Assessment of Silica Scaling Around Injection Wells of the Berlin Geothermal Field, El Salvador, Using Field Experiments and Chemical Modeling

摘要

Berlin Geothermal Field is one of the two reservoirs in exploitationfor the generation of electrical energy in El Salvador. Silica scaling in reinjection wells occurs and clogs the pores reducing their permeability. After rapid cooling within the steam separation units, the fluid circulates towards the reinjection wells in the surficial piping system and later within the well until it reaches the reservoir and moves within the rock formation. Polymerization of silica occurs when silica minerals are precipitated from the solution. During the time the fluid circulates throughout the external pipe system, precipitation of silica minerals does not occur because the induction time for polymerization (or polymerization time) has not finished. Experimental results of silica polymerization and deposition and radiotracers tests allow the calculation of polymerization and precipitation times and groundwater velocitities. This information is used in this paper to calculate the area around the wells likely to be affected by silica scaling. Using the fastest velocity found in the tracer tests a maximum radius of 72 m away from the well is found. However, only a very small fraction of the water can achieve the highest velocity. More representative of the damage zone is the radius calculated with the mean velocities for water movement between wells. Considering the fastest mean velocity, a radius of 9 m is found. The damaged zone could have an area ranging from a few centimeters up to 9 m. Comparison of these results with geochemical modeling work (Castro et al., 2006) show that for Berlin Geothermal Field an area for silica scaling around the wells of 10 m could produce 4-5percent pore reduction per year or about 30percent in 6 years. If the radius is smaller, the well will clog in a shorter time, as it probably happened in TR-1A in 1999. These results show that field experiments and geochemical modeling can help to predict or assess silica scaling in geothermal reinjection wells, and set the basis for complete groundwater flow, solute transport, and reaction modeling.