Abstract
An integrated reservoir characterization of Seminole San Andres Unit was conducted using outcrop and subsurface data. The high-frequency cycles and rock-fabric facies identified on outcrop and cores were used to correlate wireline logs. Reservoir and simulation models of the outcrop and a two-section area of the Seminole San Andres field were constructed using rock-fabric units within high-frequency cycles (HFC's) as a geologic framework. Simulations were performed using these models to investigate critical factors affecting recovery.
High-frequency cycles and rock-fabric units are the two critical scales for modeling shallow-water carbonate ramp reservoirs. Descriptions of rock-fabric facies stacked within high-frequency cycles provide the most accurate framework for constructing geologic and reservoir models because discrete petrophysical functions can be fit to rock fabrics and fluid flow can be approximated by the kh ratios among rock-fabric flow units. Permeability is calculated using rock-fabric-specific transforms between interparticle porosity and permeability. Core analysis data showed that separate-vug porosity has a very strong effect on relative permeability and capillary pressure measurements.
The stratigraphic features of carbonates can be observed in stochastic realizations only when they are constrained by rock-fabric flow units. Simulation results from these realizations are similar in recovery but different in production and injection rates. Scale-up of permeability in the vertical direction was investigated in terms of the ratio of vertical permeability to horizontal permeability (kvh). This ratio decreases exponentially with the vertical grid-block size up to the average cycle size of 20 ft (6.1 m) and remains at a value of 0.06 for a grid-block size of more than 20 ft >6.1 m), which is the average thickness of high- frequency cycles. Simulation results showed that critical factors affecting recovery efficiency are stacking patterns of rock-fabric flow units, kvh ratio, and dense mudstone distribution.
Introduction
More than 9 billion stock-tank barrels of oil has been produced from shallow-water ramp carbonates of the San Andres Formation, West Texas and New Mexico. Because reservoirs in this play are highly heterogeneous and stratified, waterflood recovery averages only 30 percent, and more oil can be recovered if reservoir characterization is done along with infill drilling and CO2 flooding programs. Major issues in characterizing carbonate reservoirs are geologic framework, interwell heterogeneity including rock-fabric facies and permeability structure, scale-up of petrophysical properties, and factors affecting recovery efficiency. Because well spacings in most San Andres fields in West Texas and New Mexico are greater than 600 ft >200 m), outcrops on the Algerita Escarpment in the Guadalupe Mountains, Texas and New Mexico, provide an opportunity to define geologic framework, to quantify interwell heterogeneity, and to develop methods for scale-up of petrophysical properties. Applying the results of outcrop investigations to subsurface reservoirs leads to the development of new methods and techniques for constructing 3-D reservoir and flow models for simulating fluid flow and forecasting performance.
The Seminole San Andres Unit (SSAU) lies on the northeastern margin of Central Basin Platform (Fig. 1) immediately south of the San Simon Channel. It covers approximately 23 mi2 and contains more than 600 wells. The field, discovered in 1936, is a solution-gas-drive reservoir with a small initial gas cap and has an estimated original oil in place of 1,100 MMSTB. Production comes from the Upper San Andres Formation and the upper part of the Lower San Andres Formation. The crude is 35 API and has an initial formation volume factor (FVF) of 1.39 and a solution-gas ratio of 684 SCF/STB.
Occupancy and habitat correlates of javelinas in the southern San Andres Mountains, New Mexico