• 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 main functionsPre-1950s oil-based 1950s water-based with GUAR 1969 First crosslinked GUAR (with 10% oil) 1970s HPG gelling agent 70% of treatments are water-based Currently 25% are energized 5% are oil-based Fracture fluids historyWater-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 (6-10% residue) 2. HPG – chemically-treated guar, cleaner (2-4% residue) 3. HEC – cellulose derivatives Fracture fluids fluid typesCrosslinkers - to increase viscosity of fluid at higher temperatures (alternative to increasing polymer loading, but expensive) Fracture fluids fluid types Borates Titanate & Zirconium Crosslinking Fast Controlled Reversible Yes No Shear degradation No Sensitive Temp limit < 225 F < 325 F Friction High Delayed system PH 8-10 required Variable• 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 typesOil-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 typesFoamed 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 typesFoamed 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 fluid types• Buffers - maintain pH • Bactericides - 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 additivesExample Fracture fluids additives• 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 mixing1. Formation temperature and fluid rheology 2. Treatment volume and rate 3. Type of formation 4. Fluid loss control requirements 5. Formation sensitivity to fluids 6. Pressure 7. Depth 8. Type of proppant 9. Fluid Breaking requirements Fracture fluids design criteriaDefinition: Science of the deformation and flow of matter Most important variable…viscosity = f(g, T, t, C) Fracture fluids Fluid Rheology Effect of temperature on the viscosity of a 40 lbm/1000 gal HPG solution (SPE Monograph Vol 12, 1989)Newtonian Fluids Apparent viscosity is constant Fracture fluids Fluid Rheology }{*}cos{}{ rateshearityvisapparentastressshearg g (sec - 1 ) .  (lb/ft 2 ) Slope = 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 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 nkgFracture fluids Fluid Rheology Non-Newtonian Fluids • Measure in lab with concentric, cylindrical viscometers…obtain n’ and k’. • n = n’ g (sec-1) .  (lb/ft2) Slope = n int = k nnnkpipeknnnkslotk413'312'log logFracture fluids Fluid Rheology Non-Newtonian Fluids • Drag reducing non-Newtonian fluids require correlations involving several experimentally determined parameters. • Bowen’s relation: • where A {lbf-secs/ft2+b}, b, and s are the required experimental constants. sdvbAdLfpdw84• Detrimental because it decreases the efficiency of the treatment • Process – Filter cake – deposition of polymer or particulates – Filtrate invasion – Uninvaded zone • where pv is fracture wall interface pressure differential and pc is invaded zone to reservoir pressure differential. Fracture fluids Fluid Loss/Leakoff fracture uninvaded zone filter cake Invasion zone pc pvLab-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 k - effective formation permeability, D f - porosity f - fracturing fluid viscosity, cp pc - differential pressure across the face of the fracture, psi; pf – pr Fracture fluids Fluid Loss/Leakoff mincpk0.0469vCftffb. Compressibility-control