Determination of True Effective In Situ Gas Permeability in Subnormally Water-Saturated Tight Gas Reservoirs

Abstract

Abstract The phenomena of subnormally saturated, very low permeability gas reservoirs, which exhibit the unique combination of very low initial water saturation (less than the expected capillary equilibrium value) combined with low to very low permeability (0.0001 - 0.1 mD) have been well-documented in the literature and exist extensively in a number of regional sedimentary basins on a worldwide basis. Previous technical papers have concentrated on the mechanism to create these types of reservoirs and how to properly measure the in situ subnormal water saturation and connate water composition. This paper provides an extension of this work, which allows the direct measurement, under in situ reservoir conditions of temperature, pressure, and overburden pressure, of the true effective permeability of gas under any possible range of porosity and water saturation conditions that can conceivably exist in the reservoir. This procedure, known as the "incremental phase trap test," provides a valuable tool in the evaluation of whether given pay zones in an ultra-tight gas reservoir situation are worthy of completion. It also provides information on how this new method can be used in the exploitation of this ever increasing area of tight gas reservoir production. Introduction The value of the initial water saturation present in gas reservoirs can strongly affect the effective in situ permeability of the gas. Many very low permeability gas reservoirs (in situ permeability of less than 0.1 mD) are prone to large reductions in effective gas permeability if normal capillary equilibrium water saturations are present in the porous media. A capillary equilibrium water saturation refers to the water saturation which would be present if the formation matrix of a given pore size distribution is in direct hydraulic contact with a free water zone at a certain height above the defined free water contact level. In many cases where these types of low permeability, gas-bearing formations are productive, theinitial water saturation exists in a subnormal capillary equilibrium condition (where the initial water saturation is lower than would be expected in a normal "equilibrium" situation. This is favourable from a reserves and productivity perspective, but makes these types of formations much more susceptible to various types of formation damage, notably phase trapping. A more detailed discussion of formation damage mechanisms in tight gas reservoirs, and the concept of subnormal initial water saturation conditions, is contained in the literature(1-14). A common problem in formations of this type is that the exact value of the initial water saturation is unknown, or, considerable variations in water saturation exist throughout the reservoir due to permeability contrasts or varying degrees of desiccation caused by the spatial location of the sediments and degree of capillary continuity that exists in the formation. Therefore, a method to accurately etermine the true effective in situ permeability of the formation at any possible combination of immobile water saturation and porosity level is very valuable.