Excessive fluid loss during drilling causes a variety of problems, including increased friction, the arrest of a drilling column, loss of information from returning mud and drill chips, and underbalanced mud pressure and formation fluid influx. Fluid loss causes substantial disruption to drilling operations and increases the risk and the cost of drilling during a geothermal project. In the field, a variety of both natural and synthetic materials have been used to reduce or stop fluid loss by inducing clogging of high-permeability fractures and fracture zones. For effective clogging, the lost circulation materials need to take a foothold within and/or at the entrance to a fracture, stopping fluid flow by reducing the permeability of the accumulated material (filter cake). In this study, we examined the clogging behavior of several commonly deployed commercial materials in single fractures and fracture zones, testing both single materials and combinations. A commercial high-temperature, high-pressure permeability plugging tester (Ofite) was modified for improved flow, pressure, and temperature control. Additionally, a new, internal test module for this tester, containing a model glass fracture or a pack of glass beads, was developed to examine the performance of the materials, rather than using standard porous plates and slotted disks. A series of clogging tests was conducted using this device at 90 ˚C. Lost circulation materials were added to a synthetic drilling fluid containing bentonite clay, which was flowed through the fracture model by rapidly increasing the differential pressure across the test module (7.62 cm long) up to 3.4 MPa. The observed behavior can be generally categorized into four types: (1) no clogging (flow through); (2) blocking but with flow through; (3) clogging (permeability blocking); and (4) dynamic partial blocking with pressure oscillations. The results generally show superior performance gained from combinations of lost circulation materials compared to single material tests. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This presentation describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

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