Proceedings of the Second International Energy 2030 Conference,
November 4-5, 2008, Abu Dhabi, U.A.E.

Hydrodynamic Transition Zone at OWC in Non-Darcy Flow

Shengkai Duan
Louisiana State University, USA

Andrew K. Wojtanowicz
Louisiana State University, UAE

Abstract
Worldwide, there are great numbers of oil reservoirs with un-recovered oil and thousands of marginal wells that are idling, not because the resources have been depleted, but because they have become unprofitable due to excessive water production. For most operators, produced water is a single most important factor controlling economics of their business. Excessive water production makes the whole economics of reservoir/well management vulnerable to ever-fluctuating crude oil prices. There are many ways water finds its way to wells – due to wells’ integrity failure, or heterogeneity of the oil bearing formation (natural fractures, high permeability channels, etc.). However, even in relatively homogenous strata, after the water breaks through the oil into a well, it progressively dominates the inflow thus leaving the oil behind (by-passed). This process is known as a progressive transition zone above the oil water contact (OWC) and applies to well-reservoir systems with water coning, water cresting and water channeling with crossflow. We postulate that one of the mechanisms contributing to transition zone expansion is a transfer of water into the oil across moving OWC– transverse dispersion. Transverse dispersion is a hydrodynamic mixing process caused by a concurrent segregated two-phase flow [1=Ewing, 2000]. One reason for the mixing is loss of stability at the flowing fluids interface. In the Hele-Shaw cell flow experiments, where the flow is constricted by solid surface, Gondret [2] showed that the interface would be more stable for smaller gap of the cell plates. Later, Duan and Wojtanowicz [3] used a Hele-Shaw cell to study the effect of shear stress on the interface stability. Their experiments revealed cyclic perturbation of the interface in segregated flow. The size of the perturbation zone would increase at higher shearing rate gradient resulting from the velocity difference of oil and water. Using granular-packed cell experiments, Blackwell [4], Bijeljic and Blunt [5], and Jha et al. [6] demonstrated the mechanisms and effect of factors controlling two dimensional transition zone in miscible flow. Transverse dispersion was quite small comparing to the longitudinal diffusion or dispersion, which would explain why transverse dispersion in miscible flow doesn’t attract enough attention. In immiscible flow, on the other hand, Perkins [7] flow experiments with a sand packed model showed progressive symmetric transition zone perpendicular to the direction of flow for two fluids having similar mobilities. The objective of this study was to demonstrate the effects of fluid viscosities’ contrast and granular size of the porous medium on the size and shape of the transition zone in linear segregated flow of oil and water, at flow velocities beyond the validity of Darcy’s Law. It was also important to understand the physical mechanisms contributing to the dispersion, develop mathematical models and formulate criteria for the process, as well as relate the transverse dispersion process to the actual oil well inflow conditions.