Drainage of Water from Sand in Developing Aquifer Storage
Authors: Briggs James E. , Katz Donald L. 
Published: December 2007

This paper was prepared for the 41st Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to held in Dallas, Tex., Oct. 2–5, 1966. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Establishment of a usable gas storage reservoir in aquifer sands often requires that injected gas rise to displace water above it in the formation. Experimental work has been done on a simplified model of this process. Calculations with and without capillary forces were made in an attempt to simulate the experimental results. These consistently predicted quicker completion of the segregation than was observed. This leads to the conclusion that, without data on dynamic capillary forces, the types of calculation described give a reasonable lower bound on the length of time necessary for the segregation to occur. Calculations are shown for two fairly realistic cases and show the marked effect of a thin low permeability layer on the rate of segregation and gas bubble formation.

Introduction
When an aquifer gas storage field is first being developed, gas is injected at or near the top of the structure. The gas displaces water in the zone or layer having the lowest displacement pressure - which generally is the zone of highest permeability. Figure 1 shows how the gas-water interface may be conceived to develop. The water in the layers between those containing gas will often drain slowly down through the gas layers, while the gas rises, until a "bubble" is formed. After some time-perhaps several years - the water in the sand at the top of the structure will have drained down to its residual content and the gas bubble will roughly correspond to the shape of the cap rock.
In order to plan an efficient storage operation, it is necessary to estimate how long a time will be required for formation of the gas bubble. Equations can be derived using various assumptions and solved analytically or numerically to simulate this countercurrent gravity segregation process. An important question is, naturally, how well the calculations describe the physical process, i.e., can the length of time required for bubble formation be estimated closely? To study this problem, experiments were done on a simplified model of the drainage process and then calculations were made in an attempt to simulate the observed results. These results, as well as two examples of calculation using typical field data, are given in subsequent sections of this paper.