Pattern Modification by Injection-Well Shut-in: A Combined Cost-Reduction and Sweep-Improvement Effort
- R.C. Collings (Chevron U.S.A. Production Company) | G.P. Hild (Chevron U.S.A. Production Company) | H.R. Abidi (Chevron U.S.A. Production Company)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1996
- Document Type
- Journal Paper
- 69 - 72
- 1996. Society of Petroleum Engineers
- 4.6 Natural Gas, 5.4.2 Gas Injection Methods, 5.4 Enhanced Recovery, 5.2 Reservoir Fluid Dynamics, 5.4.1 Waterflooding, 2.4.3 Sand/Solids Control, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.1.5 Geologic Modeling, 1.6 Drilling Operations, 5.1 Reservoir Characterisation, 5.7.2 Recovery Factors
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The Rangely Weber Sand Unit is a mature giant oil field that is currently under tertiary recovery with CO2. The Rangely Oil Field has been developed in stages, as have most large oil fields. This staged development over 50 years has had a significant impact on the distribution of remaining movable oil. Simulation studies indicated that oil cut could be improved if injection patterns were modified by shutting in injection wells to take advantage of relative well maturity. A field pilot project based on these studies has yielded results that indicate improved economic performance because of lower operating costs related to improved oil cut and reduced well maintenance.
The Rangely Field has been producing from the eolian Pennsylvanian/Permian Weber Sandstone for 50 years. In that time 899 wells have been drilled, 790 MMSTB have been produced, 4,002 MMBW and 582 Bcf of CO2 has been injected. The field is mature, and yet 1,100 MMSTB remain in place. One method to obtain better recovery is to improve areal sweep efficiency. Pattern realignment is one method to do this. Unfortunately, converting wells is expensive, so the feasibility of simply shutting in injection wells in an areally consistent fashion to modify pattern configuration was explored.
Rangely's injection pattern is, for the most part, a direct line drive with an injection to production well ratio of 1:1. Shutting in every other injection well in a line drive would produce a modified 7-spot configuration with an injector to producer ratio of 1:2. The primary concern with trying this approach in the field was loss of processing rate and therefore a loss of oil rate that would negate any financial advantage achieved by the postulated improved sweep efficiency. Simulation studies were initiated to try and quantify the gain in oil recovery by implementing this procedure. This was followed by a field trial to demonstrate its economic practicality.
Hefner and Barrow have published a comprehensive description of the Rangely Field's geologic setting and reservoir characterization for readers interested in the geologic details of the largest oil field in the Rocky Mountains.
Field Development History
Rangely's development drilling started in 1944. By 1950 the field had been drilled on 40 acre spacing with 470 wells. These original development wells are called "40-acre" wells in this paper. The field was on primary depletion (primarily solution gas drive) until 1958 when a peripheral waterflood was initiated. Infill drilling and various interior waterflood expansions followed over the next 25 years. The field is now on 20 acre spacing with some areas of 40 acre and 10 acre spacing. The second generation of wells are called "20-acre" wells in this paper. Currently, the flood pattern is direct line drive, with some 5-spot patterns in the east and south ends of the field. There are now 368 active production wells and 286 active injection wells. Tertiary recovery with CO2 injection started in October 1986. Wackowski and Masoner provide reservoir, operations and CO2 project details not presented in this paper.
Central Field Expansion Example: Interior expansions occurred in a variety of ways, but generally, within the center of the field, wells drilled on 40-acre locations were converted to injection to form 80 acre inverted five-spot patterns (Fig. 1). The first wells drilled on 20-acre locations were drilled between the 40-acre producing wells forming a modified 7- Spot (Fig. 2).
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