The permeability of proppant-packs may be significantly damaged by the gellingagents used In hydraulic fracturing fluids. Several recent fractureconductivity studies have sought to quantify the damage caused by commonfracturing fluids over a wide range of downhoIe conditions. Very little, however, has been disclosed in the way of viable methods of minimizing orcleaning up the proppant-pack damage.
Breakers have been recognized for many years as useful for degrading theviscosity of hydraulic fracturing fluids. Degrading the treating fluid to anon-viscous state was sufficient to provide for proppant-pack clean-up uponflowback. Recent studies have illustrated that the permeability issignificantly impaired by the concentrated polymer residue even thoughsufficient breaker concentrations have been added to effectively reduce thefluid viscosity.
This study is concerned with the determination of the concentrations of anoxidative breaker which are required to significantly reduce the proppant-packpermeability damage caused by concentrated aqueous hydraulic fracturing fluids.Long-term proppant-pack permeability testing was performed using a modified API-type fracture conductivity cell which permitted fluid loss through twolow-permeability cores. The test temperature was 71 °C, which is thought to bebelow the range at which effective thermal degradation of the polymer occurs.The fluids which were evaluated were linear and borate-crosslinked gels ofnatural guar and hydroxypropyl guar (HPG) and linear gels of hydroxyethylcellulose (HEC). The post-closure polymer concentrations were varied from 12.0to 53.0 kg/m3 to account for concentration due to fluid-loss.
Test results indicate that proppant-pack permeability impairment can besignificantly reduced by the addition of elevated breaker concentrations. Thebreaker concentrations required to show significant reduction in thepermeability damage were found to be strongly dependent upon the polymerconcentration and the presence of crosslinker. An understanding or the effectsof polymer and breaker concentrations on the proppant-pack permeability can bevery useful in the effective design of fracturing treatments.
References and illustrations at end of paper.
The objective of engineers designing hydraulic fracturing treatments is theplacement of a high permeability proppant-pack, through which the reservoirfluids and/or gases may be efficiently produced. Engineers designing andevaluating fracturing treatments have frequently predicted productivityincreases much greater than those actually observed. The in-situ proppant-packpermeability has long been recognized as a major factor in well productivityand is therefore of importance to the design and evaluation or fracturingtreatments1,2. Until recently, the only proppant-pack permeabilitydata provided for use in the design of fracturing treatments have beenshort-term data collected under ambient temperatures conditions without thepresence of fracturing fluid 3–5. Numerous efforts have been focusedrecently on the evaluation of long-term conductivity of proppant-packs withoutfracturing fluids. Proppant-pack permeabilities have been found to besignificantly reduced when exposed to long periods at temperature and stress.The long-term permeabilities of many of the proppants tested in these studieshave been reported to be reduced by as much as 50% compared to the previouslypublished short-term values 4,5.
The polymeric gelling agents commonly used in hydraulic fracturing fluids mayalso significantly impair the permeability of proppant-packs.
