Polyurethane Grouting Geothermal Lost Circulation Zones

Abstract

Abstract Polyurethane grouting has been successfully applied to a lost circulation zone in a geothermal well at Rye Patch, NV. Previously, failure to seal this zone resulted in the temporary abandonment of the well after twenty cement plugs, including 15 conventional, two thixotropic, and three with foam cement, were unsuccessful. Advantages of polyurethane grout are that the viscosity and setting time can be controlled to fit the job. The grout can be engineered to have a low viscosity while being pumped and then gain strength in a short period, minimizing "waiting-on-cement" and the potential for the plug to be washed out. Polyurethane has been successfully used in core drilling operations (slim holes) to stop lost circulation and stabilize boreholes; however, previous attempts to apply polyurethane grouting to large diameter geothermal boreholes have not been successful. The techniques applied to grouting with polyurethane at Rye Patch were adapted from civil engineering technology where polyurethane is becoming the grout of choice for sealing high cross flows. The success of the grouting of the loss zone at Rye Patch was a result of packing off the hole and squeezing the grout, using sufficient polyurethane to sweep away the drilling mud, and controlling the gel time. Polyurethane grout, a solution grout Chemical grouts are mixtures or solutions that react to produce a gel or foam resulting in an increase in viscosity. One common example of a solution grout is sodium silicate. Thus chemical grouting is not foreign to lost circulation control. However, sodium silicates alone have not been very successful in plugging major geothermal lost circulation zones with large open fractures. Besides the difficulty in controlling placement, a major problem with sodium silicate is its low viscosity and yield strength. One category of chemical grouts is pure solutions or resins. Resins, which include polyurethanes, are solutions of organic products in a solvent, capable of reacting. This category includes single component prepolymerized polyurethanes that require water to react and two component polyurethanes that are mixed and react with each other. Polyurethane can be engineered to have a low viscosity while being pumped and then gain strength in a short period, thereby minimizing "waiting-on-cement" and the potential for the plug to be washed out. The role of polyurethane in civil engineering grouting has been reviewed and compared to traditional cement grouting by Bruce et al.1 They noted that one of the most difficult challenges facing the grouting industry is grouting when there are high water flows into or through civil engineering structures such as dams, tunnels, and quarries. Polyurethane is rapidly becoming the civil engineering material of choice for dam remediation and for sealing boreholes with large voids and high cross flows.1,2,3 Past Work Past work on applying polyurethane grout to geothermal lost circulation zones included both "successes" and failures. Past tests deemed "successful" included work at elevated pressure and temperature (300°F or 150°C) in a test chamber4 and at ambient temperature and pressure in an engineered full-scale borehole/loss circulation zone.5 These tests were deemed a "success" because of significant expansion of the polyurethane; however expansion is not required for successful polyurethane grouting. Past tests deemed unsuccessful include high-pressure laboratory tests and field tests at the Geysers geothermal field.6,7 Since those tests, new polyurethane formulations have been developed that react better in aqueous environments. Also, techniques for controlling the process have improved. Past work targeted elevated pressure and temperature lost-circulation zones. Deployment of the chemicals was by a somewhat complex two-chamber tool lowered into the well.