The primary objective of this DOE-sponsored project is to develop a method that will predict interwell connectivity from waterflood production and injection well rate fluctuations. The work addresses reservoir characterization and the advanced diagnosis of reservoir systems. By evaluating interwell communication using universally available field data, the proposed work represents a new, low-cost method of data analysis to refine geological and engineering models for field management and tertiary oil production.

Texas Engineering and Experiment Station (TEES), Texas A&M University, College Station, TX 
The University of Texas, Austin, TX

All stages of oil recovery—primary, secondary, and tertiary—benefit from knowing the flow paths within a reservoir. That this information is particularly difficult to gain is evinced by the numerous techniques developed to infer it. Reservoir simulation is a common approach, but no technique is entirely satisfactory because they all use information from which the character of the flow is derived rather than the flow itself. There is a great need to develop a robust approach to quantitatively evaluate connectivities between injection and production wells.

Results
The project is based on improving a method of estimating interwell communication in waterflooded reservoirs, originally proposed by Albertoni and Lake (2003). Their method provided a single parameter (lambda) to estimate the interwell connectivity for each injector-producer well pair.

This project’s improvements, termed the capacitance model (CM), have resulted in a method that accommodates variations in both compressibility and swept volume and that provides values for two parameters, lambda and a time constant tau (Yousef, et al., 2006a). Researchers have:

• Combined lambda and tau values to optimize waterflood performance under several constraints (e.g., prices of oil and water disposal).
• Investigated new procedures to improve lambda and tau estimation when non-reservoir factors, such as workovers, change production rates.
• Tested the robustness of an alternative method, analyzing well rates in the frequency domain to infer connectivity. The project performers found that the presence of free gas in the reservoir strongly affected the estimated degree of connectivity.

Benefits 
This technology will increase economically producible reserves of oil and gas and will aid economic development. At the completion of this project, Texas A&M University will have a software package publicly available that maps and quantifies interwell connectivity. The software will be suitable for use on a PC using readily available complementary programs. This technology is intended for use by oil companies that produce oil from waterflooded fields where there are minimal data other than periodic production and injection volumes. The intent is to provide a tool that will be useful for operators of mature fields to improve recovery. There are four intended benefits:

• A tool to infer or validate a geologic model, specifically about the efficiency of faults or fracture patterns.
• A tool to make use of large volumes of well-rate data.
• A means to adjust injector-producer balance in displacement processes.
• A means to allocate hydrocarbon production from a given producer back to the injector causing that production.

Summary
The purpose of this project is to show that the extent of communication between injector and producer pairs can be quantitatively inferred from statistical analysis of well-rate fluctuations without a priori information. Experience with a recently developed method to analyze injection and production rates in waterflooded fields has shown some promise (Albertoni and Lake, 2003). The method, however, involves several assumptions that restrict its application in real-world cases. This project investigates procedures to remove dependence on the assumptions to give reliable measures of interwell communication.

The procedure involved uses injection and production rates, data that are almost universally available in waterflood projects. Four specific areas are targeted to make the procedure robust: fields with wells shut in for extended periods, fields with non-uniform compressibility, very heterogeneous reservoirs, and efforts to ensure that results give physically possible connectivities. Statistical and signal analysis methods are applied to the field data to estimate connectivity, and the procedure is being tested with several large field datasets.

(July 2007)
This project has been completed. Final report has been received. Two presentations to industry groups have elicited favorable responses.

Funding
This project was selected in response to DOE’s Oil Exploration and Production solicitation DE-PS26-03NT15450 (focus area: Advanced Diagnostic and Imaging Systems and Reservoir Characterization).

$384,000

$127,000 (25 percent of total)

NETL - Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 918-699-2021) 
TEES - Jerry Jensen (jensen@pe.tamu.edu or 979-845-2206)

Publications
“Interwell Connectivity and Diagnostic Using Correlation of Production and Injection Rate Data in Hydrocarbon Production,” August 2004, first annual report to DOE.

“Interwell Connectivity and Diagnostic Using Correlation of Production and Injection Rate Data in Hydrocarbon Production,” August 2005, second annual report to DOE. “Interwell Connectivity and Diagnostic Using Correlation of Production and Injection Rate Data in Hydrocarbon Production,” August 2006, third annual report to DOE.

Albertoni, A., and Lake, L.W., “Inferring Connectivity Only from Well-Rate Fluctuations in Waterfloods,” SPE Reservoir Evaluation and Engineering Journal, v. 6, pp. 6-16, 2003.

Yousef, A.A., Gentil, P., Jensen, J.L., and Lake, L.W., “A Capacitance Model to Infer Interwell Connectivity from Production and Injection Rate Fluctuations,” SPE Reservoir Evaluation and Engineering Journal, V. 9, pp. 630-646, 2006a.

Yousef, A.A., Lake, L.W., and Jensen, J.L., “Analysis and Interpretation of Interwell Connectivity From Production and Injection Rate Fluctuations Using a Capacitance Model,” SPE 99998, 15th SPE Improved Oil Recovery Symposium, Tulsa, OK, April 22-26, 2006b, 15pp.