Determining Gas-Filled Pore Volume in a Water-Drive Gas-Storage Reservoir
Authors: Hubbard R.M. , Elenbaas J.R. 
Published: December 2007

Introduction
The increased importance of underground natural gas storage as a means of solving marketing problems has dictated that engineering attention be directed to the study of storage-field behavior. As a result, considerable effort has been expended by various researchers in attempts to determine methods by which the behavior of depleted producing gas fields subject to water flux and utilized for storage could be described and predicted. One of the most extensive recent efforts in this area has resulted in the monograph, "Movement of Underground Water in Contact with Natural Gas".This paper, using as background much of the material presented in the monograph, explores the determination of gas-filled pore volume. This includes the initial, or discovery gas-filled pore volume, as well as the pore volume at any time during the field's history. Comparison of the initial and the gas-filled pore volume at some later date indicates growth of the gas bubble. The term gas bubble will be used to denote the gas-filled pore volume. The need for additional storage capacity frequently leads to storage of gas at pressures exceeding the reservoir discovery pressure. Under these conditions the gas bubble grows in size. Because there is always some safe limit of gas bubble size, knowledge of its size at any time is desirable. The ability to reliably predict gas-bubble growth from a postulated gas-in-place schedule is of considerable value when evaluating possible storage programs. An efficient and effective method for determining values of the factors governing water movement and field behavior is required. In this paper a material balance as the equation of a straight line has been utilized to evaluate these factors, which include initial gas-bubble size. This general procedure is not new, and discussions of several approaches using the linearity of the material balance to determine the factors governing the behavior of waterdrive oil and gas reservoirs are found in the literature. However, the use of the form of linear material balance equation presented here, to the authors' knowledge, has not been previously reported and will be shown to be a comprehensive and instructive procedure for determining the necessary factors. This method provides for the determination of two of the four factors simultaneously, and implicitly for values of the other two. Selection of optimized values for the factors utilizes as a criterion the minimum deviation between calculated and observed field pressures. Once the factors influencing reservoir behavior have been determined, they may be used with a gas injection-withdrawal schedule to calculate reservoir pressure and gas-filled pore volume behavior resulting from past or future storage operations.

Discussion
Material Balance Applied to Storage ReservoirsA common procedure used in an attempt to describe and predict the behavior of a petroleum reservoir is the material balance, which in its most general form relates the amount of material remaining in the reservoir to the amount originally present, less the amount produced from the reservoir.
JPT
P. 383ˆ