Capillary Behavior in Porous Solids

TLDR

Capillary behavior of fluid mixtures in reservoir rocks is essential for petroleum production, yet the theory remains incomplete, distinguishing static balance between capillary and gravitational forces from dynamic flow driven by gravity, capillarity, and pressure gradients, and assuming capillary equilibrium for fluid distribution. The paper aims to apply established thermodynamic and physical principles to capillary problems in reservoir rocks to advance the underlying theory. The study discusses static capillary problems, presents experimental data on the capillary properties of unconsolidated sands, and assumes reservoir fluids are in substantial equilibrium from long‑term undisturbed contact. The authors conclude that reservoir fluid distribution at tapping can be predicted from capillary equilibrium theory if measurable rock properties are known.

Abstract

Knowledge of the theory underlying the behavior of mixtures of fluids inreservoir rocks is essential to the proper solution of certain types ofproblems in petroleum production, but is as yet incompletely developed. Theobject of this paper is to show the application of well establishedthermodynamic and physical principles to these problems, and thus to assist inthe development of the basic theory. For convenience the problems to beconsidered here may be divided into two groups:Static problems, involving only the static balance between capillary forcesand those due to the difference in densities of the fluids; i.e., gravitationalforces.Dynamic problems, involving analysis of the motion of mixtures of immisciblefluids in porous media under the influence of forces due to gravity, capillarity, and an impressed external pressure differential.Capillary Equilibrium in Sands. Under this heading the static type of problem will be discussed and the resultsof experimental investigations on the capillary properties of unconsolidatedsands will be presented. Although the discussion of this section is, in asense, prefatory to the treatment of problems of mixture flow, the conceptsdeveloped here have considerable intrinsic importance apart from theirapplication to flow problems. For, it is reasonable to postulate that thereservoir fluids are, owing to their long existence in undisturbed mutualcontact prior to exploitation, in substantial equilibrium. It follows thattheir distribution in the reservoir at the time of tapping should be entirelypredictable from the theory of capillary equilibrium, provided certainexperimentally measurable properties of the reservoir rock are known. Knowledgeof the distribution of the several fluids in the reservoir is, of course, helpful in the estimation of reserve, and in other problems.It is to be emphasized that throughout the discussion of capillary statics itis assumed that the fluids are in equilibrium from the capillary standpoint.Thus, water, where it is referred to as being in a reservoir, will beunderstood to be interstitial water, present at the time of drilling thereservoir, commonly termed