Design and Evaluation of Lost-Circulation Materials for Severe Environments

Abstract

Summary Lost-circulation materials (LCM's) for geothermal applications were analyzedwith laboratory tools developed specifically for that purpose. Test results ofcommercial materials and mathematical models purpose. Test results ofcommercial materials and mathematical models for evaluating their performanceare presented. Physical attributes that govern the performance of LCM's ingeothermal wells are identified and correlated with test results. ". . . particles outside an effective range for a fracture of interestdo not contribute to a stable bridge, although they may act as filter material. Consequently, a tailored particle-size distribution for a narrow range offracture widths should provide the best plugging capabilities." Introduction As part of the U.S. DOE's Geothermal Technology Development Program, SandiaNatl. Laboratories conducted an extensive testing program to evaluate candidateLCM's to aid in solving the severe lost-circulation problems encountered ingeothermal-well drilling. Our work concentrated on high-temperature, fracture-dominated loss zones rather than the matrix loss zones more typical ofoil and gas drilling. The results, however, are applicable to the petroleumindustry, particularly because of the petroleum industry, particularly becauseof the inevitable progression into hotter, more fracture-dominated petroleumreservoirs. Mechanics of the Fracture-Plugging Process The effectiveness of a plug in preventing fluid loss into a fracture dependson the mechanical strength of the plug as well as its permeability. The portionof the plug permeability. The portion of the plug responsible for itsmechanical strength is the bridge, and the portion that controls the plugpermeability is the filter. The functional permeability is the filter. Thefunctional requirements for bridging and filter materials are sufficientlydifferent that few materials would be expected to perform both functionsadequately. As a result, a mixture of rigid, granular particles with morepliant flakes and/or fibers generally provides the best fracture-pluggingcharacteristics in the laboratory tests reported here and in Ref. 3. Single-Particle Bridging. If the dimension of a particle normal to thefracture direction is larger than the width of the fracture opening, b, single-particle bridging is possible (Fig. 1). This type of bridging can alsooccur inside a fracture at locations where it undergoes a sudden reduction inwidth. The particle is idealized as a beam of length L with particle isidealized as a beam of length L with a rectangular cross section of height hand depth D. The particle is held against the fracture wall by the pressuredifferential, p. The maximum bending stress in the equivalent beam occurs at midspan and iscompressive at the top of the beam and tensile at the bottom of the beam. Whenthese stresses exceed the compressive or tensile strength, S, of the bridgematerial, the material undergoes irreversible strain (i.e., plastic deformationor brittle fracturing), and plastic deformation or brittle fracturing), and thebridge fails.