A Model for the Origin of Underpressured and Overpressured Tight Gas Systems: Rate Competitive Gas Generation, Water Drainage and Gas Leakage
S. W. Burnie1, Dr. Brij-Maini2, Kaush Rakhit1, and Bruce R. Palmer1
1 Rakhit Petroleum Consulting Ltd., Calgary, AB, Canada
2 Dept. Pet. & Chem. Engineering, University of Calgary, Calgary, AB, Canada
Underpressured, normally pressured and overpressured Tight Gas Systems such as the Alberta Deep Basin, the Greater Green River Basin in southwestern Wyoming and the shallow Milk River gas reservoir in southern Alberta have common aspects. These are: low permeability (0.1 md to 0.1 ΦD), tall gas columns, variable but usually low water production and a “reservoir” in which gas is the continuous fluid on a regional scale. Investigation of these common factors, using a theoretical and practical approach lead to the development of a four stage model to explain the evolution of these tight gas systems. The four stages are: Genesis, Transition, Steady State and Imbibition (Figure 1a, 1b, 1c, 1d, respectively). In the initial stage, Genesis, the extent of water drainage from the reservoir by buoyant gas movement determines the amount of water production. As gas becomes the continuous phase, the gas column may not yet be tall enough to have pushed enough water from the rocks to reduce the water saturation to a level that would give water free gas production. In the later Transition and Steady State stages, gas has become the continuous phase on a regional scale and enough water has been drained from the system such that the seals to the system are no longer effective and gas continually leaks from the reservoir. The tall gas columns that are characteristic of the Transition and Steady State stages then become underpressured. As gas continues to migrate through the system and leak to the regional aquifer, the “reservoir” is further dried-out, by vapor extraction. Water production from the underpressured Steady State systems is usually low and generally limited to water of condensation. However, in areas where drainage has not been effective (by-passed aquifers), water production can and does occur. These wet areas are restricted in extent. The last stage, Imbibition, occurs at some time after peak gas production when much of the gas has migrated from the system and the reservoirs are at the lowest pressures. In this last stage, water is very slowly re-introduced into the Tight Gas System through the reservoirs with the lowest permeability. The gas-charged system is then very slowly returned from whence it came, to an aquitard. It is possible to observe the first three stages in the Alberta Deep Basin (Figure 2a), and the Green River Basin (Figure 2b). The last stage has not yet been documented in the field but, from a theoretical point of view, it should exist, and it has been observed in the laboratory.
A capillary model was developed in the laboratory to demonstrate the properties and confirm the developmental stages of the Tight Gas System. Experimentation showed that all four developmental stages could be reproduced and occurred as a continuum. The amount of water left in the system was governed by the extent of drainage before the system begins to leak and become underpressured. The higher gas pressures, relative to normal hydrostatic conditions, were shown to be the result of the pressure difference, due to capillarity, between the gas column and the aquifer. The higher pressures could also be a result of aquifer compression due to the rate of gas influx exceeding water drainage. Subnormal gas pressures developed by continued gas leakage from the system, beginning at the time when the gas column became tall enough to exceed the capillarity of the upper capillary tube “seal”. Also, it was necessary to reduce the influx of gas to the system and restrict the water entry to flow only through the lower capillary tube so that mass entering the system was less than the mass of gas that leaked from the system. Only then could underpressuring occur. Since capillary tubes were used to study the Tight Gas System model, the effects of relative permeability, that were theorized to occur in the early stages of development, could not be verified at this point.
In any tight gas system, it is necessary to identify the developmental stages in order to effectively develop and explore for further reserves.
Figure 1. Four-stage model: evolution of tight gas systems.Figure 2. a. Pressure-elevation plot, Alberta Deep Basin. b. Pressure-elevation plot, Green River Basin.