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Fracture fluids main functions Open the fracture Transport proppant Desirable features 1 Compatible with the formation and reservoir fluids 2 Provide good fluid loss control 3 Exhibit low friction pressures 4 stable break clean rapidly 5 Economical Fracture fluids history Pre 1950s 1950s 1969 1970s oil based water based with GUAR First crosslinked GUAR with 10 oil HPG gelling agent Currently 70 of treatments are water based 25 are energized 5 are oil based Fracture fluids fluid types Water based fluids Advantages low cost high performance ease of handling Disadvantages water sensitive formations damage due to polymers Polymers to viscosify fluids 1 GUAR high molecular weight long chained sugars natural 610 residue 2 HPG chemically treated guar cleaner 2 4 residue 3 HEC cellulose derivatives Fracture fluids fluid types Crosslinkers to increase viscosity of fluid at higher temperatures alternative to increasing polymer loading but expensive Borates Titanate Zirconium Crosslinking Fast Controlled Reversible Yes No Shear degradation No Sensitive 225 F 325 F High Delayed system 8 10 required Variable Temp limit Friction PH Fracture fluids fluid types An increase in T or pH will accelerate the crosslink reaction If crosslinking is too rapid then higher friction pressure and shear degradation occurs If crosslinking too slow then proppant may settle in wellbore Desirable to have crosslink time fluid time in wellbore Dual crosslink system Fast to ensure adequate viscosity at perfs Slow ensures viscous fluid in fractures Fracture fluids fluid types Oil based fluids Advantage Application to water sensitive formations Disadvantages Costly environmental and safety concerns Quality of gels is poor and residue is high Fracture fluids fluid types Foamed fluids Addition of CO2 or N2 to base fluid Foam Quality volume of frac fluid that is foam Range is 60 to 90 quality foam to be stable and have sufficient viscosity Typical is 70 quality Fracture fluids fluid types Foamed fluids Advantages Improved flowback cleanup performance Good proppant transport Low fluid loss thus applicable to sensitive formations CO2 enhances solubility of oil Also CO2 has higher density thus lower surface treating pressures Nitrogen is less dense however requires less to create foam thus reduction in material costs Disadvantages Costs Operational Sand concentration limit Fracture fluids Buffers Bactericides additives maintain pH prevent viscosity loss due to bacterial degradation Stabilizers enhances stability of gels at higher temperatures Breakers polymers break at defined temperature need chemical breaker if temperature below this defined temperature Surfactants promotes formation of foams and promotes cleanup of fracturing fluid in the fracture Clay stabilizers control formation of clay swelling and migration Fluid loss additives reduce excessive fluid loss thus minimize premature screenout Types silica flour emulsions Fracture fluids Example additives Fracture fluids mixing Batch Mixed together on surface Bactericide polymer salt clay stabilizer Crosslinker is borate Fly Mixed while job is pumping Crosslinker is Titanate Sodium Hydroxide to raise pH for borate crosslinkers Breakers fluid loss additives quality assurance vs cost Fracture fluids 1 2 3 4 5 6 7 8 9 design criteria Formation temperature and fluid rheology Treatment volume and rate Type of formation Fluid loss control requirements Formation sensitivity to fluids Pressure Depth Type of proppant Fluid Breaking requirements Fracture fluids Fluid Rheology Definition Science of the deformation and flow of matter Most important variable viscosity f g T t C Effect of temperature on the viscosity of a 40 lbm 1000 gal HPG solution SPE Monograph Vol 12 1989 Fracture fluids Fluid Rheology Newtonian Fluids shear stress a apparent vis cos ity g shear rate lb ft 2 Apparent viscosity is constant Slope g sec 1 Fracture fluids Fluid Rheology Non Newtonian Fluids Fracturing fluids typically follow the power law model thus apparent viscosity is dependent on shear Significant in proppant transport and friction kg n where k consistency index indicative of the pumpability of the fluid lbf secn ft2 or 47 900 Eq cp lbf secn ft2 n power index indicating the degree of non Newtonian characteristics Fracture fluids Fluid Rheology Non Newtonian Fluids Measure in lab with concentric cylindrical viscometers obtain n and k 2n 1 k slot k 3n n 3n 1 k pipe k 4n n log lb ft2 n n Slope n int k log g sec 1 Fracture fluids Fluid Rheology Non Newtonian Fluids Drag reducing non Newtonian fluids require correlations involving several experimentally determined parameters Bowen s relation b 8v s d p f w Ad 4 L d where A lbf secs ft2 b b and s are the required experimental constants Fracture fluids Fluid Loss Leakoff Detrimental because it decreases the efficiency of the treatment Process Filter cake deposition of polymer or particulates Filtrate invasion Uninvaded zone uninvaded zone pc Invasion zone pv filter cake fracture where pv is fracture wall interface pressure differential and pc is invaded zone to reservoir pressure differential Fracture fluids Fluid Loss Leakoff Lab derived a Viscosity controlled mechanism Applies to filtrate invasion Corresponds to ideal case i e no filter cake and minimum resistance between filtrate and reservoir fluids kf p c ft C v 0 0469 f min k f f pc effective formation permeability D porosity fracturing fluid viscosity cp differential pressure across the face of the fracture psi pf pr Fracture fluids Fluid Loss Leakoff b Compressibility control mechanism Fluid filtrate has similar flow properties to reservoir fluids Reservoir total compressibility affects pressure kc t ft Cc 0 0374 pc min r r ct reservoir fluid viscosity cp total reservoir compressibility psi 1 Fracture fluids Fluid Loss Leakoff c Wall building mechanism Cake building is proportional to volume passed through surface 0 0164m ft Cw Af min m Af slope of filter loss curve cc min 1 2 obtained from static filtration test filter area cm2 Pressure differential correction p m m act act p L 1 2 where pL is differential pressure of filtration test Fracture fluids Fluid Loss Leakoff Temperature correction for titanate HPG gel C 80 F C w w corr w Tres Valid until thermal degradation occurs 200 deg F for 30lbm 1000 gal loading Fracture fluids Cumulative Filtrate Volume Static filtration test Fluid Loss Leakoff Slope m b int time Dynamic filtration tests are available but complex Volume Lost static Volume Lost

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