NUMERICAL SIMULATION STUDY OF SILICA AND CALCITE DISSOLUTION AROUND A GEOTHERMAL WELL BY INJECTING HIGH pH SOLUTIONS WITH CHELATING AGENT

Dissolution of silica, silicate, and calcite minerals in the presence of a chelating agent (NTA) at a high pH has been successfully performed in the laboratory using a high-temperature flow reactor. The mineral dissolution and porosity enhancement in the laboratory experiment has been reproduced by reactive transport simulation using TOUGHREACT. The chemical stimulation method has been applied by numerical modeling to a field geothermal injection well system, to investigate its effectiveness. Parameters from the quartz monzodiorite unit at the Enhanced Geothermal System (EGS) site at Desert Peak (Nevada) were used. Results indicate that the injection of a high pH chelating solution results in dissolution of both calcite and plagioclase minerals, and avoids precipitation of calcite at high temperature conditions. Consequently reservoir porosity and permeability can be enhanced especially near the injection well. CALCITE DISSOLUTION USING CHELATING AGENTS Removal of calcite scaling from wellbores is commonly accomplished by injecting strong mineral acid (such as HCl). Injected strong acid tends to enter the formation via the first fluid entry zone, dissolving first-contacted minerals aggressively while leaving much of the rest of the wellbore untreated. An alternative to the mineral acid treatment is the use of chelating agents such as ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). Mella et al. (2006) performed lab experiments using EDTA and NTA to investigate the effectiveness of chelating agents for calcite dissolution in the formation. A laboratory reactor was designed and fabricated for investigating calcite dissolution using these agents under controlled conditions that simulate a geothermal reservoir; the setup and results will be discussed in the next section. Preliminary experimental data indicated that both EDTA and NTA are effective dissolution agents and that dissolution capacity increases with temperature. Such agents have the ability to chelate (or bind) metals such as calcium. Through the process of chelation, calcium ions would be solvated by the chelating agent, driving calcite dissolution. The kinetics of calcite dissolution using chelating agents is not as fast as that using strong mineral acids. The lower dissolution rate allows the chelating agent to take a more balanced path through the formation and more evenly dissolve calcite in all available fractures, rather than following the first fluid entry zone and leaving the rest relatively untouched. In the calcite chelating process, one EDTA molecule will associate with two Ca ions, EDTA + 2Ca = Ca2EDTA (1) allowing theoretically for the dissolution of two moles of calcite per one mole of EDTA. Two NTA molecules are required to dissolve three calcite molecules. 2NTA + 3Ca = Ca3NTA2 (2) NUMERICAL MODELING METHOD General Features The modeling of the lab experiment and field example was done with the non-isothermal reactive geochemical transport program TOUGHREACT, whose physical and chemical process capabilities and solution techniques have been discussed by Xu and Pruess (2001) and Xu et al. (2006). The program uses integral finite differences for space discretization (IFD; Narasimhan and Witherspoon, 1976). The IFD method provides for flexible discretization using irregular grids, which is well suited for simulation of flow, transport, and fluid-rock interaction in heterogeneous and fractured rock systems with varying petrology and complex model boundaries due to the presence of engineered structures. For regular grids, the IFD method is equivalent to the conventional finite difference method. An implicit time-weighting scheme is used for modeling flow, transport, and kinetic geochemical reactions. The program can be applied to one-, two-, or threedimensional porous and fractured media with physical and chemical heterogeneity, and can accommodate any number of chemical species present in liquid, gas and solid phases. A broad range of subsurface thermal-physical-chemical processes are considered under various thermohydrological and geochemical conditions of pressure, temperature, water saturation, ionic strength, and pH and Eh. Temporal changes in porosity and permeability due to mineral dissolution and precipitation are considered in the model. Changes in porosity are calculated from changes in mineral volume fractions. Several porosity-permeability relationships are considered in the simulator, including the cubic Kozeny-Carman grain model and Verma-Pruess model (1988). Reaction Kinetics Kinetics of mineral dissolution and precipitation is very important for chemical stimulation of an EGS reservoir. A general kinetic rate law for mineral dissolution and precipitation is used in TOUGHREACT (Lasaga et al., 1994)