A Technique for the Determination of Capillary Pressure Curves Using a Constantly Accelerated Centrifuge

Abstract

HOFFMAN, ROBERT N., MISSOURI SCHOOL OF MINES, ROLLA, MO. JUNIOR MEMBER AIME Abstract A new technique for determining capillary pressure curves has been developed and tested. The technique differs from previously reported centrifuge techniques in that the centrifuge is slowly accelerated from zero to the maximum desired speed rather than being held constant at particular, progressively higher speeds. An important advantage of this technique over other methods of determining capillary pressure curves is the short time required to obtain the desired amount of data over the chosen pressure range. For example, to obtain 30 data points between 1.2 and 104 psig with a 1.55-in. long, 3/4-in, diameter core using a brine-air system, 6. 6 hours were required with this technique. An equally important development of this paper is an analytic method for the conversion of the data from the centrifuge experiment to capillary pressure curve data. Previously there has been only an approximate conversion available.Although the capillary pressure curves determined by this technique appear to be as accurate as those determined by other techniques, the accuracy could be improved if certain variables, not treated in this experiment, were investigated. Among these are the dynamic distortion of the centrifuge equipment and imperfect initial saturation of the cores. Introduction Pirson defines capillary pressure in a porous medium as "the differential pressure that exists between two fluid phases at their interfaces when they are distributed under static equilibrium within a porous material". Capillary pressure in rocks is known to be a function of fluid saturation, among other things, and a capillary pressure curve is defined for the purposes of this paper as a plot of the capillary pressure-wetting phase saturation relationship for a particular rock sample.Several methods are used for determining capillary pressure curves for small rock cores. Prominent among these are the semi-permeable barrier, mercury injection, and, to a lesser extent, centrifuge methods. The semi-permeable barrier method is currently the most popular. It features simplicity in both execution and the mathematical conversion of the experimental data into a capillary pressure curve. The main disadvantages of the semi-permeable barrier method are the time required - as long as two months - to obtain several points for the curve, and a fairly low maximum pressure before breakthrough of the non- wetting phase into the barrier occurs, for example, about 32 psig for a brine-air system.It is for these reasons that other methods such as the centrifuge method have been introduced. High accelerations and the absence of a barrier result in quicker attainment of saturation equilibrium at a given pressure. However, the centrifuge method involves much more expensive equipment and more difficult procedures and calculations than does the barrier method. The purpose of this investigation has been to improve the equipment and procedures of the centrifuge method and to develop an analytic method for the conversion of the experimental data into a capillary pressure curve.Hassler and Brunner did the original work in the determination of capillary pressure using a centrifuge. In their work the centrifuge speed was increased in a step-wise manner, each speed being held constant until saturation equilibrium was reached in each core. Saturation equilibrium was indicated when the volume of liquid collected in the graduated pipette of the core holder remained constant. According to Hassler and Brunner, equilibrium was reached in "a few minutes to one-half hour or more".In the centrifuge method, as opposed to the barrier method, the fluid saturation of the core is not a constant throughout the length of the core, but varies with the radius of centrifugation. Also, the capillary pressure cannot be read directly but must be calculated from a knowledge of the centrifuge speed and other parameters. SPEJ P. 227^

Heat and mass transfer in water‐laden sandstone: Convective heating

“…Capiflury Pressure-Saturation Curue. The capillary pressure vs. liquid water saturation (defined as the fraction of pore space filled with liquid water) was determined in the presence of air by the constant-sped centrifuge method (Hoffman, 1963). A fully saturated sandstone sample, 3.4 cm long and 2.3 cm in diameter, was centrifuged in a model UV International Centrifuge until constant weight was attained, then the final weight of the sample was recorded.…”

Computation and Interpretation of Capillary Pressure From a Centrifuge

“…Hoffman 6 proposed a method based on increasing the rotation speed from zero to the maximum desired speed, rather than increasing rotational speed after reaching drainage equilibrium at each speed. Luffel 7 discusses this proposal further.…”

Measuring Capillary Pressure Tells More Than Pretty Pictures

“…Comparison with results found by porous-plate method showed good agreement of the drainage capillary pressure curves [7]. Many variations and improvements to Hassler and Brunner's method of calculating saturation and experimental procedure followed in the next decades [8][9][10][11][12]. In 1986 Rajan proposed an analytical solution to the problem of accounting for the changing centrifugal force along the sample length [13].…”