ABSTRACT

We are becoming increasingly aware of compartmentalization of reservoir sands by small-offset faults as methods for detecting these faults improve. Faults can act as hydrocarbon conduits into the compartment, escape routes, or seals; whether or not a fault-bound compartment is a good target depends on the availability of hydrocarbon charge and the contrast between permeability and capillary pressure of the reservoir and the bounding faults. To determine migration paths of oil along faults and through a series of stacked permeable reservoirs separated by impermeable shale, and quantify potential reserves, we used off-the-shelf reservoir simulation software (Eclipse) to study the effect of varying fault and reservoir properties on hydrocarbon charge. We constructed a simple model geometry (Fig. 1) to represent stacked fault-bound reservoirs observed in the Ship Shoal 274 field, Louisiana OCS, Gulf of Mexico, and used reservoir and seal properties consistent with those observed in the field. Oil flows up one bounding fault under a small but constant overpressure and is allowed to escape along a second fault after passing through one or both of a series of flat-lying reservoirs. Results discussed below focus on the role of fault permeability and capillary pressure in determining flow paths and oil retention in the reservoir.

Figure 1. Model geometry and properties. Fault with is exaggerated for illustrative purposes. Boundary conditions are described in text. Reservoir permeability (k) is on the order of 100 md for all models, reservoir porosity is 30%, and reservoir capillary pressure is shown at right as capillary pressure in a mercury-air system (pcma) with respect to water saturation (S_w). For the shale seal, permeability is 10-6 md and porosity is 15%. Fault permeability varies from 0.1 ("low k") to 1000 ("high k") md, porosity varies from 15 to 30% , and the variability in capillary pressure is shown at right

Since oil typically flows in the direction of increasing permeability and decreasing pressure, charge along a high-permeability fault can result in a primary migration pathway that bypasses the lower reservoir and favors charge of the upper reservoir (Fig. 2a). Charge along a low-permeability fault favors charge of the lower reservoir (Fig. 2b). Migration paths through either the upper reservoir or the lower reservoir are maintained throughout the charge history; however, some secondary charge of the upper reservoir can result from remigration of oil from the lower reservoir, as is seen in Figure 2b. Charge resulting from both migration and remigration is consistent with patterns of oil distribution observed in the field.

An actively charging system can maintain anomalous column heights. Our models suggest that reservoirs "trapped" by high-permeability faults may be fruitful targets either if charge is ongoing or during remigration.

Once the oil front moves across the reservoir, charge continues, but a percentage of oil begins to escape the reservoir. Both the rate of escape and the ratio of retained to escaped oil depend on bounding-fault properties. A thousand-fold increase in permeability of the escape fault can result in a ten-fold increase in escape rate (Fig. 3). However, the quantity of escaped oil also depends on charge properties.

Capillary pressures of potential seals are too often ignored; our model results indicate that it is necessary to identify fault capillary pressures to adequately estimate oil reserves. When we varied fault and reservoir capillary pressures independently of permeability, charge paths were broadly the same as in the cases described above, but differed in detail, as did oil saturation distribution (Fig. 4). Thus, while flow path can be qualitatively predicted from fault and reservoir permeability, quantification of oil charge and entrapment requires a good capillary pressure model.

End_Page 8------------------------

Figure 2. Oil saturation distribution in reservoirs after migration pathways have developed. In Figure 2a (LHRL), the left-hand fault (the supply fault) has a high permeability (1000 md) while the right-hand fault (the escape fault) has a low permeability (1 md). Refer to Figure 1 for position of faults. In Figure 2b (LLRH), the supply fault has a low permeability (1 md) while the escape fault has a high permeability (1000 md).

Figure 3. Comparison of oil escape to oil retention for two cases. In both models, the permeability of the supply fault is high (1000 md); in the LHRL model, the escape fault permeability is low (1 md) while in the LHRH model, the escape fault permeability is high (1000 md). Figure 3a show the onset of escape (x-axis intercept) and the escape rate in stock-tank barrels per day (STB/day). Figure 3b shows the ratio of escaped to retained oil for both models as a function of time.

Figure 4. Oil saturation distribution for model in which capillary pressure was zero everywhere for all values of water saturation (S_w). Other fault and reservoir properties and boundary conditions are identical to those of model LHRL (Fig. 2a)

End_of_Record - Last_Page 9-------