This paper presents a dimensionless number and its critical value for predicting the onset of instability during immiscible displacement in porous media. The critical dimensionless number obtained from a stability theory for a cylindrical system successfully predicted the onset of instability in laboratory floods. Therefore, this number can be used to classify the stability of two-phase incompressible displacements in homogeneous porous media. Introduction When a fluid displaces a more viscous fluid, the displacement front may become unstable, resulting in viscous fingering. This phenomenon raises both practical and theoretical concerns. Apart from further reducing the displacement efficiency of an already inefficient displacement arrangement, instability may invalidate the usual method of simulating immiscible displacement performance based on relative permeability and capillary pressure concepts. Also, it introduces an additional scaling requirement for using model tests to forecast prototype displacement results. Therefore, it would be most beneficial to predict the onset of instability, so as to avoid viscous fingering, or, where it is unavoidable, to be able to recognize it as a factor in the displacement.The onset of instability call be predicted by a stability analysis of the displacement. The objective of such an analysis is to determine the conditions under which small disturbances or perturbations of the displacement front will grow to become viscous fingers. Ideally, the analysis should give a universal dimensionless scaling group together with its critical value above which instability will occur. The stability classification then would entail no more than the calculation of one dimensionless number in a manner analogous to the calculation of a Reynolds number to distinguish between laminar and turbulent flow.Several stability studies of immiscible displacement have been reported in the literature. Collectively, they show that these variables are pertinent to the stability problem:mobility (or viscosity) ratio,displacement velocity, system geometry and dimensions,capillary and gravitational forces, andsystem permeability and wettability. However, none of the previous studies have combined these variables into one dimensionless number that can be used to quantify the stability classification.The objective of this study was to obtain, by means of a stability analysis, a universal dimensionless scaling group and its critical value for predicting the onset of instability during immiscible displacement in porous media. This paper shows how the stability theory of Chuoke et al. was extended to achieve this objective and presents the results of laboratory floods that confirm the predicted onset of instability in cylindrical cores. Theory The pertinent dimensionless number for predicting the onset of instability was obtained by extending the stability theory of Chuoke et al. Their theory was based on a piston-like unperturbed displacement model in which the oil and water zones are separated by a planar interface. Details of the theory and our extension of it are presented in the following sections. SPEJ P. 249^

The injection of mixtures of flue gas and steam has been pro-posed in conjunction with the development ofdownhole steam generators, the Vapor Therm process and the Wet Air Oxida-tion Boiler. The combined gas-steam injection process may be superior to steam-only injection in terms of improved oil pro-duction performance and reduced levels atmospheric pollu-tion. This paper reviews previous experimental and numerical simulation work related to gas-steam injection and presents the results of an experimental study of steam-flooding with nitrogen and carbon dioxide additives. The experiments were condtuctedin linear porous media which were saturated with a T.G. Harding Thomaas G. Harding is currently a graduate student in petroleum engineer-ing at the University of Alberta. Previously, he worked for B. P Explora-tion Canada Limited as a reservoir engineer in the Oil Sands Department. fvlr. Harding holds B.Sc. and M.Sc. degrees iii cheniical engineering, both from the University Of Calgary. His research interests include oil sands technology, thermal recovery methods and numerical simulation. * now with BP Exploration Canada Limited S.M. Farouq Ali S.M. Farouq Ali is a professor of petroleum engineering at the Universi-ty of Alberta, where he has been since 1979. Previously, he was with the Pennsylvania State University. He holds a BS degree in electrical engineering from Karachi University, a BS in petroleum engineering ftom igham University, and MS and PhD degrees in petroleum and natural gas engineering from Pennsylvania State University. moderately viscous refined oil and water. Several tests involved in the injection of slugs of gas followed by steam but the majority used the simultaneous injection of the gases and steam. It was found that for the systems studied, the addition of the gases to steam resulted in a slight improvement in over-all recovery but a marked improvement in the rate of production of oil. Donald L. Flock After serving for a short period as a research engineer and project leader with Cities Service Research & Develop-ment Co. in Tulsa, Oklahoma, Donald Flock joined the staff at the University of Alberta in Edmonton, Alberta, Canada, in 1957. As a professor of petroleum engineering, he carried out fundamental researchch in the specialized areas of secondary recovery in-cluding miscible fluid displacement and thermal methods of oil recovery.

A laboratory study was conducted to determine the recovery of high-viscosity crude oil from natural uncon-solidated sand by WATERFLOODING with a number of chem-ical solutions. Displacement tests were run using chemical solutions to displace a high-viscosity crude oil from ar-tificial packs of unconsolidated core sand. Results of the displacement tests showed that oil recovery was increased by a number of chemical solutions.Sodium hydroxide in brine at concentrations less than 0.01 per cent by weight had little effect on recovery as floods. Sodium hydroxide concentra-cent or greater increased recovery compared to brine tions of 0.1 per cent significantly. P. M. Dranchuk received his formal training in petroleum engineering at The interfacial tension between Lloydminster crude oil and various aqueous solutions was greatly reduced by the addition of a number of chemicals to the aqueous solutions. The presence of sodium chloride in the aqueous solution the amount of chemical required for the reduction of the interfacial tension between crude oil and the solution to a given level. The reduction in interfacial tension influenced the recovery of oil.The production of a very stable emulsion and evidence of reduced water mobility during displacement tests sug-gested that emulsification may have occurred within the core. The formation of an emulsion may have increased Introduction THERE ARE large known reserves of heavy oil in West-ern Canada. However, the nature of the reservoirs and oils involved are such that primary recovery is estimated at 5 to 6 per cent"', with a possibility of increasing total recovery to about 10 per cent of the oil in place by means of conventional WATERFLOODING. In order that these recoveries be increased to a more acceptable value, new secondary recovery techniques must be developed.This study represents a continuation of a series of investigations conducted at the University of Al-berta'2,:" into the effect of chemical additives on waterflood recovery from heavy oil reservoirs. It is well known that the preferential wettability of the solid surfaces in porous media influences their relative permiability, capillary pressure and water-floodability characteristics. These properties can be altered by introducing any one of a variety of liquids which alter the contact angle. However, if a surface is oil-wet, the contact angle (conventionally measured through the (denser phase) cannot be reduced below 90 degrees, so that further reductions in adhesion ten-sion and therefore energy requirements for oil dis-placement can only be realized through reductions in interfacial tension. Reductions in interfacial tension also serve to reduce capillary forces, thereby further reducing the amount of energy required for oil dis-placement. Similarly, changing a system from an oil-wet to a water-wet state improves the relative per-meability to oil and increases the efficiency of the oil displacement process.Several investigators have reported obtaining higher waterflood recoveries from preferentially water-wet sys...

A mathematical model was formulated to describe the injection of saturatedsteam down oilwell tubing under constant inlet conditions. The system was divided into three parts, consisting of the fluid, thewellbore and the formation. Each part was considered to be a separate system, with the heat flux across the boundaries acting as the coupling parameter. Thewellbore consisted of a tubing string enclosed by either one or two casingstrings. The heat transfer in the wellbore was considered as steady state. Theheat transfer in the formation was treated as unsteady-state radial conduction, and the fluid flow was described by a modified two-phase flow correlation. The analysis resulted in three equations, two of them implicit, which weresubsequently solved simultaneously on the 7040 computer. The solution did notpresent any convergence problems. Although a complete verification of themathematical model was not possible, the calculated temperature profiles weresimilar in shape to an observed temperature profile. Introduction With the cost of finding and developing high-grade crude reservescontinually increasing, more attention is being focused on the development ofhigh-viscosity or "heavy" oil reserves. As the oil viscosity is highlydependent on temperature, a small increase in reservoir temperature decreasesthe oil viscosity markedly, facilitating increased production rates andultimate recoveries. One of the more popular means of increasing the reservoirtemperature is steam injection. To evaluate the feasibility of a thermalproject, a reasonably accurate estimate of the heat losses from the well bore, the sandface pressure and the sandface quality of the steam would be desirable, if not necessary. As the injection times are generally quite small, steady-state solutions would not be very representative and unsteady-statesolutions are required.

This study investigated the nature of formation plugging with bacteria and attempted to relate its characteristics to physical rock properties. Fifteen core samples of four specific formation types were defined and plugged using a commonly occurring, uniform sized dead bacteria, Bacillus subtilis. Two of the formation types were fairly uniform-grained sandstones and two were heterogeneous carbonates. The injection rate and concentration of solids in the brine were held constant during the test runs. The pore geometry factor G was a significant petro-physical rock characteristic for correlation with depth of plugging, but lacked importance as a parameter for rate of plugging. Although the G factor may not completely describe porous rock geometry for plugging predictions, for reservoirs with multi-pore systems, there is evidence that the pore geometry factor might form a basis for establishing injection water specification in more homogeneous reservoirs with only primary porosity development. Introduction GENERAL PURPOSE Injecting water into oil-bearing formations to increase recovery is common practice in the petroleum industry. One of the most serious operational problems involved with water injection wells is the tendency of the porous rock immediately adjacent to the injection wellbore to plug partially. This results in increased injection pressures and/or a decrease in the rate of injection. This plugging in injection wells can result from a number of causes, including bacteria contained in the injected water. The purpose of this work was to investigate the nature of injection plugging with bacteria and to attempt to relate its characteristics to physical rock properties. Many variables exist in research of this type, so, to reduce them, injection rate and concentration of bacteria in brine were held constant. The bacteria used for plugging was a uniform-sized, dead bacteria, Bacillus subtilis, a micro-organism commonly found in water, air and soils. Four specific porous rock types were plugged-two consolidated sandstones and two heterogeneous carbonates. The experimental work involved two distinct phases. The first concerned determining the physical rock properties and the second was the experimental work of actually plugging the selected core samples. PORE GEOMETRY FACTOR Capillary pressure is necessary to displace a wetting fluid from a capillary opening with a nonwetting fluid. The factors which govern capillary pressure are the surface or interfacial tension of the fluids involved, the system wettability and the equivalent pore radius. Thus: (1) Thomeer used the idea that the location and shape of the capillary pressure curve reflect characteristics of the pore structure of any porous media sample. He presented the mathematical description of capillary pressure curves and of indicated differences in pore geometry of samples. (2) where G = pore geometry factorpc = capillary pressurepd = displacement pressure(Vb)pc = fractional bulk volume occupied by thedisplacing fluid at any capillary pressure(Vb)px = fractional bulk volume occupied by thedisplacing fluid at infinite pressure ortotal inter-connected pore volumes. The shape of the curve is defined by the pore geometry factor G. The curve shape is related directly to the sorting and interconnection of the pores of the sample. The location of an infinite number of curves can be defined by the same asymptotes. However, these curves differ in shape, each curve being described by a specific value of G. Thus, the description of a curve is unique when, in addition to the position of its asymptotes, its shape is defined. These curves can be used graphically to determine the characteristic parameters of any arbitrarily located curve. Those parameters that can be approximated from any capillary pressure curve are:total interconnected pore volume, represented by (Vb)px;extrapolated displacement pressure, the abscissa axis; andpore geometry factor, by the curve shape. PROPERTIES OF BRINE AND BACTERIA The fluid used throughout this study was an aqueous solution of sodium chloride, 25,000 ppm by weight. The addition of salt to fresh water was intended to eliminate clay swelling in the sandstone cores and to retard leaching in the limestone cores. JPT P. 201ˆ