INTEGRATED RESERVOIR CHARACTERIZATION STUDY OF A CARBONATE RAMP RESERVOIR - SEMINOLE SAN-ANDRES UNIT, GAINES COUNTY, TEXAS

One of the important issues in constructing geologic and reservoir mod els is to define geologic frameworks. A geologic framework is fundamen tal to defining flow units, to interpolating well data, and thereby to modeling fluid flow. For the Seminole San Andres Unit (SSAU), the hig h-frequency cycles (HFC's) and rock-fabric facies identified on outcro p analogs and cores were used to correlate wireline logs. Reservoir an d simulation models of the outcrop and a two-section area of SSAU were constructed with rock-fabric units within the HFC's as a geologic fra mework. Simulations were performed using these models to investigate c ritical factors affecting recovery. HFC's and rock-fabric units are th e two critical scales for modeling shallow-water carbonate ramp reserv oirs. Descriptions of rock-fabric facies stacked within HFC's provide the most accurate framework for constructing geologic and reservoir mo dels, because discrete petrophysical functions can be fit to rock fabr ics and fluid flow can be approximated by the kh ratios among rock-fab ric flow units. Permeability is calculated using rock-fabric-specific transforms between interparticle porosity and permeability. Core analy sis data showed that separate-vug porosity has a very strong effect on relative permeability and capillary pressure measurements. The effect of stratigraphic constraints on stochastic simulation was studied. Ge ologic models generated by a conventional linear interpolation, a stoc hastic simulation with stratigraphic constraints, and a stochastic sim ulation without stratigraphic constraints were compared. The stratigra phic 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 ver tical direction was investigated in terms of the ratio of vertical per meability to horizontal permeability (k(vh)). This ratio decreases exp onentially with the vertical gridblock size up to the average cycle si ze of 20 ft (6.1 m) and remains at a value of 0.06 for a gridblock siz e of more than 20 ft (>6.1 m), which is the average thickness of HFC's . Simulation results showed that the critical factors affecting recove ry efficiency are stacking patterns of rock-fabric flow units, k(vh) r atio, and dense mudstone distribution.