A Study Of Chemical Stimulation On Enhanced Geothermal Systems(EGS)in Yingcheng Formation Of Songliao Basin,China

The Enhanced Geothermal System(EGS)develops in last four decades,which involves the injection of cold fluid into low-permeability hot dry rocks to open the fractures,heat exchange between fluid and the created thermal reservoir,hot water extraction from the production wells,and the electricity generation by hot water.Due to the poor fractures in the reservoir,the weak hydraulic connection between the production/injection wells often limits the heat extraction from the hot dry rocks.Therefore,efforts have been made to create fracture networks and open the existed fractures to improve performance of EGS.Chemical stimulation techniques increase the permeability of the reservoir by injecting aqueous chemical agents(such as mineral acids)into the formation under a relatively low injection pressure.The chemical reaction between the injected fluid and the hot rocks allows the mineral dissolution within the distance of several meters surrounding the injection point,and thus results in the increase of porosity and permeability and the improvement of reservoir frustration.When compared to the traditional hydraulic/thermal stimulation,the chemical stimulation takes the advantages of lower cost and lower risk of induced earthquake.As a consequence,chemical stimulation have been proposed and employed as an important method for reservoir stimulation.This paper regards Yingcheng formation as geothermal reservoir and investigates the effects of traditional chemical agents then develops new chemical agents.Both laboratory tests and numerical modeling approaches are employed to understand the interaction between chemical agents and the minerals and the effect of chemical stimulation in the reservoir.The focus of our study was to document the dissolution and precipitation of minerals result from acid-rock interactions.Based on above study,new acid system and chemical stimulation technology have been presented to improve the effect of stimulation.The core-flow experiments were performed to evaluate the processes of chemical reaction by injecting different chemical agents into the core under different conditions for investigating chemical stimulation of an EGS reservoir.The core of volcanic rock was recovered from the research thermal reservoir of the Yingcheng Formation,in the Songliao Basin,China.The permeability in the hot dry rocks can be enhanced by chemical stimulation using low concentration mud acid as chemical stimulator.The permeability increases with the injection rate.The enhancement of permeability is mainly induced by dissolution of k-feldspar and albite.However,the precipitation of amorphous silica may block the fractures in the reservoir during the acid-rock reactions.We should boost the mode of "high flow rate,no well shut-in" as a rule in optimizing operation cases.The tradition mud acid system is likely to be spent immediately next to the wellbore,and will not penetrate deeper into the formation.Moreover,high temperatures reduce the efficiency of corrosion inhibitors(and increase their cost)as well as increasing the acid/rock reaction rate.Therefore,a new retarded acid system has been developed to improve the effect of chemical stimulation in the study.In this study,we also illustrate how reactive transport models are used to evaluate the impact of chemical stimulation on the enhanced geothermal(EGS)reservoir when developing such models.Chemical reaction processes in whole system will be predicted with reactive transport simulator TOUGHREACT.We also discuss the effect of various factors on acid-rock interactions through sensitivity analysises.Geophysical and geochemical parameters of Yingcheng Formation of EGS reservoir of central depression in Xujiaweizi area,Songliao Basin were used.The injections of HC1 and mud acid often result in dissolution of minerals in the EGS reservoir.Reservoir porosity and permeability can be enhanced around the injection well.The enhancement of permeability is mainly caused by dissolution of calcite,and k-feldspar and albit in the reservoir.Mover,the precipitation of amorphous silica in the core was indented to cause acid-induced formation damage in the reservoir during the acid-rock reactions.When the calcite content increased,the maximum enhancement of porosity and permeability is improved but the effective penetration is reduced.When the reactive surface of minerals increased,the dissolution of k-feldspar and albite,and the precipitation of amorphous silica are increased.When the fluid injection rate increases,the precipitation of amorphous silica is weaken.The CO2-conventional acid stimulation and hydraulic-chemical stimulation is proposed in this study because of the disadvantage of conventional mud acid stimulatio.The effect of above chemical stimulation techno logies have been investigated with stimulator TO UGHREACT and TOUGH2Biot.In the CO2-conventional acid stimulation,the acidity of CO2 stimulation agent is weaker and it can penetrate larger extent along the flow path due to the dissolution of calcite in the reservoir.The hydraulic stimulation through cold fresh water injection can enhance the effect of the chemical stimulation in an EGS reservoir.Hydraulic stimulation could extend the effect distance of chemical agent and inhibit the precipitation of amorphous silica in the chemical stimulation.The effect of hydraulic stimulation during short period on chemical stimulation is similar with hydraulic stimulation during longperiod.Based on the simulation results,an alternate scheme of hydraulic stimulation and chemical stimulation is proposed.