Abstract

A waterflood pattern realignment project in the Grayburg / San Andres McElroy Field is improving the waterflood performance. This paper presents a case history of a 640-acre [259 paper presents a case history of a 640-acre [259 ha] section of the field that was realigned in 1988. Irregular and widely spaced patterns were developed into smaller and more uniform patterns. The results of the realignment are patterns. The results of the realignment are proving the economic viability of realignment proving the economic viability of realignment work at McElroy and are improving reservoir characterization.

Introduction

The McElroy Field has been under waterflood for 26 years. Portions of the field are not performing as a mature waterflood. The Section performing as a mature waterflood. The Section 205 project area exhibited low bottom hole pressures (BHP's), low water-oil ratios (WOR's), pressures (BHP's), low water-oil ratios (WOR's), declining oil and water production, and irregular pattern geometry. A pattern realignment was chosen to improve the performance by improving the geometry, performance by improving the geometry, decreasing the spacing of the patterns, reducing the producer to injector ratio, and hence improving areal and vertical sweep efficiency and injection support. The historical background of the section area, the support for the realignment, and the results of project implementation are presented below.

Background
Overview of the Geology

The McElroy Field is located on the eastern edge of the Central Basin Platform in Crane and Upton Counties, Texas (see Fig. 1). Section 205 is located in the southwest portion of the field. It produces from Grayburg and San Andres shelf dolomites. The Grayburg is the main formation under waterflood and is at a depth of approximately 3000 ft [914.4 m].

The field is an example of a classic strato-structural trap. A structure map of the top of the McElroy shows a North-Northwest trending asymmetric anticline with steeply dipping east flank and a gently dipping west flank (see Fig. 2). This structure, along with a permeability barrier located on the gently dipping west limb, provides the trapping mechanism.

The average porosity of the Section 205 main pay, corrected fo gypsum, generally increases pay, corrected fo gypsum, generally increases from west to east and is shown in Fig. 3. The zones between th marker "E" and "M" are referred to as the main pay. Average permeability values also increase from west to permeability values also increase from west to east (see Fig. 4). However, neither of these maps shows the spatial variability of permeability. Cross-section B-B' (see Fig. permeability. Cross-section B-B' (see Fig. 5) give a clearer picture of the heterogeneity of the reservoir. The shaded curves are core permeability. Even though permeability increase from west to east along permeability increase from west to east along the cross section, there is grea dea of vertical discontinuity. Many of the individual permeability streaks are very difficult to permeability streaks are very difficult to correlate and disappear before they get to the next well on the cross section.

The permeability, not just porosity, must be considered when evaluating the Grayburg / San Andres reservoir for realignment. The porosity may indicate continuous zones even though the zones are discontinuous due to gypsum plugging.

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