Aquifer Chemistry From Wellhead Measurements

Papazian, Harold A., Consultant Papazian, Harold A., Consultant Copyright 1979, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Petroleum Engineers, Inc. This paper was presented at the 1979 SPE of AIME International Symposium on Oilfield and Geothermal Chemistry held in Houston, Texas, January 22–24, 1979. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expy., Dallas, Tx. 75206. Abstract In order to control scaling in a geothermal well, brine chemistry from the aquifer through production temperatures and pressures must be production temperatures and pressures must be understood. Using equilibrium constants at appropriate conditions and measuring chemical concentrations at the surface, it is possible to deduce some of the chemistry in the aquifer. Introduction One of the major problems in the exploitation of geothermal energy from hot brines is scale formation during production and utilization of such fluids. An understanding of how and under what conditions scale formation takes place may lead to methods of inhibition of the problem. Fundamental to such an understanding requires knowledge of the brine chemistry at production temperatures and pressures and the changes in chemistry concomitant with reduction in both T and P during utilization. The present communication discusses chemistry observed at the surface and how such measurements may be used to interpret some of the chemistry of the aquifer. Although the discussion is based on the East Mesa KGRA, Holtville, CA, the approach is applicable to other geothermal brine systems. The predominant scale from wells at East Mesa is calcite (CaCO3). The important parameters are pH and the constituents controlling it, the concentration of Ca++ in solution and its temporal variations, and the chemical reaction mechanism that occurs during flashing leading to calcite scale. In the discussion below it will be demonstrated that (1) the pH of the wells at East Mesa are controlled by CO2; (2) the Ca++ concentration in the brine is determined by the reaction of CO2 with CaCO3 in the aquifer; (3) the formation of CO3 flashing that precipitates the Ca++ as calcite scale is brought about precipitates the Ca++ as calcite scale is brought about by the disproportionation of HCO3; (4) several different equilibria may be used to calculate a consistent value for the pH in the aquifer, and the success of this calculation indicates some of the minerals to be found in the aquifer. Discussion The wells at East Mesa are designated as Mesa 6-1, Mesa 6-1, Mesa 8-1, and Mesa 31-1. The bottomhole temperatures were measured after well completion and were found to be 204, 188, 179, and 154 degrees C, respectively. The total dissolved solids are about 22,000, 4500, 2000, and 2500 ppm, respectively. With the exception of Mesa 6-1, they are approximately 6000 ft. deep. Mesa 6-1 was originally drilled to about 8000 ft, but presently it is several hundred feet less for unknown reasons. Since the wells are not of high salinity, only concentrations will be used in calculations below. This will serve to demonstrate the approach, and refinements using activities can be made when appropriate. The equilibrium constants used in the calculations are taken from Helgeson. Control of pH by CO2 These wells have considerable amounts of CO2 with large temporal variations in concentration. Fig. 1 shows the results obtained from Mesa 6-2 over a period of several months. The method for CO2 analysis is outlined below. Most samples were collected from Mesa 6-2 during low flows (50 gpm). Two samples were taken when the well was flowing at 146 gpm and 186 gpm. The measured pH indicated that at these flow rates the well was not pH indicated that at these flow rates the well was not flashing below the wellhead. Mesa 8-1 and 31-1 wells were flowed through a 2-in line for 24 hours prior to sampling. Sampling was done at ground level at a valve in a 1-in, pipe that connected to the well in the "well cellar" some 6 ft below ground level. In order to collect "unflashed" brine a 3/8-in. stainless steel cooling coil, immersed in an ice slush, was attached to the valve on the 1-in. pipe.