Curry Unit: A Successful Waterflood in a Depleted Carbonate Reservoir With High Gas Saturation
Authors: Hasan Syed M. 
Published: December 2007

When waterflooding was begun in 1966, most of the wells in the old Curry Pool had been abandoned and a vacuum had been pulled for some 30 years. The pilot injection program was expanded and in 1971 the fieldwide flood pattern was changed to a five-spot on 40-acre spacing. Production increased from 80 BOPD in 1966 to 3,650 BOPD in 1972, and Production increased from 80 BOPD in 1966 to 3,650 BOPD in 1972, and the incremental oil recovery due to waterflooding is estimated at 7.5 million barrels.

Introduction

The Curry Unit waterflood project is about 4 miles southwest of Breckenridge, Stephens County, Tex. (see Fig. 1). The reservoir formerly known as "Curry Pool" is currently classified as the Stephens County Pool" is currently classified as the Stephens County Regular field by the Texas Railroad Commission. The producing formation is Caddo Reef, which lies at an average depth of 3,200 ft. The field is about 3.6 miles long and about 2.5 miles wide and encompasses more than 3,200 productive acres. The field was discovered in 1918, and by 1925 there were about 195 wells producing from this reservoir. The initial potentials ranged from 15 to 2,000 BOPD. The early wells were drilled by cable tools and completed open hole. Each contained several strings of uncemented casing. The wells were stimulated with 80 to 250 qt of nitroglycerine. Remedial work consisted of clean-out, shooting with nitroglycerine, and acidizing. Many of the wells were plugged in the late 1930's and early 1940's, but how and with they were plugged is unknown. In the 1950's a number of wells were drilled. Their potentials ranged from 10 to 55 BOPD, and as far potentials ranged from 10 to 55 BOPD, and as far as the primary production is concerned, they were economic failures. The reservoir properties (high porosity, low water saturation, low oil viscosity) do indicate a good candidate for waterflood. However the gravity drainage was the main producing mechanism at the start of the waterflood and the gas saturation was thought to be dispersed throughout the formation. This was substantiated by the fact that throughout the reservoir, regardless of the structural position or vicinity of the original gas-cap area, the GOR's were high. In addition, a number of wells produced water that was suspected to be from the lower limestones or from shallow water sands through casing leaks or channels behind casing. The field had to be practically redrilled, requiring significant capital investment. Thus conditions were not conducive to instituting a waterflood in this reservoir.

The reservoir thickness was estimated mostly from driller's logs and secondary recovery was estimated with an empirical resaturation equation. The performance to date indicates that the recovery estimate performance to date indicates that the recovery estimate is still valid.

Geology and Reservoir Description

The Bend Arch is the main structural feature controlling the occurrence of oil in the North Texas area. The Bend Arch is a broad anticlinal feature dipping to the northwest and extending from the Llano-Burnet uplift in Central Texas to about the center of Archer County in North Central Texas. It separates the West Texas Permian Basin to the west from the Fort Worth Syncline to the east.

JPT

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