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.

Data Analysis of Barnett Shale Completions

“…SWF began in 1997-98, and the assumption was that the Barnett would respond to a sand concentration of less than a monolayer and yield commercial production (Grieser et al 2003). Our data set begins approximately 4 years before the first SWF was attempted.…”

Comparison of Single- and Dual-Array Microseismic Mapping Techniques in the Barnett Shale

“…1,2 In particular, the migration from crosslinked-gel, proppedfracture treatments to waterfracs with higher rates and lighter proppant loadings has provided both technical and economic benefits by improving connectivity within the reservoir while reducing costs. While these benefits have been seen directly in both gas production and AFE reductions, a full understanding of the mechanisms for these improvements has primarily been provided by microseismic monitoring of the treatments.…”

Experimental and Modeling Study of Water Imbibition and Flowback in Shale: Prediction of Relative Permeability and Capillary Pressure Curves

“…Multistage hydraulic fracturing is the main stimulation technique, , widely applied for economic gas extraction from such ultra-tight reservoirs. Water has remained the main source of hydraulic fracturing fluid (HFF), which has successfully been used to increase tight gas and shale gas recovery among different fracturing methods. − However, shale formations are highly prone to blockage by water due to the contribution of nanopores to the texture of rock. Therefore, understanding the water penetration and flowback processes is crucial for the proper evaluation and development of these highly complex formations.…”