Inversion Of Stress Field In Deep Geothermal Field Based On Multiple Conditions

The deep high-temperature geothermal field attracts more and more attention.According to the water content and development model,its thermal reservoir can be divided into hydrothermal type and dry hot rock type.At present,enhanced geothermal system(EGS)is the main model for the development of deep dry hot rocks,which has been applied in the European Union,Japan,South Korea,Australia and the United States.The case study shows that the characteristics of in-situ stress field,especially the direction and magnitude of the maximum principal stress,are the key factors for the successful development of deep dry hot rock.The stress field analysis of deep geothermal field in foreign countries began to carry out structural analysis before logging,so as to determine the direction of the field stress field.For example,Milford geothermal field in the United States uses small fault tendency,fault cliff direction,water resistance and focal mechanism solution to determine the stress field;Dikili geothermal field in Turkey uses 279 fault slip data from 33 locations to determine the direction of the ancient stress field;Bessel geothermal field in Switzerland uses focal mechanism solution to determine the fault extension direction of the reservoir,so as to determine the deep thermal reservoir In situ stress field.This paper mainly analyzes the direction and numerical value of the stress field of the geothermal field in Gonghe basin.Firstly,the mechanical analysis of the main faults in Gonghe basin is carried out by using the traditional geological method.We assume that the main fault causes the discontinuity of the regional block,then,between the faults,the geological body is continuous,and the corresponding stress state is gradual.Based on this,we use surfer software to carry out the regression analysis of the main fault acceptance mechanism.Compared with the traditional geological method,this method puts forward an accurate quantitative analysis to ensure the accuracy of regional stress analysis.Under the background that the main fault analysis is not accurate enough,the method of auxiliary small structure analysis is used.The main faults near the geothermal field in Gonghe basin are investigated.Based on the fault kinematics,the stress mechanism of the small faults is analyzed,and the maximum principal stress tendency and dip angle near the site are regressed.A joint fracture method based on shear failure criterion is proposed to determine the in-situ stress field.In order to reduce the error to the greatest extent,a total of 80 joints were measured in the field.The length,width,mechanical properties and filling of each joint were recorded in detail.In order to reduce the influence of multi-stage superposition of stress on joint fissures,we mainly analyzed the joints with a length of more than 1 m,and compared the statistical results of Triassic granite and Neogene mudstone sandstone.Due to the large statistical quantity of joint fissures,the error is reduced to a certain extent.An analysis method combining surface faults with recent earthquakes is proposed.In order to invert the geostress field in the dry and hot rock mass at the depth of 2500-4000 m,it is not enough to only analyze the surface data,so we analyzed the main earthquakes in Gonghe Basin in the past 70 years.The deep geostress regression is carried out.By comparing the stress mechanism and offset between the surface fault and the source,the relationship between the source and the fault is obtained,and the deep extension of the fault is obtained.At the same time,through seismic analysis and statistics,we know that the hidden Chaka layihai fault in the basin is extremely active.The west,East and north main faults are relatively active.The main fault in the south is relatively stable.After the completion of the above work,the boundary load method is used to find the appropriate in-situ stress boundary by establishing the model and constantly changing the boundary conditions.Different from the previous assumed boundary conditions,this paper obtains the load boundary based on the field geological survey,which greatly reduces the multiplicity of inversion and reduces the inversion cost while controlling the accuracy.At the same time,according to the existing logging data,the temperature of Gr2 well in the dry hot rock site is 236 ? at the depth of 3705 M.In order to better fit the geological conditions of the geothermal field in Gonghe basin,in addition to the vertical load,the stress gradient change caused by the increase of temperature is also considered.Through the analysis and calculation of MIDAS software,the main stress curve of the geothermal field in Gonghe basin is obtained.At 3500 m,the maximum principal stress is 82 MPa,the minimum principal stress is 45 MPa and the middle principal stress is 70 MPa.The analysis results are compared with the existing in-situ stress test data in China to ensure the rationality of the inversion of in-situ stress curve.In this paper,through the combination of field geological investigation and laboratory test,the stress field analysis of deep high temperature geothermal field is carried out.This method has the advantages of large analysis range,relatively accurate direction,economic reliability and so on.Compared with the traditional qualitative analysis method of geology,the analysis is more accurate.In the follow-up well logging work,this method can reduce the workload of in-situ stress measurement,and obtain the distribution characteristics of the continuous in-situ stress field.Especially,with the support of sufficient in-situ stress survey data in the field,the reliability of in-situ stress loading conditions is improved,and the in-situ stress field value obtained from load condition inversion analysis is more accurate.Since our analysis data are from the whole geothermal field of Gonghe basin,the stress field of dry hot rock development site is obtained by regression analysis and numerical simulation.In the process of tectonic movement,there are many fractures in the actual stratum,which will release the local stress field in the stratum,resulting in the sudden change of stress direction and size in the local area.Under the background of this heterogeneous reservoir,based on the stress field analysis of regional geothermal field,the change trend of regional stress field is mainly clarified,and the stress field including size and direction of dry hot rock field is different.Because there are only Gr2 wells in the site,the core is relatively precious,and the differential strain test was not conducted immediately when the core was taken out.Now the stress release of the core is over,and the differential strain measurement has lost its function.In the subsequent development of the site,further work is needed.