Fractured Reservoir Discrete Feature Network Technologies

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The project consists of five major task areas carried out over three years. These are illustrated in Figure 1-2 and described briefly below.

• Task 1 - Fundamental research into the relation between the stress field on a fracture and its fluid-flow properties. This task will be carried out during years 1 and 2.
• Task 2 - Reservoir characterization technology development to devise new methods for creating three-dimensional models of fractured reservoirs from geological and well test/production information. This task will be carried out during years 1 and 2.
• Task 3 - Three-dimensional discrete fracture model creation, in which the fundamental relations developed in Task 1 and the characterization tools developed in Task 2 will be integrated to produce a reservoir model for the Yates Field. This task will be carried out in years 2 and 3.
• Task 4 - Application and demonstration of the model to predict TAGS processes in the Yates Field. This work will be carried out during years 2 and 3.
• Task 5 - Technology transfer, to present the preliminary finding to industry and academic experts during the course of the project to both help guide the research through feedback, and to disseminate useful technology to industry. This task is on-going throughout all three years.
• Task 6 - Project management.

The project outline shown in Figure 1-2 presents several unique and innovative approaches for characterization of fractured reservoirs, synthesis of reservoir data into geologically realistic reservoir models, and the integration and use of these fractured reservoir models with existing computer codes to design fractured reservoir-specific oil recovery processes. The project combines:

• Fundamental research on fractured rock hydraulic processes and fracture network models specific to fractured reservoir site data,
• Technology development for interpretation of reservoir characterization data using quantitative, stochastic approaches,
• Integration of site characterization and reservoir data into an innovative discrete fracture conceptual model,
• Application and demonstration of discrete fracture analysis, modeling, and production technologies in an active fractured oil reservoir, and
• Technology transfer via publications, conferences, workshops, and World Wide Web (WWW) internet distribution services.

The fundamental basis of this project is the concept of discrete fracture network modeling (Figure 2-1). In the discrete fracture network modeling approach, the fractured reservoir is realistically depicted by a network of discrete features representing fractures, combined with a background permeability. The discrete fracture network approach provides unique abilities to evaluate the pressures, flows, and connectivities of fractured oil reservoirs.

The analysis of flow in fractured reservoirs continues to present a major technical challenge. All geologic systems have some degree of heterogeneity, and fractured systems with a low permeability matrix have the most heterogeneity. The assumptions of continuum behavior break down in highly heterogeneous media. Unlike a porous continuum, there are not connections between all points in the reservoir. The effects of activities in one borehole may bypass nearby wells and strongly affect distant points. Flow is restricted to discrete pathways, and networks themselves may be finite. The fracture porosity reflects a complicated mix of tectonic and geochemical processes.

Although there has been considerable progress in the past twenty years of studying fracture flow systems, a validated methodology for collecting fracture geometric data and simulating fracture flow is not yet within the bounds of standard practice. The most comprehensive fracture flow study to date has been the OECD’s international Stripa Project (Olsson, et al. 1992), which was funded by several of the world’s radioactive waste programs including the DOE. Stripa showed the importance of integrating information from a broad range of earth science disciplines.

Fractured reservoirs have often been low-priority candidates for oil recovery processes designed to frontally-displace fluids in conventional non-fractured reservoirs. To date, industry efforts have been directed at making the reservoirs behave more like homogeneous systems rather than specifically designing recovery processes that benefit from the unique characteristics of fracture network flow. This industry-wide practice has left numerous large and small fractured reservoirs near abandonment producing at high water-oil ratios while retaining most of their original oil in place. Although field, laboratory and computer simulation efforts are ongoing to improve the definition and design of fractured reservoir recovery processes, co-operative funding of these efforts would supplement and accelerate the integration and development of these tools. The philosophy of the proposed scope of work addresses six essential points:

1. The formation matrix is recognized as a critical oil storage volume, but equally important, is serves as a barrier to flow in the fracture network;
2. The fracture network effective flow capacity is highest in the vertical direction and varies directionally at a lower capacity in the horizontal plane;
3. Oil recovery is improved through the management of fracture fluid contacts and near-well perturbations of the unconfined oil column;
4. The segregation of oil for efficient withdrawal can be assisted by maximizing the oil-phase driving force and minimizing the resistance to flow;
5. Thermally-Assisted Gravity Segregation (TAGS) synergistically merges the natural tendencies of areal fracture network flow segregation, gravity segregation in the fracture network, and compositional/thermal phase behavior into a cost-effective fractured reservoir thermal EOR process;
6. Oil recovery can be greatly accelerated and project cost effectiveness significantly improved through “real” three-dimensional definition of the effective fracture network connectivity and flow capacity.

The work will target construction of a technically-supported fracture network characterization which spans the range of variability indicated by field data. The work will also require the development of new fundamental understandings and new analytical tools. Although the work will be closely tied to a specific field site, the techniques and sequences of evaluation are applicable to other fractured reservoirs, and will assure that the proposed concepts can be explored with field testing and computer simulations. The multi-specialty approach is required to assure comprehensive recognition and analysis of evaluation techniques, and is a strength in the overall approach.