Abstract

A case study comparing the performance of wells fractured with crosslinked guar and hydroxypropyl guar (HPG) is presented. Data indicates that for a given crosslinked system, guar and HPG polymers yield comparable proppant pack impairment and fracture polymers yield comparable proppant pack impairment and fracture conductivity. Detailed laboratory conductivity and well performance data are presented for both low and high temperature conditions. Laboratory conductivity data is reported for proppant placed with guar and HPG crosslinked fracturing fluids under in-situ conditions. The effects of proppant embedment. the gel filter cake, and long-term exposure to reservoir temperature and closure stress were determined in this study. Additionally, the effect of crosslinker, breaker system, and polymer load on fracture conductivity is discussed in detail. Subsequent to laboratory testing, numerous fracture design and production simulations were performed to determine the effect of production simulations were performed to determine the effect of guar and HPG crosslinked fluids on well performance. The data indicates the forecasted cumulative production of wells fractured with guar and HPG are similar. However, due to the relatively lower cost of guar, the Net Present Value (NPV) of wells fractured using guar is greater due to lower treatment costs. A case study of sixteen high-temperature wells located in Weld County, Colorado was performed to determine the "actual" effect of the fracturing fluid on well performance. A comparison of post-frac well production data indicates guar and HPG crosslinked fluid yield comparable proppant pack impairment, and thus similar well performance. A detailed discussion is presented. performance. A detailed discussion is presented

Introduction

Previously researchers have reported natural polymers utilized in Previously researchers have reported natural polymers utilized in water-based fracturing fluids have adverse effects on proppant permeability. However, these viscous fluids provide an permeability. However, these viscous fluids provide an efficient means to create and propagate a fracture, and effectively transport proppant into the created fracture. Thus, it would be beneficial to develop methods to minimize proppant permeability impairment caused by the polymers. One of the earliest attempts to decrease proppant permeability damage was directed at the development of a "cleaner" polymer system. Guar, a naturally occurring polysaccharide, was reacted with proplylene oxide to form HPG. During this process, a large quantity of plant material (residue) was removed to yield a "cleaner" polymer. Laboratory tests indicated that HPG had only 1 to 3 weight percent (%w/w) residue, whereas guar exhibited 8 to 13 % w/w residue. Based on this information, it was concluded that HPG creates less proppant pack damage than guar polymers. Additionally, HPG offered improved compatibility with methanol and greater high-temperature stability. Consequently, in the early 1970's, the fracturing industry became very conscious of the fact that HPG had a lower residue than guar. Usage of guar fracturing fluids decreased dramatically (Fig. 1) while HPG usage increased. Thus, guar was perceived to be a "dirty" fluid and HPG became the standard frac fluid polymer. In 1984 a conductivity study was performed by Almond and Bland that indicated no correlation could be drawn between % w/w residue and proppant pack permeability impairment. In fact, it was reported that guar and HPG produced similar proppant pack damage (18 % HPG vs. 20 % guar @ 120 degrees F), even though the intermediate residue guar had 5 to 6%w/w residue compared to HPG's 1 to 2%w/w residue. Recently, conductivity studies performed under in-situ conditions by Pennys have shown that both guar and HPG yield similar conductivity impairment. Additionally, Penny's study indicates that polymer loading, type of crosslinker, and breaker system dramatically affect the conductivity of a proppant pack. In today's oilfield environment with depressed hydrocarbon prices, the optimization of all costs associated with the production prices, the optimization of all costs associated with the production of oil and gas is required. Since the fluid polymer costs may constitute 15 to 20% of hydraulic fracturing costs, guar warrants reevaluation as a cost effective alternative to more expensive, so-called "cleaner" polymers. This paper presents the results of extensive laboratory conductivity testing of proppants placed with various guar and HPG crosslinked systems. The laboratory data is supported by a case study of 16 wells fractured with guar and HPG crosslinked systems.

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