Fractured Reservoir Discrete Feature Network Technologies

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Intoduction

The Yates Field is located in Pecos and Crocket Counties of West Texas, approximately 80 miles south of Midland. It was discovered in 1926 and had produced nearly 1.3 billion barrels of oil until the beginning of 1993. The recovery at that time was approximately 30% of the estimated OOIP. At the end of 1992 a total of 1100 production and 57 injection wells were active.

Geology

The Yates Field produces from four formations (Seven Rivers, Queen, Grayburg and San Andres). The Seven Rivers interval is composed of interbedded sandstone, siltstone and some dolomite. The Queen consists of siltstones and fine sandstones interbedded with silty dolomites. The Grayburg is composed of interbedded dolomite and siltstone. Unlike the overlying Seven Rivers and Queen, the Grayburg produces from fractured dolomite.

The San Andres is the thickest and most prolific formation within the Yates Field. The unique reservoir quality of the San Andres is a function of depositional carbonate shoal facies, extensive natural fracturing, karst, dolomitization, and precipitation of secondary calcite cement. 

Early Production History

Before 1976, the Yates Field was operated under depletion, producing mainly from the fractured Grayburg and San Andres formations. As pressure depleted free gas evolved and migrated rapidly through the natural fracture system to the structural top to form a slow expanding gas cap.

Following the Yates Field unitization in 1976, a gas injection pressure maintenance program was instituted to retard the invasion of water into the oil producing portion of the reservoir and to conserve energy. This allowed greater utilization of the efficient gas cap gravity drainage mechanism.

Additional pressure maintenance and infill development well programs were implemented during the 1980's to improve recovery. Starting with a pilot waterflood on the western flanks of the field in 1979, a pattern polymer flood was expanded into additional portions of the westside oil column from 1983 to 1986. The polymer flood operations were suspended on schedule in 1989 while pattern waterflooding continued. During late 1985, carbon dioxide injection commenced in the northern, eastern and crestal areas of the field. Carbon dioxide injection was abandoned in 1991.

The Co-Production Project

All programs mentioned above only addressed the westside oil column and the eastside gas cap. Recognizing the need to recover the large remaining fieldwide reservoir target contained in the portions of the reservoir bypassed by water encroachment, a fieldwide Co-Production Project was initiated in late 1992. The main goal of the project was to de-water reservoir areas containing oil bypassed by water encroachment by lowering the gas-oil and water-oil contacts using high-volume water withdrawals and gas cap inflation (increase reservoir pressure using methane and nitrogen injection). A second effect is the initiation of gas-oil gravity drainage within the expanded gas cap.

The water invasion occured in the field's fracture network bypassing the oil located in the highly oil saturated matrix. The oil can only be recovered if the water is removed from the fracture network allowing the oil to flow from the matrix into the fracture system.

During the Co-Production Project the aquifer encroachment was reversed and a significant amount of oil was recovered that could not have been produced by conventional primary or secondary production methods (see Tract 17 oil production).

Fracture Network

In addition to the Co-Production Project, since 1992 the natural fracture network within the Yates Field has been studied in detail. It was shown that all the wells in the field receive matrix fluids from conduits in the extensive natural fracture network. By focusing on the areas where the fracture network is most developed, it was possible to take advantage of the natural drainage system by maximizing withdrawals from high rate, high efficiency wells in these areas. Between 1992 and 1994 almost 400 wells where shut in while keeping a stable total daily oil production rate.

For the existing wells it was possible to identify those located in highly fractured areas of the reservoir based on their production history. In 1993 and 1994 more than 30 new short-radius horizontal wells were drilled for which such data was not available. A discrete fracture network modeling approach using FracMan was used to study the spatial distribution of fractures in the field and to optimize location and orientation of the new horizontal wells.

Thermal Recovery Process

 The application of the Co-Production Reservoir Management plan has significantly improved oil production from the Yates Field. An additional opportunity exists to explore the oil potential remaining beyond the scope of the Co-Production Process. The reservoir has a large gas cap containing slowly draining oil in the matrix system, above a thin fracture oil column. Improved recovery processes to take advantage of this situation are limited by the fractured nature of the reservoir which makes displacement processes ineffective.

Oil flow from the the matrix into the the fracture system is controlled by gravity drainage. The physical properties controlling this process are matrix permeability, density contrast, relative permeability and viscosity. The relative permeability and the viscosity are parameters that can be changed using EOR methods. Heat flow through the fracture system (and thus heat transfer from the fractures into the matrix system) will assist oil drainage by reducing oil viscosity and improving the oil relative permeability. Because of the fractured nature of the reservoir this process can not be designed as a classic thermal flood. Instead Marathon invented a process called Thermally Assisted Gravity Segregation (TAGS).

Click here for more about TAGS.