Initiation of an In-Situ Combustion Project in a Thin Oil Column Underlain by Water
Authors: Horne Jerrold , Bousaid Issam , Dore Timothy L. , Smith Luther B. 
Published: December 2007

Summary
An in-situ combustion project was initiated in the Caddo Pine Island field in a thin oil column underlain by a water zone. An inverted five-spot well pattern was selected for the project, and the formation was ignited in Sept. 1980 to evaluate the potential of enhanced oil recovery by a combustion process. This paper presents engineering and laboratory procedures used in evaluating the reservoir for in-situ combustion. The program used to monitor the project performance, including both surface and subsurface measurements, is discussed. The paper reviews the design and installation procedures for the project wells and surface facilities, and presents project results.

Introduction
An in-situ combustion project has been initiated in the Nacatoch sand underlying the Harrell Fee lease in the Caddo Pine Island field, approximately 30 miles (48.2 km) northwest of Shreveport, LA (Fig. 1). The Nacatoch formation is an Upper Cretaceous semiconsolidated, friable, fine-grained sandstone reservoir located approximately 1,000 ft (304.8 m) below ground surface [800 ft (243.84 m) subseal. The formation contains a thin [29-ft (8.83-m)] column of high-viscosity, low-API-gravity oil that is underlain by a thick [100-ft (30.48-m)] ‘water zone. Primary recovery from the reservoir was low because of the absence of reservoir energy and because of the unfavorable mobility ratio between the viscous crude and the underlying water. Reservoir fluids and matrix properties are presented in Table 1. A typical well log and a structure map of the Nacatoch sand in the project area are shown in Fig. 2.
The inverted five-spot well pattern (Fig. 2) encompasses five acres (20.23×10 m) and is centrally located within the Harrell Fee tract. The project was initiated by injecting air in Well 6, which was ignited on Sept. 17, 1980. Since ignition, daily gas analysis from the four producing wells indicates that an efficient combustion front is in progress within the pattern.
A reservoir simulator was utilized in selecting and evaluating the pattern size and configuration for the insitu combustion project. Inverted five-spot, seven-spot and nine-spot well patterns ranging in size from 3 to 10 acres (12.14×10-3 to 40.47×10-3 m2 were considered. A 5-acre (20.23×10-3m2) pattern was chosen to obtain results from the project at an early date.
All five wells in the pattern were drilled through the Nacatoch sand, and the casing was cemented by using a special heat-resistant slurry. Conventional and sidewall cores were taken from all wells (1) to aid in reservoir definition, and (2) to provide core material for laboratory tests. Logs from all five wells indicated sand continuity throughout the project area. Well completions were designed to accommodate both dry and wet combustion.
Laboratory studies consisted of dry and wet combustion tube runs performed on core material from injection Well 6. The optimal injection water/air ratio (WAR) for the project was determined from the laboratory tests.
The production facility is designed to handle normal oilfield problems as well as difficult emulsions and sand production associated with in-situ combustion projects. Air injection is supplied by a compressor rated at MMcf/d (28.32×10(3) M3/d) and a second standby compressor that was used during the ignition phase of the project. Ignition of the reservoir was accomplished by a downhole gas burner in the injection well. Ignition was confirmed by a downhole thermocouple. A program was developed to monitor project performance and combustion zone efficiency. The program includes bottomhole temperature and pressure (BHT and BHP) surveys as well as chromatographic analysis of the produced gas. Generally, the project response has been satisfactory.

Process Selection
The Nacatoch sand was selected as a candidate for thermal recovery methods. The reservoir contains a thin column of high-viscosity oil underlain by water.
JPT
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