Development And Performance Evaluation Of A Recoverable Clean Fracturing Fluid With Good Thermal Stability

Hydraulic fracturing is a critical technology for efficient development of the poor reservoirs.However,fracturing fluid which is called“the blood,of the fracturing still has some disadvantages.Poor heat and shearing resistance will result low viscosity retention rate of traditional polymer fracturing fluid in the case of high temperature shearing.Higher initial viscosity is essential for meeting the construction requirement,which will against the fracture height containment.Moreover,rich residues in polymer fracturing fluid will extremely harm the matrix permeability and fracturing conductivity.Clean fracturing fluids based on VES(Viscoelastic surfactant)are believed to be the best altermatives of traditional polymer fracturing fluids,while their heat resistances need to be further improved.Especially the higher costs limited their application in current low price of oil.Besides,environment pollution comes up with the hydraulic fracturing increasingly attracted people’s attention in the world wide.The flowback liquid must be treated before discharge.Environmental protection law of China encourages industries green manufacturing and resource reutilization.In comprehensive consideration of those problems,a clean fracturing fluid technology which aims at high temperature resistance and reutilization was proposed with the purpose of reducing the cost,promoting the flowback liquid treatment and environmental protection by this way.Thus,some results and knowledges were achieved via theoretical studies and indoor experiments.(i)A novel VES thickener(J-22)of Gemini quatermary ammonium was designed and syethesized from dicarboxylic acid,N5 N-Dimethyl-1,3-propane diamine,epoxy propane and methylamine.It has a stable molecular structure,low kraft point below zero.The CMC of this thickener is 6.5×10-4 mol/L,saturated adsorbing capacity is 10-6 mol/m2.the minimal cross-sectional area in saturated adsorption condition is 10-18 m 2 and the adsorption follows Langmuir Isothermal Adsorption.This thickener has no nessary to be pured in practical application and it can be dilliuted in to 60 wt%of ethanol solution in prior inorder to meet the requirement of fast preparing in the field.(ii)The aqueous solution of J-22 thickener behaves as a viscoelastic fluid,which shows an obvious elasticity.The fluid is in line with Bingham fluid of non-Temporal and non-Newton fluid,which can be well fitted with Herschel-Bulkley、Cross、Carreau and Carreau-Yasuda rheological models.Counter-ion of potassium chloride and thickener concentration almost has no influence on its fluid type and rheological models.However,the viscoelasticity will be affected,mainly as the increase of viscosity,viscoelastic modulus and relax time.(iii)The J-22 Clean fracturing prepared from 5 wt%of J-22 thickener and 1.2 wt%of potassium chloride can withstand 150℃ high temperatures.Evluation results showed that this fluid performed well,the drag reduction rate was 61%,and no obvisous settlement was found after 3 hours of static proppant suspension.It could be quickly broken without residues.Damage of the gel breaking liquid to matrix permeability is only 5.17-6.73%which was 14.17～18.44%of damage of guar fracturing fluid(36.49%)under the same conditions.(iv)The molecular structure of the J-22 thickener is much stable,which made it can matain a good integrity under the conditions of high temperature shearing.The J-22 fracturing fluid behaved as a dynamic equilibrium of reversible damage.The viscosity reduction was caused by the faster micellar dissociation come up with the increase of molecular thermodynamic and the solubility.The counter-ion of potassium chloride firstly promotes the micelle grovth and then make the micelles tighter,which enhanced the resistance of damage caused by high temperature shearing and then improved heat resistance of the J-22 clean fracturing fluid.(v)Futher improved J-22 clean fracturing fluid with formulas of 5 wt%of J-22 thickener,0.5 wt%of potassium chloride and 0.25 wt%of potassium bromide matained a viscosity over 40 mPa·s after being sheared 2 hours under the conditions of 160“C and 170 s’1,which meets the requirement of hydraulic fracturing constructions and updated the suitable temperature of clean fracturing fluid to 160℃.(vi)Thickener of J-22 can be recycled from the flowback liquid by means of phase separation.This method not only has advantages of simple operation,high recovery and low cost,but also the recovered product has a very high purity.Fracturing fluid prepared from reeovered thiekener ean meet the requirement of 120 ℃.This teehnique ean largely inerease the utilization rate of J-22 thickener,reducing the cost of clean fracturing fluid,making the treatment of flowback iquid more practical.This paper take the thickener of clean fracturing fluid as entry point,achieved a clean fracturing fluid system with simple formula and good performance,which updated the suitable temperature of clean fracturing fluid to 160“C.Moreover,the viscosity reduction under high temperature shearing condition was analyzed from the molecular scale.For the first time,a novel technique of thickener recycling by phase separation was proposed and proved from the molecular scale.This technique not only can largely increase the utilization rate of thickener of clean fracturing but also responsed the national call for the green manufacturing and resource reutilization.It can speed up the progress of zero emission in hydraulic fracturing construction.