentration, a plugging-while-drilling technique was developed for fractured strata. The results show that calcite particles tend to form the filling layer at the fracture inlet, which cuts off the leakage of the drilling fluid into the fracture, eliminating the drilling fluid pressure applied on the fracture surface. Thus, a stable sealing for the fractured formation is achieved, and the pressure-bearing capacity of the borehole wall is increased. The result also reveals the optimal mixing and concentration models for calcite particles with various diameters. The plugging technique based on calcite particles for fractured formations is applied in a field experiment. The results confirm that the technique can improve the chance of lost circulation prevention and thief-zone plugging in fractured strata and remarkably reduce both the event quantity of lost circulation and the volume of circulation loss. The findings of this research lay a theoretical basis to address lost circulation in fractured formations and, thus, have important practical significance.