Acid Stimulation of Geothermal Wells in Central America

Abstract

Unconventional energy sources have become one of the keys to keep the world energy supply along with a sustainable development. In Central America, geothermal power is one of the most important energy sources, given the limited fossil fuel reserves in the area. The region imports 75% of its energy requirements, and geothermal-generated electricity in many countries represents 10 - 25% of the national supply. In fact, Central America is one of the world's richest regions in geothermal resources, according the world energy council. Despite that the region has an estimated potential for geothermal electricity generation of 4,000 MW, the actual installed capacity is only 508 MW (CIA world fact book). Most of the geothermal projects in Central America produce below their potential due to two main factors: depletion and low productivity (or injectivity) due to scaling in tubulars and formation damage. The damage is typically due to several causes, including drilling mud damage, production fines migration, silica plugging and scales in the reservoir. This paper describes the results of a five-year well stimulation campaign in El Salvador and Nicaragua, in several geothermal fields, run by different operators. More than 20 stimulation treatments were performed using different acid systems in volcanic and metamorphic rocks. The laboratory observations, their analyses and corroboration with the field data show that the combination of a properly designed treatment and special acid blends has lead to significant improvements in the stimulation of such reservoirs. Comparison of results in different reservoirs with varying stimulation treatments is also included as reference. Introduction Geothermal power consists in sourcing energy from earth's heat. This heat comes ultimately from the earth's core (estimated temperature 4,400 to 6,100°C), which heats the earth's mantle, as shown in the figure 1. In locations where magma from the mantle is closer to the surface (e.g. in volcanoes, mid ocean ridges, hotspots, etc.), an abnormally high temperature gradient is observed. Geothermal reservoirs are usually found in such high geothermal gradient locations. Subsurface water is heated by hot rock in these locations. If this water is able to flow (e.g. the subsurface rock is permeable or fissured), then a well can be drilled to enable this hot water to be produced to surface, as represented in the figure 2. Then, if the water is hot enough, steam from it is able to drive an electric power generator (Aboud). A very common example of such hot subsurface water locations are geysers. History of geothermal power generation started using steam directly from such geysers. With time, in order to increase the steam flow rate and the steam temperature, geothermal wells were drilled. Today, several geothermal fields are located in areas where no natural surface steam flow (e.g. geysers) is present.