Research On The Mechanism Of Microbial And Enzymatic Remediation Of Fracturing Fluid Formation Damage

As the demand of oil and gas increasing,the unconventional reservoirs such as low permeability and tight oil reservoirs have been a promising point in the oil and gas development.Now,hydraulic fracturing is the essential stimulation technology for the unconventional reservoirs.Hydraulic fracturing can increase the reservoir permeability leading to the improvement of the oil and gas production.However,the residue of fracturing fluids after hydraulic fracturing can plug the pore-throat in the fracture surfaces,causing heavy formation damage,which leads to the decrease of oil and gas production.Microbial and enzymatic remediation of formation damage that caused by guar-based fracturing fluids is a novel technology.In this technology,microbes are injected into the formation and the microbes will grow and transport in the porous media by taking advantage of the guar residue.In this process,the insoluble residue and filter cake will be degraded and flowed back.Then,the permeability will be recovered and the oil and gas production will be improved.Then microbial remediation in the porous media of oil reservoir is very complicated while the method and mechanism of this technology are the key for its application.Firstly,the physical and chemical properties of guar-based fracturing fluid were systematically studied,and a new method was explored to improve the thermal stability of guar gum in order to design the temperature-resistant microbial culture medium in which guar gum was the sole carbon source.Isolation and identification of gum-degrading bacteria were carried out to establish a guar-degrading bacterial strain system.The effects of formation environmental factors on the growth of bacteria strains were studied and the growth curves were used to study the grow characteristics in different media.Secondly,the biochemical mechanism of microbial and enzymatic degradation of guar gum was revealed through the change of physical and chemical properties of guar gum during the degradation.According to the main causes of fracturing fluid damage formation,the degradation of insoluble residue after fracturing fluid gel-breaking by microorganisms and their enzymes was studied through the change of weight and particle size during the microbial and enzymatic degradation.The thickness and desorption of guar gum filter cake during the microbial and enzymatic degradation were also measured to study the mechanism of filter cake degradation.Finally,with the PDMS microfluidic model,the injection pressure changes of the microscopic models with or with enzyme were studied to study the enzymatic degradation of guar gum in porous media.The formation and remediation process of fracturing fluid formation damage were simulated by artificial core experiments to evaluate the technical feasibility and efficiency of microbial and enzymatic remediation of guar-based fracturing fluid formation damage.The results show that guar gum solution is a shear thinning pseudoplastic fluid,and its viscosity increases exponentially with the increase of guar gum concentration,and guar gum has biodegradability and guar gum has thermal instability when the temperature is over 85?.It is found that low concentration of phosphate can significantly improve the thermal stability of guar gum and keep the guar gum at a high level of average molecular weight after high temperature treatment.In the phosphate treatment,the physical and chemical properties such as crystal structure,chemical structure,crosslinking property are not changed;27 strains of guar gum degrading bacteria are isolated and five of them including Bacillus aerius(3-2),Bacillus paralicheniformis(CGS),Bacillus sonorensis(XSJ),Anoxybacillus rupiensis(M1),and Geobacillus stearothermophilus(S1)are screened as the high-efficiency guar-degrading bacteria;For these guar-degrading bacteria,guar gum can be used as the sole carbon source.The optimum temperature and p H value of guar gum degrading bacteria are 50-70?and 4.84-11.00,respectively;The growth curves of bacteria in guar gum media show that microorganisms in guar gum have high growth amount and longer delay time;Bacillus aerius(3-2)is screened as a high-efficiency enzyme-producing strain;The highest activity of guar-degrading enzyme can be 3.4 U and the optimum time for guar-degrading enzyme producing is22 hours;The concentration of guar substrate and guar-degrading enzyme can significantly affect the rate of enzymatic reaction;Guar-degrading enzyme is a typical extracellular inducible enzyme.Guar and starch can be used as the carbon source for enzyme producing.The producing yield of enzyme is higher using organic nitrogen source,and the optimum p H for enzyme producing was 6.0-7.0;The enzyme has high activity in the application environment of 30-80?and p H 4.0-8.0,and this enzyme also has catalytic specificity for guar and its derivatives;Ba²⁺can activate the enzyme,but the enzyme is sensitive to high concentration formation water,glutaraldehyde fungicides and cross-linking agents;Microbial and enzyme degradation can significantly reduce the apparent viscosity and average molecular weight of guar gum;The main microbial degradation products of guar gum are small molecular polysaccharides,a small amount of reducing sugar and CO₂.The enzyme can degrade the guar into small guar gum,oligosaccharide,disaccharide and reducing sugar step by step.Microorganisms and enzyme can effectively degrade guar gum residue,the weight and particle size of residue decrease significantly;Microorganisms and enzyme can effectively degrade guar gum filter cake,that the thickness of filter cake decreases significantly,and the desorption effect of filter cake is obvious;Guar-degrading enzyme can effectively degrade the guar-based fracturing fluid in the porous media and it also can clean the residual fracturing fluid in the porous media up.Five bacterial strains have good remediation efficiency on the fracturing fluid damaged cores and the recovery rate was 43.46%-96.57%.Among them,Bacillus paralicheniformis(CGS)has the highest repairing efficiency and the recovery rate is above 85%,while the recovery rate of enzyme 3-2 is 73.93%-94.18%,which prove that microbial and enzymatic remediation of guar-based fracturing fluid damage is reasonable and feasible.