Tracer-test Based Methods To Evaluate The Effect Of Reservoir Stimulation In Enhanced Geothermal Systems

In recent years,China has significantly increased the proportion of geothermal energy in energy consumption,which is both an urgent need for national energy structure adjustment and an important way for the country to achieve the dual carbon goal.Currently,the total amount of geothermal energy development and utilization in China has jumped to the top in the world,but its proportion in energy consumption is less than 1%.The vast majority of geothermal energy is stored in deep high-temperature and low-permeability rock masses with a buried depth of more than 3 km.It is urgent to vigorously develop deep geothermal resources and promote the stepwise increase in the utilization of geothermal energy resources.Strengthening the permeability of deep high-temperature rock mass through hydraulic fracturing and other methods and forming an enhanced geothermal system is necessary.We also need the test to accurate evaluation of reservoir fracturing effects.Cross well tracer testing is the most effective and commonly used technical approach to evaluate the connectivity between injection and production wells and the permeability of reservoirs after fracturing and reconstruction.However,in the early stage of deep geothermal reservoir fracturing,there is usually only one geothermal well used for reservoir fracturing,and it is not possible to conduct cross well tracer tests.At the early stage of reservoir reconstruction,there is still a lack of effective methods for permeability evaluation;In deep geothermal reservoirs,cross hole tracer tests usually only obtain a single tracer breakthrough curve,and their interpretation is often based on the assumption of uniform media(single equivalent permeability),but there is still a lack of permeability heterogeneity distribution and uncertainty analysis.Based on the actual geological and construction conditions of the first dry hot rock development demonstration site in the Republic of Qinghai Province,this study proposes a single well injection and extraction tracer test process and interpretation method system that can be used for permeability evaluation in the early stage of fracturing reconstruction;An improved inversion method for interwell connectivity tracer test data is proposed for quantitative evaluation of the spatial distribution of heterogeneous permeability between wells;The proposed method has achieved good results in the application at Gonghe site,providing scientific guidance for the formulation of fracturing construction and thermal energy acquisition plans at Gonghe site.Research shows:(1)During the initial stage of fracturing at Gonghe Site,a single well injection and pumping test was conducted,and the tracer breakthrough curve obtained through monitoring was used to calculate the average weekly permeability value of about 0.8D at the initial stage of fracturing;(2)A multi-phase pulse type cross hole tracer test was conducted at Gonghe site.The average seepage velocity,average permeability,and dispersion in the three phase tests were calculated using analytical methods.Based on the second phase tracer test data after fracturing,the estimated permeability zone value was inversely estimated to be between 1 D～10~2 D;After calculating the permeability,it is predicted that the root mean square error between the tracer concentration and the true value in the second phase of the tracer test is less than 0.1 mg/L,confirming the rationality of the interpretation of the spatial distribution of permeability.(3)Based on the comparison of the estimated permeability around the fracturing well before and after the fracturing transformation,the permeability of the high permeability area around the fracturing well in Gonghe Dry Hot Rock Field will be increased by 6.25 times after the reservoir fracturing transformation in 2020Relying on the research on the tracer test process measured and the data from the dry hot rock development demonstration,a method system for evaluating the effect of deep reservoir reconstruction based on the tracer test has been formed.By comparing the initial and later stages,it is possible to quantify the effect of fracturing modification on improving reservoir permeability.