Measurements of the Viscosities of Oils under Reservoir Conditions

Abstract

The viscosity of the oil in the reservoir is one of the properties thatinfluence its movement through the sand to producing wells. Measurements ofviscosity, therefore, are pertinent to problems associated with well behaviorand with the estimation of recoveries, and afford an indirect means for partialevaluation of various methods of controlling reservoir behavior. The effect ofdissolved gases on the viscosity of crude oil has been determined, but no datahave been published on the viscosity of representative samples of reservoiroils, This paper describes a simple instrument that has been used to determinethe viscosities of a number of subsurface oil samples at the temperatures andpressures existing in the reservoirs, and presents the results of thedeterminations for typical fields. Construction of the Apparatus The principal requirements of any instrument used for the examination ofsubsurface samples are that it be strong and simple both in design and inmethod of operation. Accuracy beyond that of the degree of reproducibility ofsubsurface samples from various wells in a reservoir or exceeding that of thecommon measurements of reservoir temperatures and pressures is not required.Because of the expense of procuring subsurface samples, it is necessary alsothat the instrument operate on a relatively small fraction of a sample, leavingthe remainder for other tests, and that there be few or no failures of the equipment to cause undue delay orloss of a sample. After preliminary experiments with a falling bullet, the results of which werenot satisfactory, a simple viscosimeter was built of the rolling ball typefirst proposed by Flowers and later used by several investigators. Theapparatus consists essentially of a removable, accurately bored cylindricalbarrel of 1/4-in. nominal internal diameter, 8 in. long, in which a closely fitting steel ball rolls through the oil with thebarrel inclined at a definite angle, The ball makes contact at one end of thebarrel with an insulated electrode, closing an electrical circuit, whichactuates a buzzer. The measurements consist essentially in determining the timerequired for the ball to travel the length of the barrel. The details of the construction are shown in Fig. 1. The barrel in which theball rolls was made from a section of 2s-caliber blank rifle barrel, speciallybored to an exact uniform diameter and polished. The barrel slides snugly intoa hole bored in a solid stainless-steel cylinder, an upper external shoulder ofthe barrel compressing a small spring, and is held in place by a hollow nut.The spring prevents the barrel from seating against the bottom of the boredhole in the cylinder, while narrow external longitudinal slots in the barrelpermit fluid to flow around it and through the bottom. T.P. 1220