Research On Low Injury Water-base Fracturing Fluid

Along with the development of Daqing Oilfield, the marginal lowporosity and low permeability oil layers becomes the main strength ofcompensating oil production decline. Reducing the fracturing fluid damageto matrix and propped fractures is precondition and key point of lowpermeable reservoir fracturing stimulation.On the basis of fracturing fluid injury reason analysis, the key point oflow injury fracturing fluid system research has determined: to reduce theresidual content, to reduce the fluid loss, to improve the gel breakingperformance, to enhance the compatibility between fracturing fluid, thereservoir and the formation fluid. Through massive theoretical analysis andthe indoor experiments, optimal low injury fracturing fluid system that suitsDaqing marginal low seepage oilfield was determined, and good fracturingstimulation effect has obtained in the field application.1.Damage reason analysis of fracturing fluid on formation andpropped fractures1.1 Damage reason of fracturing fluid on matrixThe damage of fracturing fluid filtrate on formation: clay hydrationswelling, dispersion, migration, water blocking, emulsification, and so on.The damage caused by solid phase in the fracturing fluid, mainlyincluding small particle size residual in broken liquid and the impurity inthe chemical additives blocking the hole throat, lead to matrixpermeability drop.1.2 Damage reason analysis of fracturing fluid on propped fractures1.2.1Damage of filter cake to fracture conductivity1.2.2 Damage of fracturing fluid concentration to fracture conductivity1.2.3 Damage of fracturing fluid residue to fracture conductivity2. Research on low injury water-base fracturing fluid systemAccording to analyzing the reasons that the fracturing fluid injuresthe formation and fracture conductivity, we focus on reducing residualcontent, decreasing the filter loss, improving the gel broken ability,enhancing the concertedness between fracturing fluid and thereservoir bed and the reservoir fluids during the research of low injurywater-base fracturing fluid system. Based on many experiments inlaboratory, low injury water-base fracturing fluid that is suitable to theDaqing low permeable periphery-oilfield is studied and applied.2.1 GelatinizerAfter evaluating on some performances, such as the ability toenhance viscosity, the content of water-insoluble substance, heat andshear resistance capability, CHJ-05 gelatinizer was chosen, and the primedensity of the gelatinizer in different temperature has been determined.CHJ-05 gelatinizer features low using amount, low cost, lowwater-insoluble substance and residual content, good heat and shearresistance capability. The gelatinizer's using amount is only 0.35%;viscosity remains are above 100mPa.s after 1 hour's shear;the residualcontent is only 167mg/l in 90℃.2.2 CrosslinkerBased on evaluating the gellant speed, thermo-stability in thecrosslinking system, shear stability and its influence to the formation'spermeability, the organic boron crosslinker WT – 04 was chosen. WT –04 can improve network structure of the fracturing fluid gellant aftercrosslinking;enhance the visco-elasticity and sheer resistance of thefracturing fluid gellant;reduce the gelatinizer's density greatly;reduce thefracturing fluid residual's absolute content;improve the fracturing fluid'sbroken gel performance;and reduce the injury of the fracturing fluidresidual and the concentrated gellant. Moreover, the WT-04 density is lowand does not need to adjust the base fluid's pH value. The gallant's flowperformance is good, and the confection is rather simple.2.3 Broken masticThrough analyzing the influence of the best time for the brokenmastic to the fracturing fluid's viscosity, capsule broken mastic, whichcan be used with high density and does not cause the fracturing fluid'sviscosity loss ahead, was selected. The binary broken mastic systemcombining the capsule broken mastic with the conventional broken masticwas adopted according to the infield tests, which effectively solve thecontradiction between the fracturing fluid gel carrying the sands' viscosityand the fast gellant broken. After making the artificial fracture, thefracturing fluid can be fast gellant broken and flow out of formation,which reduce the injury of the fracturing fluid to the reservoir greatly. Theinfield experiments indicated that the binary broken mastic system couldguarantee the fracturing fluid's viscosity in the injecting process of sands.The maximum sand ratio may reach 50%. After 40 minutes in thefracturing operation, the fracturing fluid can be let out from the formation,and the returning fluid's viscosity is less than 5mPa.S.2.4 clay stabilizerThrough the dispersion rate's and the expansion rate's experiments,against-swollen the system matching KCL with the low molecular weightpositive ion polymer was optimized. By using this system, the rockdebris' crash rate drop by 65%, debris' expansion rate drops by 54%.Whether preventing the clay disperse or the clay swelling, clay stabilizerhas the good effect to the periphery reservoirs. It also has goodcompatibility with the main agents of fracturing fluid and little influenceto changing the broken gellant.2.5 Cleanup additivecleanup additive The main functions of cleanup additive are toreduce returning resistance of the fracturing fluid, that is to reduce thesurface and interfacial tension of the fracturing fluid. Through evaluatingthe surface and interfacial tension of the fracturing fluid and the fracturingfluid performance, ZP-4 was chosen. It features low surface andinterfacial tension and good compatibility with fracturing fluid. Bymeasuring CMC value, the optimal density of ZP-4 was determined.When the density is 0.16%, the water surface tension is dropped from72.8mN/M to 25.12mN/M. The interfacial tension of ZP -4 solutions anddehydrated petroleum is dropped from 30.2mN/M to 0.37 mN/M.2.6 Fluid-loss agent's choiceAll kinds of fluid-loss agents have the certain influence to keeppermeability. In order to reduce the fracturing fluid's injury, one kind ofoil soluble HY106 was chosen as a low injury fracturing fluid-loss agent.It neither dissolves in the water nor in the oil. After the fracture operationis completed, HY106 can dissolve in the liquid state hydrocarbon, thusavoid injuring the strata and the proppant packing intervals. Adding 1%HY106 in 90 ℃ , fluid-loss coefficient is dropped from 4.08 ×10-4m/ min to 1.41×10-4m/ min , and the core sample's injury rate isdropped from 24% to to 20%.3.Low injury fracturing fluid performance evaluation3.1 Indoor evaluationLow injury fracturing fluid performance evaluation was conductedaccording to petroleum professional standard "SY/T5107-1995Water-base Fracturing Fluid Performance evaluation Method" and "SY/T6376-1998 Fracturing Fluid General Engineering Factor"The features of this system are as follows:(1) The fracturing fluid has good heat and shear resistance capability,the viscosity remains above 100mPa.s after 1 hour's shear under thereservoir temperature;it also has quick gel breaking speed, definitely, thegel can break completely in 4 hours by using binary breaking agent.(2) The broken liquid of low injury fracturing fluid has lowsurface/interfacial tension, good swelling inhibition effect, and gooddemulsion effect. It also has good fluid-loss reducing property by usingoil-dissolution fluid-loss reducing agent, which can dissolve in crude oilafter operation and cause no permanent damage to the reservoir.(3) The broken liquid of low injury fracturing fluid causes 20% lowerdamage to the reservoir permeability and 16.8% lower to formationconductivity compared to conventional fracturing fluid.3.2Field application11 wells in Gu137 block of west Pu oilfield and 23 wells in south Xingoilfield were fractured with this low injury fracturing fluid. Compared tothe conventional fractured wells, the average single well oil production isincreased by 2t/d. This indicated low injury fracturing can obtain bettereffect.4.Conclusion1. Based on the damage analysis of fracturing fluid filtrate to the matrixpermeability and the fracturing fluid residual and filter cake to fractureconductivity, this technology focuses on low injury fracturing fluid.2. New thickening agent is developed, and the performance of fracturingfluid is improved with less thickening agent. The thickening agentconcentration in the formula for 90 ℃ is reduced by 30% compared to theconventional fracturing fluid, the residual content reduces 48.9%, soformation damage is mitigated. The cost of new formula is close to theconventional one.3. Delayed crosslinker and breaking agent are screened, and according tothe effectiveness period of fracturing fluid viscosity, binary gel breakingsystem is determined. The breaking agent concentration and the reactiontime are adjusted to ensure fracturing fluid to have enough viscosity makingfractures, and also break completely to reduce reservoir damage.4. Based on low permeable reservoir geologic characteristics, rock claymineral content and reservoir sensitivity analysis, additives are screened,and the obtained fracturing fluid has features of complete demulsion,good clay stability, quick discharge, small fluid loss and low conductivityinjury.5. This technology has been applied on 34 development wells in Daqinglow permeable reservoir with 100% effectiveness ratio and very goodresults. Compared to the conventional fractured wells, the average singlewell oil production is increased by 2t/d.