Integration of geologic, geophysical and petroleum engineering disciplines has facilitated a reservoir characterization of Cedar Hill field, a naturally-fractured coalbed gas reservoir. A 3-D multicomponent (3-D, 3-C) seismic survey was collected over a portion of Cedar Hill field. The seismic survey was successful in defining detailed structural features in addition to identifying zones of high fracture permeability within the reservoir. By incorporating the results from the seismic interpretation with the detailed vertical mapping of the reservoir into a simulation model, improved reservoir exploration and exploitation can be achieved. Reservoir simulation techniques verify faults and areas of increased reservoir pressure in the study area. These areas of higher pressure indicate blocks of the reservoir which are not currently adequately drained by the present well spacing. Three-dimensional reservoir flow simulation used in combination with 3-D, 3-C seismic gives a more complete description of this coalbed reservoir and may provide a cost effective alternative in the early development of coalbed gas fields.
Cedar Hill field is located 25 miles northeast of the city of Farmington in San Juan County, New Mexico, U.S.A. (Figure 1). It is the oldest producing coalbed gas field in the San Juan Basin and has an aerial extent of approximately 15 square miles. Production is extremely variable ranging from 0.1 to 5 MMSCFGPD per well. Wells of greatest productivity produce from zones of high fracture permeability in the Fruitland coal. Due to the high compressibility of the coal, natural fractures or cleats are generally closed at reservoir depths. Local stress relief by faulting, folding, and over-pressuring can cause fracture enhanced permeability in the coal.
A portion of the western edge of the field, centered on Section 30, T32N, R10W, was selected for a detailed study. This area was chosen to include a portion of the basin which lies between two known pressure regimes, a high pressure area and a transitional pressure zone. A three-dimensional (3-D), three-component (3-C), seismic survey was acquired from October 25 - November 6, 1991 over Section 30. The area used for engineering analysis and reservoir simulation was enlarged beyond the seismic survey area to eliminate boundary effects in the primary area of interest.
A large quantity of geological, geophysical, and engineering data has been acquired in Cedar Hill field. The Hamilton No. 3 well (SW1/4 Section 30) has one of the most complete suites of well logs and cores available within the field. Geophysical data collected include a 3-C Vertical Seismic Profile (VSP) in the Hamilton No. 3 Well and a 3-D, 3-C seismic survey encompassing 1- 1/4 square miles, with a bin size of 55x55 feet (Figure 2). Three-component receivers as well as compressional (P) wave and two orthogonal shear (S) wave vibrator sources, were used in the acquisition.
Amoco began production at Cedar Hill field in May, 1977. Since then, 23 production wells have been completed in the field. The eastern portion of the field was drilled on irregular spacing until 1984 when regulations required that wells be drilled on 320-acre spacing with the first well drilled in the NE1/4 or SW1/4 of a section. Thirty-one wells are completed in the Fruitland coals in the study area. While there is a long documented production history, there is limited pressure data available from the Fruitland coals. A total of 46.6 BSCFG has been produced from the study area through December 1993.
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