Threshold Pressure Profiling by Continuous Injection
Authors: Rudd Neilson , Pandey G.N. 
Published: December 2007

This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. 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.
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Abstract
Conventional methods for threshold pressure determination in gas storage cap pressure determination in gas storage cap rock measure the pressure at which gas displaces the wetting phase only in the zone immediately adjacent to the input end of the core sample. Since cap rock is rarely lithologically homogeneous, the significance of threshold pressures so measured is uncertain, dependent largely upon the lithology lying adjacent to the input end of the sample.
If an incompressible, non-wetting liquid is injected into the sample at a constant rate, sufficiently slow that the pressure resulting from frictional flow resistance is small relative to capillary displacement pressures, the injection pressure increases pressures, the injection pressure increases to a maximum as the non-wetting phase approaches high threshold pressure zones and then decreases as the threshold pressure is exceeded and fluid continuity to the nonwetting phase is established. Discrete threshold pressure events ranging from less than 100 to over 2,000 psi may be observed as the core is penetrated by the non-wetting phase and may be correlated with visible phase and may be correlated with visible lithologic zonation providing specific threshold pressure characteristics for each lithology.
Analysis of the volume of the nonwetting phase injected together with the characteristics of the threshold pressure events themselves provide insight into the nature of the break-through of the injected liquid.

Introduction
Both the conventional method for determination of threshold pressure in gas storage cap rock and the more recently developed pressure leveling technique measure the pressure at which gas displaces the wetting phase (water) only in the zone immediately adjacent to the input end of the core sample, the threshold pressure being commonly read as the first displacement of water in the conventional technique or the equilibrium pressure of a small volume of injected gas in the pressure leveling technique. Since most gas storage cap rock is heterogeneous, the significance of threshold pressures measured by either of the prior techniques is uncertain, being a function of the sample preparation process which controls the lithology adjacent process which controls the lithology adjacent to the input end of the sample.