The Rocket Science Behind Water Frac Design

Abstract

Abstract The 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 (Cfd). 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 model-predicted volume for the mapped microseism geometry is compared to actual volume pumped. Introduction Water fracs have had various names through the years. From the mid 1970s to early 1980s, "river fracs" were performed on many Hugoton wells in Kansas. Water and sand from the Cimarron River was pumped at high rates (200 to 300 bbl/min) with little more than a few gallons of friction reducer, 20 to 30 dump trucks of river sand, and an occasional frog or turtle. During the same time, "pit fracs" were pumped into the Hunton and Mississippi formation in Canadian County, Oklahoma. The term "pit" comes from the water-storage container, which was an earthen pit, sometimes lined. Frac volumes ranged from 4,000 to 38,000 bbl. Averages of 1,200 gal/ft and 0.425 lb/gal were most common. From 1986 to 1988, UPRC performed water fracs in the Austin Chalk in both vertical and horizontal wells. Typical volumes were 400 bbl of acid pumped in stages with 30,000 bbl of water and wax beads diverter. In 1997, Mitchell Energy (now DEVON) experimented with light sand fracs (LSF) in the Barnett shale. The company continued reducing polymer gel loadings to the point where little more than friction reducer and biocide were used. Average job size is 2,000 to 2,500 gal/ft or 24,000 bbl for a 400-ft section. Average proppant concentration is 0.3 lb/gal. Other terms or descriptive mnemonics used to describe water fracs, including:LSF - light sand fracsSWF - slick water fracsLPF - low proppant fracsTWF - treated water fracsMHF - massive hydraulic fracs Many rules of thumb are offered for water-frac design methods. The following "rules" are among the most common:Frac tanks per 100 ft of pay (tanks/100 ft)Barrels per ft (bbl/ft)lbm of proppant per ft (lbm/ft)Rate per ft (bpm/ft)