Gravity monitoring of a water-flooded zone in areal sweep

1. A method of characterizing a water-flooded zone in an earth formation resulting from injection of water to the formation through an injector well borehole intersecting the formation, the method comprising:

estimating horizontal elongation of the water-flooded zone using a gravity field change over time measured in the injector well borehole, comprising:

predicting a gravity field change due to the injection of water for each of a plurality of model water-flooded zones for the borehole, wherein each model water-flooded zone is an elliptical cylinder having finite height and semi-axes; and

fitting the measured gravity field change to one of the predicted gravity field changes corresponding to one of the plurality of elliptical cylinder model water-flooded zones;

creating a model of the formation by using the estimated horizontal elongation of the water-flooded zone to estimate dimensions of the water-flooded zone; and

conducting secondary recovery operations in dependence upon the model, wherein the secondary recovery operations comprise at least one of: i) drilling operations in the formation, ii) injection operations in the formation, and iii) production operations in the formation;

wherein the horizontal elongation is an elongation of the water-flooded zone in one axis of a plane normal to a vertical longitudinal axis.

2. The method of claim 1 , wherein the gravity field change comprises a difference between a first estimated gravity field in the injector well borehole before injection and a second estimated gravity field in the injector well borehole after injection.

3. The method of claim 2 , further comprising taking a set of gravity measurements at a plurality of depths in the injector well borehole to estimate at least one of i) the first estimated gravity field; and ii) the second estimated gravity field.

4. The method of claim 1 , further comprising estimating a measure of uncertainty of the estimated horizontal elongation.

5. The method of claim 1 , further comprising estimating the dimensions of the water-flooded zone.

6. The method of claim 1 , further comprising creating a model of the formation using the estimated horizontal elongation of the water-flooded zone.

7. The method of claim 6 , further comprising conducting secondary recovery operations in dependence upon the model.

8. A non-transitory computer-readable medium product for characterizing a water-flooded zone in an earth formation resulting from injection of water to the formation through an injector well borehole intersecting the formation, the medium having instructions thereon that, when executed, cause at least one processor to perform a method, the method comprising:

estimating horizontal elongation of the water-flooded zone using a gravity field change in the injector well borehole, comprising:

predicting a gravity field change due to the injection of water for each of a plurality of model water-flooded zones for the borehole, wherein each model water-flooded zone is an elliptical cylinder having finite height and semi-axes; and

fitting the measured gravity field change to one of the predicted gravity field changes corresponding to one of the plurality of elliptical cylinder model water-flooded zones;

creating a model of the formation by using the estimated horizontal elongation of the water-flooded zone to estimate dimensions of the water-flooded zone; and

conducting secondary recovery operations in dependence upon the model, wherein the secondary recovery operations comprise at least one of: i) drilling operations in the formation, ii) injection operations in the formation, and iii) production operations in the formation;

wherein the horizontal elongation is an elongation of the water-flooded zone in one axis of a plane normal to a vertical longitudinal axis.

9. The non-transitory computer-readable medium product of claim 8 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a flash memory, or (v) an optical disk.

10. A method of characterizing a water-flooded zone in an earth formation intersected by an injector well borehole, the method comprising:

estimating elongation of a water-flooded zone by monitoring density changes in the formation from the injector well borehole, wherein the water-flooded zone is modeled as an elliptical cylinder with finite height and semi-axes, comprising:

predicting a density change due to the injection of water for each of a plurality of model water-flooded zones for the borehole, wherein each model water-flooded zone is an elliptical cylinder having finite height and semi-axes; and

fitting the density change to one of the predicted density changes corresponding to one of the plurality of elliptical cylinder model water-flooded zones fitting the density change to one of the predicted density changes corresponding to one of the plurality of elliptical cylinder model water-flooded zones;

creating a model of the formation by using the estimated horizontal elongation of the water-flooded zone to estimate dimensions of the water-flooded zone; and

conducting secondary recovery operations in dependence upon the model, wherein the secondary recovery operations comprise at least one of: i) drilling operations in the formation, ii) injection operations in the formation, and iii) production operations in the formation.

11. The method of claim 10 , further comprising estimating the dimensions of the water-flooded zone.

12. The method of claim 10 , further comprising conducting secondary recovery operations in dependence upon the estimated horizontal elongation of the water-flooded zone.

13. The method of claim 10 , further comprising creating a model of the formation using the estimated horizontal elongation of the water-flooded zone.

14. The method of claim 13 , further comprising conducting secondary recovery operations in dependence upon the model.