Study On The Chemical Stimulation Techniques For Enhanced Geothermal Systems (EGS)

In the context of global warming,governments are very great importance to energy conservation and emission reduction.Geothermal energy as a kind of renewable and clean new energy,has become one of the effective substitutes for traditional fossil fuels.Research shows that in developed countries,the use of Enhanced Geothermal Systems(EGS Enhanced Geothermal Systems)power generation,almost not affected by the external environment,almost no human environment pollution and destruction,and this energy is very rich,has great potential for exploitation and utilization.Enhanced geothermal systems(Enhanced Geothermal System,EGS)is an engineered reservoirs that has been created to extract economical amounts of heat from low permeability geothermal resources,formed by artificial geothermal reservoir in deep low permeability rock mass in the economical production of heat energy of geothermal system.In the EGS project,the hydraulic contact between the production well / injection well and the artificial geothermal reservoir fissure network is often poor,which can not meet the production capacity demand of EGS.In order to expand the fracture network,increasing fracture permeability,usually using EGS chemical stimulation,that is use lower than the fracture pressure injection pressure injection chemical stimulus to the heat reservoir fracture,depends on its chemical dissolution to increase the permeability of the formation.In this study,we focus on the granodiorite rock samples collected from Gonghe basin in Qinhai province,through the static and dynamic two groups of chemical stimulation experiments,research chemical stimulus dissolution of rock and formation of altered minerals,reveal the chemical stimulus mechanism to improve the penetration rate of the rock.It will provide a theoretical basis for the implementation of chemical stimulation technology for the enhanced geothermal system of Gonghe basin in the future,and has some guiding significance.Static chemical stimulation experiments on high temperature and high pressure reaction kettle as experimental apparatus,carried out under the different temperature,different time and different chemical stimulus.The results show that:(1)sodium hydroxide,hydrochloric acid and mud acid all can improve the permeability of the rock,and mud acid effect the most significant.(2)In the experimental process with the increase of temperature and time,for sodium hydroxide,reaction after the rock sample permeability increases;for the hydrochloric acid stimulant,reaction after the rock sample permeability increased slightly,but not significantly;the soil acid concentration is less than 5% HF,elevated temperature?reaction time and the concentration of HF,permeability of rock samples were significantly increased,but when the concentration of HF was 7%,rock mineral acid strong chemical reaction,altered minerals block micro cracks formed in the reaction previously,sometimes the permeability of rock samples have lower.(3)Sodium hydroxide dissolution ability strongest of quartz,after the reaction mainly dissolved ions in the solution for Si,metallic elements such as Ca,Mg,Al and Fe in alkaline solution is easy to produce precipitation,the concentration of these ions in the solution is very low.After reaction,Na and Si and Al combine to form analcite,and the concentration of Na decreases,and the decrease of Na concentration increases with the increase of reaction temperature.(4)hydrochloric acid mainly dissolution feldspar minerals,most K and Na in the solution after reaction;Hydrochloric acid react to quartz dissolution ability is very weak,the concentration of Si is the lowest compared with other metal elements.In addition,altered mineral was not detected in the solution.(5)Mud acid can severe dissolution of quartz and feldspar,K mainly comes from potash feldspar dissolved in the solution,K is easy to combination of Al,Si and F to generate sheet potassium fluoroaluminate andvcube shaped potassium fluosilicate,with the increase of HF concentration,the greater the amount of the alteration minerals.Si mainly comes from quartz and feldspar dissolved,the concentration of Si increases with the concentration of HF,in solution Si supersaturated generated spherical silicon dioxide.To simulate the chemical reaction after chemical stimulus injecting into the well drilling and permeating the rock fracture in the actual project,this article ran dynamically chemical stimulation experiment by core flow instrument.The experimental results showed that:(1)the rock fracture rate was enhanced after injecting hydrochloric acid and mud acid.The increased efficiency of rock fracture rate is obviously bigger with mud acid than hydrochloric acid.When injected with alkaline stimulus such as sodium hydeoxide,the rock fracture rate did not increase but reduced.(2)For the chemical stimulus with low dissolving capacity such as hydrochloric acid,the slower injection flow rate can increase the contact time between the acid with the rock.This could result in the mineral dissolving.For the mud acid with stronger dissolving capacity,The degree of corrosion of rock samples can be highest in relatively low injection flow rate.The dissolved constituent is more likely to be deposited from the liquid phase and deposited in the sample fissures,causing the permeability of the sample to relatively decrease.(3)Hydrochloric acid and mud acid improved the permeability of core by depositing the feldspar minerals on the surface of the fissure.Mud acid is significantly more soluble in feldspathic minerals than hydrochloric acid.The dissolving capacity of sodium hydroxide is limited in depositing the feldspar minerals,but significantly better in depositing quartz mineral than hydrochloric acid and mud acid.After the reaction with the rock,the dissolved component was precipitated in an unsteady form,and caused the core permeability to decrease.The decrease of permeability increases with the increase of injection flow rate.According to the static and dynamic flow of chemical stimulation,known that the stimulation effect of soil acid on rock sample is better than hydrochloric acid and sodium hydroxide.The concentration of HF in acid soil seriously affects the stimulating effect,when the concentration of HF is 3~5%,which not only can effectively dissolve rock,to produce a new fracture and permeability increase,but also not to produce a large number of alteration minerals which in turn reduces the permeability of rock,is the best chemical stimuls for Gonghe basin implementation of enhanced geothermal system.