A Coupled Simulation of Fracture Sealing of Lost Circulation Materials at Elevated Temperatures

Lost circulation is one of the major problems encountered in drilling geothermal and HTHP (high temperature and high pressure) formations. A part of these problems can be attributed to the high temperature and presence of hard and fractured rocks. Consequences and treatment of these lost circulation events can be extremely costly. Underbalanced drilling and LCMs (lost circulation materials) are the two most common methods to address the lost circulation problem. However, some LCMs have proved to be unsuccessful due to the unpredictable nature of fractures, complicated dynamics of LCM, as well as the change of mechanical properties of LCMs under high-temperature conditions. Long-distance circulations in high-temperature environments usually lead to the decline of size, strength, and friction coefficient of LCMs in parallel to the reduction of drilling fluid viscosity, and eventually failure in sealing fractures. In this paper, we developed a coupled CFD-DEM by combining computational fluid dynamics with discrete element methods to simulate the fracture sealing process by LCMs. The viscosity of drilling fluid, particle size, Young’s Modulus, Poisson's ratio, and friction coefficient of LCMs were determined to represent possible variations under geothermal conditions. A series of sensitivity studies were conducted to further understand the fracture sealing efficiency of LCMs at elevated temperatures. The results show that a reduction in particle size, friction coefficient, and Young’s modulus lowers bridging’s probability, and slows down bridging initiation, but deepens the sealing depth, and tighter but the result is an unstable sealing zone. We noticed that the bridging mechanism changes from single-particle bridging to dual-particle bridging as particle size reduces. Also, the reduction of drilling fluid viscosity makes the sealing zone form faster and shallower.