The Scope and Perspective of ROS Measurement and Flood Monitoring
Authors: Thomas E.C. , Richardson J.E. , Shannon M.T. , Williams M.R. 
Published: December 2007

In this paper, we deal only with the scope and perspective of the methodology and how they interrelate with the overall objective of successful supplemental recovery. Our plan is to discuss remaining oil saturation (ROS) measurements and follow up with flood monitoring methods.

ROS Measurement

This familiar topic has been covered at some length. We chose not to repeat the efforts of Refs. 1 through 3, but rather to build on them. The specifics on any particular technique are explained fully in Refs. 4 through 9. The development, implementation, and evaluation of tertiary recovery methods depend on knowing the target in-situ oil volume before and after such methods are applied. Success or failure is determined by several factors:the feasibility of the particular EOR method,whether the value of ROS X, where X is a function of volume and distribution of oil in a given reservoir, andthe economics of the project.

When an oil reservoir is discovered, the saturation distribution is normally assumed to be at gravity/capillary, or hydrodynamic. equilibrium. The initial oil saturation, Soi, is defined as the oil saturation at discovery and is denoted in Fig. 1 on a typical capillary pressure for a rock containing hydrocarbon and water. Remaining pressure for a rock containing hydrocarbon and water. Remaining oil saturation, ROS, is defined as the oil saturation at any time during depletion of a reservoir and ranges from near zero to Soi. Residual oil saturation, Sor, is defined as an immobile oil saturationi.e., the saturation where the relative permeability of oil vanishes under the conditions that exist in the reservoir. Sor may be the residual left behind after EORe.g., waterflood, chemical, or miscible flood. Sor is a special case of ROS. The amount and spatial distribution of oil remaining in a reservoir are critical for planning future oil recovery and are very difficult to determine. Reservoirs are geologically complex, and production mechanisms are complex, thus the resultant fluid distributions production mechanisms are complex, thus the resultant fluid distributions are complex. The precision required for an estimate of the target volume is a function of the risk that can be tolerated; the higher the precision of this determination, the better we can forecast our target and thus reduce risk. As the margin between economic success and failure shrinks, precision becomes critical. The number of applicable techniques for determining ROS rests on the accuracy required. For high accuracy, there are fewer applicable techniques, and these usually require greater engineering skill and expense. No single method can handle all situations. From the arsenal of techniques, we must select cost-effective methods that yield the information and accuracy required.

What Must Be Measured. Assume that ROS values are desired at a specific location in a reservoir. Available formation evaluation techniques cannot measure the target volume directly. Instead, we must rely on indirect methods to determine ROS by measurement of physical properties. A unit volume of formation contains at least a rock matrix, brine, and we hope, oil. It may also contain a gas phase and the matrix may be composed of more than one mineral. Lithology, porosity. oil, and water saturations are primary properties important to all target volume estimations at one location. Because none of these can be measured directly and independently in situ, there are at least three unknowns requiring three equations relating these to measurable properties. In addition, a variety of subsidiary physical constraints in the form of inequalities (0 1, etc.) must be satisfied. The physical properties of the rock usually depend on more than just the fractional volumes of its constituentse.g., the effect of stress-and the resultant problem is usually underdefined. Therefore, we often resort to models of the physical response of the system that involve secondary parameters depending implicitly on the structure and composition of the formation. For example, the Archie equation for shale-free rocks,

Sw=(phi-mRw/Rt)1/n,

relates water saturation, Sw, porosity, phi, and measured resistivity, Rt, but contains three secondary parameters that first must be known to determine water saturation. Brine resistivity, Rw, lithological exponent, m, and saturation exponent, n, are needed to describe the rock/fluid system. The brine resistivity is a property of the fluid, m depends on the pore geometry, and n depends on the arrangement of conducting fluids within the pore space. Other equationsi.e., the Waxman-Smits equation-have similar parameters. These parameters, which characterize the fluid, mineralogical content, and pore geometry, can be called secondary formation properties. properties. The need for measurement of secondary properties complicates the determination of ROS. The number of variables is increased greatly, as well as the introduction of measurement error for each secondary property. Many relations between primary and secondary properties are empirical and have inherently limiting assumptions. properties are empirical and have inherently limiting assumptions. We must be careful not to apply relations routinely to specific rocks or reservoirs where they may not apply. To handle these obstacles, one mustmeasure the secondary properties at the time of the ROS determination, thus under the specific conditions that apply for the particular rock under study at that time;construct a concept of the rock/fluid system; andmodel the system mathematically to determine the propagation of error in the value of ROS and to predict values of secondary properties under conditions not measured.

When To Determine ROS. We must determine ROS when the value is needed for economic or operational planning.

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