Geothermal Drilling: A Review of Drilling Challenges with Mud Design and Lost Circulation Problem

Abstract

The most expensive problem routinely encountered in geothermal drilling is lost circulation, which is the loss of drilling fluid to pores or fractures in the rock formations being drilled. Lost circulation represents an average of 10 percent of total well costs in mature geothermal areas, and often accounts for more than 20 percent of the costs in exploratory wells and developing fields. Well costs, in turn, represent 35 percent to 50 percent of the total capital costs of a typical geothermal project. Therefore, roughly 10 percent of the total costs of a geothermal project can be attributable to lost circulation. Geothermal reservoirs are typically underpressured, and this increases their susceptibility to lost circulation. In many cases, geothermal wells have been abandoned because of lost circulation issues, which can quickly put a geothermal project into economic difficulty. Thus, developing improved methods and smart materials that can effectively seal the cavernous-type rocks in geothermal wells can positively impact the total cost of the project. In the second part of this paper, different methods of curing lost circulation are discussed. The types of materials, classifications, application conditions, and limitations are reviewed. The fluid additives and LCM’s that are used for drilling high or low permeability sandstones are different from those that can be used to effectively drill carbonate rocks, vuggy, and cavernous formations. In addition, the fluids that is suitable for drilling wells with bottom hole temperature (BHT) up to 200 oF may not be suitable for drilling wells with BHT up to 400 oF. Placement of lost-circulation materials (LCM) is difficult because the top and bottom of the loss zone often are not well known. The LCM or cement being used to heal the loss zone are especially likely to migrate away from the targeted placement zone if drilling has continued well past it into another loss zone, or if there is considerable rat hole below the original loss zone. Typical drilling fluids additives (cellulose materials, calcium carbonate, graphite) used in drilling conventional and unconventional oil and gas wells may not be suitable for drilling geothermal wells because of temperature limitations. Some of the physical attributes that govern the performance of LCM’s in geothermal wells were identified and correlated with laboratory test results