The Rocket Science Behind Water Frac Design

Abstract

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThe popularity of water fracs has increased in recent years. The reduction in fluid cost and overall fracture stimulation cost has in some cases revived exploration in low-permeability reservoirs like the Barnett shale in north central Texas. Water fracs have also been used effectively in reservoirs with low permeability and large net pays, which require large volumes of fluid to attain adequate fracture half-lengths to achieve commercial production.In the past, the design of water fracs has been more of an art than a science. While the term "water frac" implies that the fluid is proppant-free, in most cases some proppant is usually pumped. The amount and concentration is usually low when compared to conventional fracture treatments. Water-frac designs are further complicated by the fact that fracture geometry, conductivity, and proppant transport are not easily modeled.Despite these difficulties, the attractiveness of water fracs requires the implementation of a design methodology. This paper discusses a design procedure for water fracs from a field operation/design standpoint. Volume and rate requirements are discussed for a specific zone height, desired fracture length, and aerial width. A fracture width vs. proppant size requirement is applied, and a simple material balance calculation is performed to generate a fracture volume taking fluid leakoff into account. Fracture conductivity of a low proppant-concentration, high fluid-volume fracture is estimated to optimize proppant length and fracture conductivity ratio (C fd ). A pump schedule is generated based on the results of the previous calculations. All design calculations are simple and require only a handheld calculator or simple spreadsheet.The design model was calibrated to a microseism-mapped Cotton Valley Lime test well. A leakoff coefficient multiplier was used to calibrate the model. The model-predicted volume was then compared to actual volume on a second Cotton Valley Sand test well and on a 10-well average Barnett shale microseism fracture-mapping data set. The overall modelpredicted volume for the mapped microseism geometry is compared to actual volume pumped.

Evaluation of Oil-Industry Stimulation Practices for Engineered Geothermal Systems

“…The stunning success of gas development in the Barnett Shale of north central Texas is largely because of the optimization of hydraulic fracturing in this unusual reservoir environment [Warpinski et al, 2005a;Coulter et al, 2004;Grieser et al, 2003]. In particular, the migration from crosslinked gel, propped-fracture treatments to water fractures with higher rates and lighter proppant loadings has provided both technical and economic benefits by improving connectivity within the reservoir while reducing costs.…”

“…Water fracturing is defined as the application of a water-based hydraulic fracturing fluid with a low-viscosity (e.g., ≤ 10 cP) [Grieser et al, 2003]. Average concentrations of proppant are less than 0.5 lbm/gal, which is very low compared to conventional hydraulic fractures.…”

“…This results in an improved productivity [Fisher et al, 2004]. Currently, nearly every treatment is performed by use of light sand fracturing with high injection rates of water, plus a friction reducer to enable reasonable rates of injection given the completion, scale inhibitor and biocide, or some permutation of this technique [Grieser et al, 2003]. This has proven to be the most important advancement in the resurgence of development programs in this basin [East et al, 2004].…”

“…The basic methodology for the design of fractures has not been aggressively applied to water fracturing [Grieser et al, 2003]. This could be because: 1) The suspected complex geometry indicated by microseismic imaging, and the uncertainty of the deposition of proppant in fluids with low viscosity; and 2) water fracturing may have been considered a low technology practice of completion, used mainly in reservoirs with marginal capability of production.…”

A new analytical multi-linear solution for gas flow toward fractured horizontal wells with different fracture intensity

“…Fracturing of horizontal wells with hybrid fluid or slick water (SWF) (Grieser et al, 2003;Fisher et al, 2004;Britt and Smith, 2009) not only increase fracture-formation contact areas by creating SRVs, but also reactivate natural fractures to some extent, which is vital to tight gas production (Cipolla, 2009;King, 2010;Ozkan et al, 2009;Stalgorova and Mattar, 2012;Clarkson, 2013).…”