East Vacuum Grayburg San Andres Unit CO2 Flood Ten Year Performance Review: Evolution of a Reservoir Management Strategy and Results of WAG Optimization
Authors: Harpole K.J. , Hallenbeck L.D. 
Published: December 2007

Abstract
The East Vacuum Grayburg San Andres Unit (EVGSAU) recently completed ten years of successful CO2 miscible WAG injection. This paper briefly reviews the original CO2 project design and field performance over the past ten years, and discusses the evolution of a CO2 reservoir management strategy from the original, fixed 2:1 WAG design to the current flexible, performance-driven WAG strategy. Variations in the magnitude and character of CO2 flood response across the Unit due to variability in local reservoir geology presented numerous reservoir management challenges. Problems were encountered in areas such as injection conformance, pattern balancing and sweep efficiency; managing large swings in gas production rates, and changes in injection gas composition and MMP due to construction of an NGL recovery facility.
These challenges required a re-evaluation of our understanding of the reservoir and prompted a review of the original project design and operating philosophy by an interdisciplinary study team. Significant elements of this effort included surveillance and data collection on selected infill wells, extensive reservoir characterization work, and use of operations-oriented simulation modeling. This work resulted in the evolution of a more flexible reservoir management strategy for EVGSAU utilizing selective, geologically-targeted infill drilling, well conversions, pattern realignment, and a performance-driven WAG management strategy. Operating changes implemented over the past two years have produced significant improvements in profitability and performance in terms of both increased oil production and reduced gas handling problems and expenses.

Introduction
The East Vacuum Grayburg San Andres Unit (EVGSAU) is located 15 miles northwest of Hobbs, New Mexico. It is one of several large units within the Vacuum Field area. Initial development of the Vacuum Field began in 1938 and was essentially complete by 1941. Waterflood development in the Vacuum area began in 1958 and gradually spread across the field. EVGSAU was one of the last areas in the Vacuum Field to be unitized (December, 1978). Ref. 1 presents details of the early field development and unitization of EVGSAU for the purpose of infill drilling to 20-acre spacing followed by implementation of full pattern waterflood operations in 1980. The initial waterflood at EVGSAU was developed using 80-acre inverted ninespot patterns.

Original CO2 Project Design
CO2 injection was initiated at EVGSAU in September, 1985. The CO2 flood used the same 80-acre inverted ninespot pattern configuration established for waterflooding Ref. 1 presents details of the original CO2 project design and facilities, describes the CO2 project implementation, and discusses early field response to CO2 injection. The designated CO2 project area at EVGSAU covers about 5000 acres (about 70% of the total Unit area) and contained an estimated 260 MMSTB OOIP out of the nearly 300 MMSTB OOIP estimated for the total Unit. The initial CO2 project development consisted of forty-five (45), 80-acre WAG injection patterns, divided into three operational WAG areas. These three areas, designated as WAG Area A, B, and C in Fig. 1, were selected to have approximately equal floodable pore volume and injection capacity.
The original project design called for injection of a total volume of 230 BCF of CO2 (=30% HCPV) into the project area using a fixed 2:1 time WAG.
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