Austin Chalk Fracturing Design Using a Crosslinked Natural Polymer as a Diverting Agent
Authors: Hill Orvel F. , Ward A. Jack , Clark C. 
Published: December 2007

This paper discusses a successful fracturing technique used to stimulate the Austin Chalk formation, located in Dimmit and Lee counties in south Texas. Austin Chalk geology and properties of reservoir fluids are described, and other fracturing practices are discussed. Production results from various stimulation techniques are compared.

Introduction

During the past 2 years, considerable interest has been shown in producing the Austin Chalk formation in south Texas because of higher oil paces and increased demand. The Austin Chalk formation in the Pearsall Field of Frio County, TX, was discovered in 1933 and partially developed through 1941. Activity briefly resumed between 1948 and 1956. Recent activity began in late 1974 and continues today.

Austin Chalk operations cover an area from Dimmit County to the west through Robertson County to the east (Fig. 1). Within this area, Pearsall Field (Frio County) and West Dilley Field (located in parts of Zavala, Dimmit, LaSalle, and Frio counties) are the most active locations. Operations also have begun in Lee and Burleson counties.

Stimulation practices during the first two operating periods primarily consisted of detonating nitroglycerin periods primarily consisted of detonating nitroglycerin through the heavily fractured intervals, followed by 2,000 to 5,000 gal HCl acid. Although a limited amount of hydraulic fracturing began in 1955, activity in the Pearsall Field terminated shortly after because of Pearsall Field terminated shortly after because of unsatisfactory economics. However, recent interest in the Austin Chalk has caused a demand for stimulation technology to make production from this formation economically feasible.

This paper describes a technique of hydraulically fracturing the Austin Chalk formation to increase productivity by assuring that the complete vertical formation productivity by assuring that the complete vertical formation interval is fractured. By improving productivity using this fracturing technique, the economics of producing the Austin Chalk formation also are improved greatly.

Theory

To obtain maximum production from thick, low-productivity formations such as the Austin Chalk, the entire interval must be fractured. This exposes more fracture surface area to the drainage system than when fracturing only limited intervals of the thick producing section. High pump rates and large fluid volumes alone cannot fracture pump rates and large fluid volumes alone cannot fracture the entire vertical length of this formation. However, fracture diversion techniques combined with high rates and volumes can do this and can be verified by temperature logs.

The technique of using a crosslinked natural polymer as a diverting agent is an attempt at both casing and formation diversions. High-rate, high-volume stages of fracturing fluid are pumped for maximum penetration, followed by volumes of a crosslinked natural polymer having the properties of a Bingham plastic. This gel allows increased hydraulic pressure to be applied to adjacent intervals, thus diverting the fracture propagation so that extensive vertical intervals may be fractured. Ball sealers also are used after one stage of the procedure to assure further casing diversion than that achieved with gel only.

Using crosslinked natural polymers for diversion in massive, highly fractured formations is not unique to the Austin Chalk formation.

JPT

P. 1795