GRAPH FACILITATES TRACKING WATER AND GAS INFLUX

H.J. Gruy
Gruy Engineering Corp.
Houston

Graphing the vertical distribution of reservoir volume is an easy method for estimating the acre-ft remaining to be exploited in reservoirs with water or gas encroachment.

To evaluate reservoir performance and estimate oil and gas reserves in waterdrive reservoirs or oil reservoirs with a gas cap, it is necessary to determine the magnitude of the movement of oil-water and gas-oil contact surfaces.

In reviewing reserve estimates and reservoir studies done by others, we have found that very few reservoir engineers or geologists have an easy method for tracking the movement of these surfaces and estimating the volumes of oil displaced water encroachment, gas cap expansion, or the volumes of oil lost by wetting the gas cap. The following method evolved from the author's studies of the East Texas field starting in 1942, and it took this form in the early 1950s.

MAPS NEEDED

To study an oil or gas reservoir, it is necessary to map the top and base of the net pay interval (Figs. 1 and 2) and to construct an isopachous map (Fig. 3) using the method presented by Jay B. Whorton Jr. in the AAPG Bulletin of July 1948.

This method also requires constructing a net sand map (Fig. 4) without regard to whether the sand carries oil, gas, or water. The net sand map should cover an area considerably larger than the reservoir to show sand thinning or pinchout that might not be apparent from the producing wells.

The oil-water contact lines on the top and base of the pay are traced on this map. An overlay is then used to construct the net hydrocarbon isopach (Fig. 3). The contours inside the water contact with the base of pay will remain as mapped on the "total net sand" map (Fig. 4).

The next step is to contour the wedge area which is underlain by water. This is the area between the water contact on the base of the pay and the water contact with the top of the pay. A proper treatment of this will result in sharp points where the wedge contour intersects the trace of water on the sand base.

Geologists do not like to draw sharp points saying that points don't occur in nature. However, the intersection of water with the sand base does result in a sharp change of direction by the isopachous lines.

We find that few people map the base of the pay although it is necessary in preparing a proper isopachous map showing the correct sand volume, unless the oil or gas is completely unlain by water so that the water level is not in contact with the base of the net sand.

VERTICAL DISTRIBUTION

After having prepared these maps, the following seven steps are needed to construct a vertical distribution of reservoir volume:

Planimeter the area enclosed by each contour on the structure maps on the top and on the base of the pay. Go at least one contour below the water contact.
Planimeter the net hydrocarbon isopachous map.
Plot the area enclosed by the contours vs. subsea depth as shown in Fig. 5.
Planimeter the area on Fig. 5 above the oil-water contact and between the top and base of the sand.
Divide the net acre-feet shown by the net hydrocarbon ispachous map by the gross acre-feet obtained in Step 4. The answer is the average net-to-gross sand ratio. If this is a reasonable number, one can have confidence that the calculations are correct so far.
Planimeter the gross sand area as in Step 5 by 10 or 20 ft increments to construct the curve for the percent of reservoir volume above water contact (right side of Fig. 5).
Place wells on Fig. 5 at their proper sand top. Show interval open to production along with dates of first water and current water cut.
From Fig. 5, it is then possible to:
- Estimate the water-level position at various times
- Calculate the percent of acre-feet flooded to those times and the barrels per acre-foot recovered
- Estimate the cumulative production to the flood out of the various wells and thus estimate their oil or gas reserves.
If gas-oil contacts are present, they can be handled in similar fashion. One can see immediately from these graphs if there is a sufficient volume of attic oil or gas to justify additional drilling.