The Impact Of Water-rock-gas Interaction On The Formation Porosity And Permeability In CO₂-EGS

With the rapid economic development of community, the problems of global warming andenergy shortages are given close attention. CO₂-EGS （use CO₂as heat transmission fluid ofenhanced geothermal system） extracts heat from geothermal reservoir is a system imitating thedeep natural geothermal loop system. It uses CO₂as a geological resource and has importantsignificance on CO₂emission reduction.The effect of water-rock-gas interactions on the formation porosity and permeability inCO₂-EGS increase its instability.This paper focus on the effect of water-rock-gas interactionsof CO₂-EGS has important theoretical significance and practical value.This paper regards a typical sandstone （the4th group of Quantou group in Songliao Basin）as geothermal reservoir and use a combination of numerical simulation and laboratoryexperiments to explore the effect of water-rock-gas interactions on formation porosity andpermeability as follows.1.We use numerical simulation technology to study the characteristic of interaction ofwater-rock-gas on CO₂-EGS then discuss its effect on formation porosity and permeabilityand heat transfer fluid flow.We also discuss the effect of various factors on water-rock-gasinteractions through sensitivity analysises.2.We select the mineral playing a dominate role on the effect of water-rock-gasinteractions by analyzing the numerical simulation results. Then we explore the variation of it inwater-rock-gas interaction and the impact of multiple factors on it through laboratoryexperiments simulating the environment of the geothermal reservoir.We can obtain the following conclusions through numerical simulation techniques.1.When CO₂is injected into the reservoir,the dissolution and precipitation of rock mineralshas following phenomenons.In the fractured channel, calcite, chlorite, K-feldspar dissolve and quartz, illite, kaolinite,ankerite precipitate. Siderite firstly precipitate then dissolve.In the matrix, calcite dissolves slightly first then precipitates. The pattern of other mineraldissolution and precipitation in the matrix is generally similar to the fracture domain, but theamounts are much larger than those in the fractures. Precipitation of carbonate minerals causes asubstantial decrease in porosity in the matrix which is favorable for CO₂mineralization. 2.Calcite playes a dominate role on the effect of water-rock-gas interactions Thedissolution of calcite has a decisive influence on the porosity and a small increases can causepermeability change considerably and increase thermal carrier fluid flow rate3.Rock mineral composition, the temperature of injection CO₂and the inter well pressuregradient of CO₂–EGS greatly influence changes of formation porosity and permeability in thewater-rock-gas interactions. The role of ground water chemical composition is not significantto the long time–scale gas-fluid-rock interactions. When the temperature of injection CO₂isincreased, the inter well pressure gradient is reduced and the calcite content of minerals isdecreased, the amounts of calcite dissolution is reduced which is not conducive to the increaseof permeability. The increase of chlorite content can enhance the precipitation of ankerite whichexacerbates the reduction of porosity in the matrix.4.If the pH of formation water is between5to8, the main source of H⁺is CO₂（aq）decomposition. Therefore the role of initial H⁺content of formation water on water-rock-gasinteractions is less affected. In the long time–scale water-rock-gas interactions, liquid-phasecomponents of the water such as Ca²⁺manly come from mineral dissolutions in the geothermalreservoir. Therefore the affection of non-H⁺component content on gas-fluid-rock interactionsis little.Calcite is a mineral that has a dominant influence on the change of porosity andpermeability characteristics of fracture channel in the thermal reservoir on CO₂-EGS. Theresults are shown as below..1.When the temperature of formation is reduced, the calcite dissolution is weaken. Whenthe pressure of formation is increased, the dissolved CO₂content is increased and the calcitedissolution is exacerbated.2.Within the impacts of salt effect and common ion effect, calcite dissolution increaseswith increasing NaCl concentration and reduction on Ca²⁺concentration.In order to decreasethe instability of CO₂-EGS at the early stage of system operation, the reservoir which haslower NaCl and high Ca²⁺concentration of groundwater is appropriate as the CO₂-EGSgeothermal reservoir.