Injectivity Enhancement In Tar Sands A Physical Model Study
Authors: Sufi Arshad H. 
Published: March 2010

Introduction

Thermal recovery continues to be the most successful tertiary oil recovery technique in use today. The method bas been successful in numerous California heavy oil reservoirs, where heat from the injected steam is used to reduce the oil viscosity in situ, allowing it to flow more easily. The Canadian heavy oil deposits contain oil that is generally more viscous. Consequently, he Canadian heavy oil deposits present additional problems associated with injection and production. The extreme case is tar sands where the initial oil viscosity exceeds 1000 PaS (1 000 000 cp).

An experimental program was developed to investigate the steamflood performance of tar sand reservoirs. This program included the building of a scaled physical model with which to study the various aspects of steam injection into a tar sand reservoir. This was done in conjunction with numerical studies dealing with steamflooding of tar sands. The next section describes the strengths and weaknesses of each type of study.

Numerical vs Physical Simulation

Numerical simulation is based on discretizing a reservoir into finite blocks. Heat and fluid flow between the blocks is governed by equations that are coded into the simulator. These equations and their corresponding approximations have been tested, in general, for lighter oil reservoirs. When extremely viscous oil is encountered, it is not certain that all of the conventional concepts of fluid flow are valid. In a scaled experiment, the predominant forces interact and move fluid and heat independent of equational constraints. This allows the experiment to reveal behaviour which might otherwise go ndetected in a numerical model.

In numerical simulation errors can be introduced due to the size and the number of grid blocks used. These errors have been minimized using various schemes. However, the solutions exhibit fewer problems for mobility ratios less then 50. With heavy oils, the mobility ratio is invariably greater than that, making the solution more difficult and increasing the possibility of errors. A physical model does not have errors associated with grid block size.

The numerical simulator approximates the actual dimensions of the reservoir. The physical model is a scaled-downversion of the reservoir. In scaling down, compromises have to be made regarding a few of the parameters. An example of this is that viscous, gravity and capillary forces cannot be scaled simultaneously in a steamflood model. Any two can be selected for the scaling. In general, capillary forces are neglected. In a numerical simulator, equations for all three of the forces can be programmed into the model.

Experiments with a physical model are time-consuming. For each run, model loading, running and subsequent cleaning can take up to three weeks. A numerical simulator can be used without these delays. Hence, more parameters can be studied using the numerical simulator in a limited time period. The two types of studies can be used interactively. This was considered when making the decision to build a scaled physical steamflood model.