Pressure Maintenance by Water Injection In the Twofreds (Delaware) Field Unit
Authors: Jones Randall W. 
Published: December 2007

Abstract
The Twofreds [Delaware] field unit is located in the Delaware Basin in Loving, Ward and Reeves Counties, Tex. Production is from the upper sand of the Bell Canyon section of the Delaware Mountain group at a depth of 4,820 ft. A pilot water injection project was started in the unit in 1963 and excellent results led to full-scale injection in 1966. Substantial oil production response has occurred in areas with water saturations as high as 45 percent. No water breakthrough has occurred anywhere in the unit. Performance of the Twofreds unit to date indicates that an oil bank can be built and maintained in the high water saturation Delaware sand, that a satisfactory volumetric sweep efficiency will be achieved, and that abnormal injectivity decline will not be a problem.
Introduction
The Delaware formation has been developed in many fields throughout the Delaware Basin. Most of these fields produce oil, and many wells produce water along with the oil. The high water saturation has caused concern in evaluating Delaware reservoirs for water-injection programs. The Twofreds [Delaware] field unit, with a field average water saturation of 43.5 percent, has had a pilot water-injection project for three and one-half years and full-scale injection for one year. The purpose of this paper is to describe the performance of the Twofreds unit under water injection for possible use as a guide in evaluating other Delaware reservoirs for increased recovery projects.
GEOLOGY AND STRATIGRAPHY
The Twofreds [Delaware] field unit is located in the east central part of the Delaware Basin and extends into Loving, Ward and Reeves of the Delaware Mountain group of Permian Age. The sand is found at an average depth of 4,820 ft. Beginning at the surface, a well in this area will penetrate about 600 ft of alluvium and shales and gravels of the Santa Rosa and Dewey Lake formations, approximately 600 ft of the Rustler formation, and about 5,500 ft of salt and anhydrite in the Castile formation. In the Bell Canyon section, the 55-ft, black, calcareous Lamar shale is found immediately above the unitized sand interval. Fig. 1 is a type log showing the unitized interval.
The unitized interval is composed of a uniform, fine-grained quartz sand in the productive part of the field, grading into a laminated shale and shaley sand section in nonproductive areas. The net sand thickness averages about 16 ft over the unit area. Even in the better parts of the field, the sand often contains shale or shaley sand streaks of low permeability ranging in thickness from a few inches to 5 ft. It is not possible to determine if these shale streaks are continuous from well to well. Fig. 2 is a net sand isopach map of the Twofreds field showing the unit outline.
The structure of the Twofreds field is monoclinal with no apparent closure. The productive limits are controlled by a lack of clean sand along the northwestern edge of the field and by high water cuts and lack of clean sand along most of the southeastern edge. The field is divided by a partially effective permeability barrier just northeast of the center of the field. There is a possibility of other barriers in the field, but this is the only one which has been definitely established. The trapping mechanism is stratigraphic, with the reservoir limits defined by changes in porosity and permeability. The field is approximately 7.5 miles long and one mile wide and contains 4,500 productive acres.