Estimating Gas-Water Contacts in Aquifer Gas Storage Fields Using Shut-In Wellhead Pressures
Authors: Burnett Peter , Ryan Robert 
Published: December 2007

Abstract
Continuous and accurate information on gas-water contact is desirable in the initial filling of aquifer storages to prevent possible loss of gas. Some information on the extent of the gas bubble is obtained as gas reaches observation wells completed in the storage zone and from periodic neutron logging. However, sometimes an insufficient number of wells are drilled for accurate structural control and for adequate knowledge of the gas bubble growth, particularly during the initial stages of development. This article presents two simple methods of estimating gas-water contacts in aquifer gas storage fields using unstabilized shut-in wellhead pressures. The first method involves setting up a shut-in wellhead pressure-vs-depth relationship for stabilized conditions. For unstabilized conditions an aquifer pressurization correction is applied enabling calculations to be made without the necessity of shutting in a field for the long period of time required for stabilization. The amount of pressurization of the aquifer over its original pressure can be obtained from a water observation well located near the edge of the gas bubble. This method gives the lowest point of the gas-water contact at the time the pressures are measured. In the second method, a pressure fall-off curve is extrapolated to stabilized conditions. It was found that shut-in time required to obtain a reliable extrapolation increased as the storage volume increased and pressurization of the aquifer occurred. Accordingly, the usefulness of this method is largely restricted to the early injection stage of field development or when no water observation well is available for calculations by the first method. Presented here are calculations of gas-water contacts based on pressure data from two storage fields which agree closely with other gas-water contact information.
Introduction
In developing gas storages in aquifers, experience indicates that initially the injected gas has a tendency to finger out rapidly through thin zones of high permeability, and often travels considerable distances horizontally and vertically. Since gas tends to move parallel to bedding planes, the gas-water contact is usually lower at the edge of the gas bubble than in the center. Often the injection pressure differential is high enough that it could force gas to spill out below the lowest closing contour between observation wells. Knowledge of the gas-water level is especially important during early stages of development. Some information on the horizontal and vertical movement of gas can be obtained as gas reaches observation wells completed in the storage formation and from neutron logs run periodically in both injection and observation wells. Katz proposed a method of calculating the horizontal distance from a water observation well to the edge of the gas bubble. However, none of these methods indicates how far the gas may extend down the structure between observation wells. when structural control is poor or unpredictable, as in the case of an erosional surface, an inherent danger exists that gas may escape at unknown spill points. During initial development, information on the movement of the gas-water contact is valuable in indicating whether operations are proceeding satisfactorily. Under normal operations the gas-water contact should progressively deepen as the volume of gas in storage is increased. If it does not, spillage or leakage is indicated. It is possible for the gas-water contact to remain constant at low rates of injection as a result of gravity drainage of water out of the gas bubble. However, periodic determinations of the gas-water contact as the reservoir is filled with gas at a uniform rate should reveal any abrupt decrease in movement which would indicate that spillage or leakage has developed. Gas-water contact data also provide a means of calculating water displacement efficiency being achieved. By determining the amount of gas stored in a unit rock volume, storage volumes at various levels of the gas-water contact can be more accurately estimated and arrival time of gas at an observation well can be predicted more closely.
JPT
P. 877ˆ