Breaker Concentrations Required To Improve the Permeability of Proppant Packs Damaged by Concentrated Linear and Borate-Crosslinked Fracturing Fluids

Abstract

Summary The concentrations of an oxidative breaker required to reduce significantlythe proppant-pack permeability damage caused by aqueous hydraulic fracturingfluids have been determined. Long-term, proppant-pack permeability testing wasused to evaluate linear and borate-crosslinked gels. Results indicate thatincreasing the breaker concentration can reduce proppant-pack permeabilitydamage very effectively. permeability damage very effectively. Introduction Long-term, proppant-pack permeability testing was performed with a modified API-type fracture conductivity cell that permitted fluid loss through twolow-permeability cores. The fluids evaluated were linear and borate-crosslinkedgels of natural guar and hydroxypropyl guar (HPG). The effective polymerconcentrations were varied from 100 to 440 lbm/1,000 gal to account forconcentration caused by static fluid loss. Test results indicate that increasing the breaker concentrations can reduceproppant-pack permeability damage very effectively. The degree of permeabilityimprovement was found to be influenced strongly by the polymer concentrationand the presence of a crosslinker. The breaker concentrations necessary topresence of a crosslinker. The breaker concentrations necessary to improvepermeability significantly were far above those typically used in fieldapplications because of the degrading effect of breakers on fluid transportcapabilities. The knowledge that elevated breaker concentrations can greatlyimprove proppant-pack permeability provided the impetus to develop delayedbreakers to permeability provided the impetus to develop delayed breakers toprotect fluids in the proppant transport stage and degrade them after protectfluids in the proppant transport stage and degrade them after closure. Also, anunderstanding of the relationship of these parameters and the resultantretained proppant-pack permeability parameters and the resultant retainedproppant-pack permeability can be useful in fracturing treatment design andwell performance prediction. prediction. Background Engineers who design and evaluate hydraulic fracturing treatments frequentlypredict productivity increases much greater than those actually observed. Thein-situ proppant-pack permeability is known to be a primary factor affectingthe productivity of fractured wells, and therefore is important to fracturingtreatment design and evaluation. 1.2 The proppant-pack permeability dataprovided for the design of fracturing treatments typically are short-term datacollected under ambient temperature conditions without a fracturing fluid. Recent efforts have focused on the evaluation of long-ten-n proppant-packconductivity. Proppant-pack permeabilities were reduced significantly when packs Proppant-pack permeabilities were reduced significantly when packs weresubjected to long periods at temperature and stress. The long-termpermeabilities of many of the proppants tested in these studies are as much as50% less than were indicated by the previously published short-term values. Proppant-pack permeability also may be published short-term values. Proppant-pack permeability also may be impaired significantly by the gellingagents common in hydraulic fracturing fluids. Among the parameters known toaffect the degree of damage are the types and concentrations of the gellingagent, crosslinker, and breaker, as well as the reservoir closure stress andtemperature. Many researchers have attempted to quantify the damaging effectsof unconcentrated fracturing fluids on proppant-pack permeability. Cookecorrelated the effects of the proppant-pack permeability. Cooke correlated theeffects of the residue of various unconcentrated gelling agents to thepermeability reduction observed in short-term stressed proppant permeabilityreduction observed in short-term stressed proppant packs. That studydemonstrated a decrease in proppant-pack packs. That study demonstrated adecrease in proppant-pack permeability with increasing gelling-agent-residueconcentration. permeability with increasing gelling-agent-residueconcentration. Kim and Losacano examined the effects of unconcentratedcrosslinked fracturing fluids on the permeability of short-term stressedproppant packs in an API conductivity cell without leakoff. proppant packs inan API conductivity cell without leakoff. Permeability reductions of 30% to 50%were reported for polymer Permeability reductions of 30% to 50% were reportedfor polymer concentrations of 40 to 100 lbm/1,000 gal. The gelling agents common in hydraulic fracturing treatments possessmolecular sizes too large to penetrate the matrix of possess molecular sizestoo large to penetrate the matrix of low-permeability formations. Therefore, the gelling agents are concentrated in the proppant pack as a result of fluidloss during the treatment and the volume reduction experienced during fractureclosure. Assuming that all the polymer remains in the proppant pack, postclosure polymer concentration factors may be calculated pack, postclosurepolymer concentration factors may be calculated from the PV with a Gasificationof Cooke's method: (1) where Cf = final polymer concentration, lbm/gal; Ci = initial polymerconcentration, lbm/gal (Cf/ci = polymer concentration factor, dimensionless); =proppant density, lbm/gal; Cs = proppant concentration in fluid, lbm/gal added;and = proppant-pack porosity, as a percent. The final polymer concentration may be calculated by multiplying the initialpolymer concentration by the polymer concentration factor. Fig. 1 shows thepolymer concentration factor as a function of proppant concentration forvarying proppant-pack porosities. The fracture width is reduced by increasingthe closure stress, thus reducing the ratio of PV to proppant volume within thefracture and therefore the proppant-pack proppant volume within the fractureand therefore the proppant-pack porosity. For example, assume that, for a giventreatment, 50,000 gal of porosity. For example, assume that, for a giventreatment, 50,000 gal of 40-lbm/1,000-gal fracturing fluid was pumped to place150,000 lbm of 20/40-mesh proppant and that the proppant-pack porosity is 33.5%. Therefore, the average proppant concentration would be 3 lbm/gal. Fig. 1shows that, for a 3-lbm/gal proppant concentration, the postclosure polymerconcentration factor is about 15. Thus, the postclosure polymer polymerconcentration factor is about 15. Thus, the postclosure polymer concentrationwithin the proppant pack would be an average of about 15 times the initialpolymer concentration, or 600 lbm/1,000 gal. It is apparent from Fig. 1 thatthe polymer concentration in the proppant pack generally is much greater thanthe 40 to 100-lbm/1,000-gal polymer concentration evaluated in previousstudies. Recent fracture conductivity studies have sought to evaluate theproppant-pack damage caused by concentrated treatment fluids when exposed torealistic environmental conditions. Penny and Parker and McDaniel reported theresults of sophisticated fracture conductivity tests that incorporated dynamicfluid loss, two-core leakoff, and long-term exposure to temperature and stress. The measured permeability reduction was a function of the dynamically formedfilter cake and the concentrated bulk fluid. Large reductions in theproppant-pack permeability were reported, particularly for crosslinked fluids.permeability were reported, particularly for crosslinked fluids. SPEPE p. 338