| As one of the research hotspots in the new energy field,geothermal energy is of great significance to mitigating climate change and accelerating energy transformation.The Xiong’an New Area is located in the middle of North China Basin and is abundant in geothermal resources.The Gaoyuzhuang Formation is the recently discovered second hydrothermal reservoir with higher temperature and better water quality.However,regional tectonic movements created a great quantities of natural fractures,making the reservoir non-homogeneous and strongly insufficient to satisfy the development requirements.Therefore,the quantitative characterization of fractures in reservoirs is the basis for geothermal development and utilization.The acidzing improvement implemented for low porosity-permeability zones is necessary.This doctoral thesis focuses on the problems of difficult characterization of deep carbonate geothermal reservoir and the further research on the mechanism and kinetic parameters of carbonate acid-rock reaction under the influence of multiple factors.This paper takes the carbonate geothermal reservoir of the Gaoyuzhuang Formation in the Rongcheng Bulge of Xiong’an New Area as the research target,and systematically studys the regional geothermal geological and hydrogeological characteristics and propose the geothermal energy convergence mechanism,adopting the integrated workflow of data collation and analysis-field investigation-indoor experiment-field test-geological modeling-numerical modeling-comprehensive study.Based on the characteristics of natural fractures,this thesis statistics the fracture network geometry parameters and length distribution patterns,establishes a three-dimensional discrete fracture network model,quantitatively portrays the natural fracture distribution,and proposes a power law function of fracture apeture and fracture length of the Gaoyuzhuang Formation.Acid rock reaction kinetic experiments were conducted to reveal the reaction mechanism and obtain the multi-mineral reaction kinetic parameters.A geothermal well was selected for acidzing tests,and the monitoring curves of injection flow rate and bottomhole pressure were analyzed to clarify the permeability enhancement.A site-scale acidification prediction model is established to simulate the geochemical reactions caused by acid injection,quantify the mineral dissolution and precipitation law,study the spatial and temporal evolution of porosity and permeability,and obtain the porosity-permeability evolution ontogenetic relationship of acidification modification.The effect of reservoir acidification modification under different permeability,temperature,rock mineral composition,and supercritical CO2 fluid conditions was comprehensively evaluated.Based on geomechanical theory,the activation possibility of natural faults under acidification modification conditions is quantified and the risk of induced seismicity was evaluated.The research results can not only provide support for the development and utilization of geothermal resources in Xiong’an New Area,furthermore,they can help grasp the natural fracture distribution pattern of carbonate geothermal reservoirs and promote the related research work on water-rock reaction mechanism.The Gaoyuzhuang Formation belongs to the lower part of Jixian System,which is distributed in the whole region of the Rongcheng Geothermal Field,and the cap layers are the Quaternary,the Neoproterozoic and the Paleoproterozoic.The top plate of the reservoir is buried at a depth of about 2000m,and the geo-temperature at a depth of3000m is 70 oC～80 oC,and the average geo-temperature gradient within the layer is 0.688℃/hundred meters due to the influence of convection.The average rock density is 2.81g/cm3,the average thermal conductivity is 5.28 W/m K,the average specific heat capacity is 840 J/kg K,the average radioactive heat generation rate is 0.29μW/m3,and the average radioactive heat generation rate of intrusive rocks is 1.07μW/m3.The geothermal water is originated from the atmospheric precipitation in the Taihang Mountains west of Baoding,and the age of the water is between 20,000 and 40,000 years.High-angle fractures are predominant in the Gaoyuzhuang reservoir,which is favorable to the vertical transport of hot water.The reservoir fractures are significantly influenced by regional tectonics.The reservoir fractures in the research area are divided into the EW fracture set and the NW fracture set,and the former has a more concentrated distribution of strike and dip angle,showing the characteristics of influenced by the Xushui Fault.The three-dimensional discrete fracture model quantitatively characterized the reservoir fracture distribution,with an average fracture line density of 0.3894fractures/m for the entire reservoir section,a predicted maximum fracture opening of 0.9mm,and a maximum fracture permeability of 5×10-8 m2.The reservoir permeability predicted by the ODA method and the hydrothermal coupled model inversion method varied between 1×10-15 m2 and 2.5×10-13 m2,and the spatial distribution of permeability was heterogeneous,and the distribution tendency was in good agreement with the logging results.The main constituent minerals of the Gaoyuzhuang reservoir are dolomite,followed by quartz and clay minerals.Under the action of hydrochloric acid,dolomite,calcite and illite in the rocks dissolved,while potassium feldspar and plagioclase dissolved inconspicuously.The acidification reconstruction test at the site shows that the unit water surge is increased from 0.024 m3/h m to 0.024 m3/h m,and production increase is very significant.The influence magnitude of each factor on acidification reconstruction is:temperature and mineral composition>local permeability>acid concentration.（1）The influence of acid concentration on the dissolution ratio of rock chips is relatively small,and the dissolution ratio of 15 wt.%HCl can reach 84.1%.The dissolution ratio of 20 wt.%HCl was only increased by 2.1%.Considering the acidification results and acid consumption,15 wt.%HCl can be selected as the main acid for acidification reconstruction.（2）The higher the temperature,the faster the mineral dissolution rate,from 40 oC to 100 oC,the core dissolution of the time required reduction from 20 min to5 min.In terms of reservoir response,the higher the temperature,the better the reconstruction in the area near the well.The reservoir temperature increased from 70°C to 130°C and the permeability change increased from 30×10-15m2 to 54×10-15m2 after the acidification reconstruction.And with the increase of injection temperature and reservoir temperature difference,the bottom of the layer section can obtain better transformation,and the maximum permeability change of the bottom can reach 108×10-15m2 under the condition of 130°C.（3）In terms of rock mineral composition,the acidification results of reservoirs with high clay are poor,and the average permeability increases by 15.9×10-15m2.The reservoir with high calcite is better acidized and modified with an average permeability increase of 91×10-15m2.High calcite is more likely to generate acid-etching wormholes,with a maximum permeability increase of up to 184×10-15m2.Calcite dissolution has a restricting effect on the dissolution of dolomite.（4）The local permeability of the reservoir affects the result of acidification modification by controlling the seepage field,and the scope of modification increases with increasing permeability.The single acidizing method is not suitable for low permeability reservoirs(<5×10-15m2),and the modification results are poor and the scope of modification is very small.Under higher permeability(>10×10-15m2)conditions,the acidizing and modification results are ideal.（5）From the perspective of acid rock chemistry,supercritical CO2 cannot achieve production improvement of Gaoyuzhuang reservoir in a short time（20 days）,but it may be a better attempt as fracturing fluid.It is suggested that acid discharge can be increased for reservoirs with high temperature and high calcite to enhance the scope of reconstruction.A combination of hydraulic fracturing and acidizing is needed for the low permeability segment(<5×10-15m2).The kinetic model of mineral reaction based on transition state theory can well describe the dissolution process of minerals.The reaction rate constants of dolomite,calcite and illite can reach 2.4×10-4 mol/m2/s,5.3×10-1 mol/m2/s and 9.5×10-2 mol/m2/s under the acidic regime,which are 4,5 and 15 orders of magnitude higher than in the natural state,respectively.Based on the field acidification modification test,the pore-permeability evolution relationship for the Gaoyuzhuang reservoir was established as follows:permeability=initial permeability×（porosity/initial porosity）7.The analysis based on Monte Carlo simulation shows that the faults in the Xiong’an New Area are very stable in natural state,with no sliding caused by acidzing modification and exploitation.Some specific fault surfaces（strike NWW,dip 60°）have a high sliding tendency and require more refined investigation and long-term monitoring jobs. |
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