Logging Observation Wells in Gas Storage
Authors: Lovan T.E. 
Published: December 2007

The underground storage of gas is relatively new in certain parts of the country, and the problems involved are unique. Some of these problems can be solved through use of electrical wireline logging services. Information concerning the actual gas saturation, growth of the gas bubble, and leakage into thief zones can be obtained with a logging program of formation density logs and gamma ray-neutron logs. The selection of logging services for observation is governed by the completion method. If the hole through the storage zone is uncased, a formation density log can be used to determine gas saturation. If casing is set through the storage zone, the neutron log is employed. Field examples of projects located in Illinois, Indiana and Kentucky are used to illustrate the interpretation techniques.

Introduction

Gas is stored in permeable and porous formations which have a suitable cap rock and sufficient closure, caused either by structure or stratigraphy. The storage zones are of two main types:

those that were originally aquifers, containing only water, and those that contained gas or oil as well as water.

The well logs recorded in a gas storage observation well are, in many cases, the same as those used by the oil industry. There is, however, a significant difference in the interpretation and application of the logs. In a storage field the gas saturation is quite often changing daily as gas is injected or withdrawn, and occasionally gas moves from the storage zone to a different permeable and porous horizon and builds up there in considerable quantities. It is important to know how much gas is available in the storage zone and what quantity has migrated to another horizon. At the present time there are two tools used to determine gas saturations in the storage zone:

the formation density log, when casing is set on top of the zone, and the neutron log, when the casing has been set through the storage zone.

Formation Density Logging of Observation Wells

The formation density log measures the average bulk density of the formation investigated by the tool. The bulk density p, of a formation having a grain density pa and a porosity filled with fluid of density p, is given by:

(1)

The fluids of interest in gas storage reservoirs are formation water and gas. Formation water density p. ranges from about 1 to 1.15 gm/cc depending upon its temperature, pressure and salinity. The density of gas p, in these relatively shallow low-pressure reservoirs is negligible and may be considered to be zero. Thus, p, in Eq. 1 may be written as:

In a reservoir containing only water and gas, Eq. 1 for the bulk density may therefore be written:

where Sw is that fraction of pore volume filled with-water of density p. (the remaining fraction of pore volume, 1 - Sw, or Sg, being filled with gas whose density, pg is assumed to be zero).Solving the last equation for Sw and setting pw equal to 1:

Since(2)

Eq. 2 gives the gas saturation when, phi and p are known. may be taken as 2.65 gm/cc for most sandstones, as 2.71 gm/cc for most limestones and as 2.87 gm/cc for dolomites. The value of phi is known from core analysis or the density log or sonic log responses when the reservoir contained only water (prior to gas injection).The value p is the true bulk density of the zone when it contains some gas. Eq. 2 may be used when the true p is known.

JPT

P. 745ˆ