Mechanism Of Chemical Interactions Between Water And Rocks During Hydraulic Fracturing In Tight Sandstone Reservoirs

Hydraulic fracturing has enabled the development of unconventional oil/gas,leading to an unconventional oil and gas revolution around the world.Under the background of global climate change and achieving carbon peak and carbon neutrality,unconventional natural gas,represented by tight sandstone gas,shale gas and coalbed methane,will play a key role in replacing coal and reducing CO₂ emissions.Numerous external fluids enter the gas reservoir and cause a series of water-rock interactions with the reservoir rock during hydraulic fracturing.These interactions can directly or indirectly affect unconventional oil/gas production.However,the type,processes and mechanisms of water-rock interaction during hydraulic fracturing,especially for tight sandstone gas reservoir,are not clear.Taking the Sulige gas field located in the Ordos Basin as an example,based on field hydraulic fracturing and laboratory water-rock experiments,the water-rock interaction mechanisms during hydraulic fracturing in tight sandstone reservoirs have been systematically and deeply studied by X-ray fluorescence spectroscopy（XRF）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,hydrogeochemical modeling,end-member mixing analysis,and isotopic tracing.The main conclusions include:（1）Based on the testing results of the drilled tight sandstone core samples in the study area,the petrology,physical properties characteristics and diagenetic process of tight sandstone reservoirs in the Sulige gas field are clarified.The results indicate that the tight sandstones of the gas-bearing reservoir are mainly composed of lithic sandstones and lithic quartz sandstones.The minerals are mainly composed of quartz（45.9%～81.6%）and clay minerals（16.5%～47.4%）,which are characterized by poor feldspar and rich kaolinite.Intergranular pores,intragranular solution voids,and intercrystal pores are developed in the reservoir,caused by the diagenetic process of compaction,cementation,and dissolution.The porosity ranges from 7.7%to 12.6%and the permeability from 0.16 to 1.42 m D.（2）Based on the experiment results of hydraulic fracturing in the field and water-rock interactions in the laboratory,the processes and mechanisms of water-rock interactions during hydraulic fracturing are identified and revealed in detail.The mixing between fracturing fluid and formation water dominates the chemical compositions of flowback fluid,which also are affected by water-rock interactions between fracturing fluid and tight sandstones.Field and laboratory experiments at different scales identified that mineral dissolution（calcite,salt rock,clastics）,mineral precipitation（barite,gibbsite）,redox reaction（pyrite oxidation）,and ion exchange and adsorption(Ca²⁺,Sr²⁺,Na⁺,B)occurred during hydraulic fracturing of tight sandstone reservoir.Additionally,the contributions of different water-rock interactions to the chemical components of fracturing flowback fluid were quantified.The concentration of Fe in flowback fluid increased by 40 mg/L due to pyrite oxidation,and Ba²⁺decreased by up to 28mg/L due to mineral precipitation,while about 20%～60%of Sr²⁺was adsorbed from water to rock by cation exchange.（3）Water-rock interactions during hydraulic fracturing will not only cause significant changes in the chemical components of flowback fluid,but also affect the tight reservoir.Influenced by water-rock interaction,fracturing fluid injected into the reservoir will dissolve and corrode tight sandstone,resulting in the increase of reservoir pores.However,it also causes formation damage such as the instability of clay minerals,precipitation and scaling.After water-rock interactions,illite minerals expand,disperse,and migrate,while kaolinite crystals break,fall off,disperse,and migrate in an alkaline fracturing fluid environment.The instability of clay minerals easily induces plugging damage to reservoir pores.In addition,the water-rock interactions can also cause secondary minerals precipitation such as barite and gibbsite,resulting in reservoir blocking.The simulation results show that gibbsite and barite precipitation can reach 2.5 mg/L and 11.7 mg/L during hydraulic fracturing,respectively.（4）In view of the groundwater pollution that may be induced by large-scale hydraulic fracturing activities,an identification model of groundwater contamination caused by fracturing flowback fluid was established.Based on the data of fracturing flowback fluid and groundwater quality in the study area,a groundwater pollution identification curve using strontium isotope(⁸⁷Sr/⁸⁶Sr)was proposed for the identification,evaluation and prediction of potential groundwater pollution.For the desert plateau in the northern Sulige area,⁸⁷Sr/⁸⁶Sr can identify groundwater contamination when only 0.49%～2.15%fracturing flowback fluid contaminated groundwater.According to the pollution and resource characteristics of fracturing flowback water,the diversified treatment and reuse methods such as storage at a different flowback time,reuse for making up fracturing fluid,and chemical elements recycling are put forward.Some suggestions are also put forward to strengthen groundwater monitoring in the study area,including establishing a groundwater quality baseline and the hydrochemical data set of hydraulic fracturing flowback water to prevent groundwater pollution.These studies and suggestions can provide guidance for the green and sustainable development of the Sulige gas field.