Improving Volumetric Sweep Efficiency by Optimizing Viscous-Gravity Forces in Water-Miscible Gas Injection

Abstract

Water-alternating-gas injection is an enhanced oil recovery method that has been used for many years. Field applications of this method had good performance and the recovery factor was quite promising in the majority of cases. Applications of this method in the North Sea are only part of this successful experience. Even though this method has been used in many fields, there is still good potential for improving the recovery factor, especially through improving volumetric sweep efficiency that was the primary goal for introducing this method. There are also many non-horizontal and unconventional oil reservoirs that might be good candidates to implement this method.

In a gas injection process the microscopic sweep efficiency is usually excellent, but it is the volumetric sweep efficiency that might be unfavorable, as this depends on the heterogeneity and other reservoir characteristics. Water-alternating-gas injection was proposed to improve this volumetric sweep efficiency. Further studies have proposed different combinations and injection strategies for water and gas to enhance the advantages.

This thesis focuses on the reactions of water-miscible gas injection on a macroscopic level by means of studying viscous and gravity forces which are main forces present on this level. The techniques that can help to improve this volumetric sweep have been analyzed and also, the new potentials for implementing this method have been discussed.

It is still a question if implementing water-alternating-miscible gas injection is advantageous in a non-horizontal pattern. Analytic developments have been used to answer this question. Also, simulation studies have been coupled to show the reliability of the results. The type of the numerical simulation which is applicable in this process has been discussed.

Among all practical developments and proposed techniques for injecting these two phases, water above gas and particularly, water above miscible gas injection is expected to be a very efficient alternative. This occurs through improving volumetric sweep efficiency by means of maximized viscous force to the gravity force ratio. However, this could not be proved without experimental verification and it is obvious that scaling is a main challenge in this type of experimental study which investigates a reservoir-scale process. A scaled experimental setup was implemented to study this technique. The results have been compared with analytic equation, previously verified for a water-alternating-miscible gas injection process.

Finally, in a supplementary discussion that also shows the importance of the subject, this thesis investigates if water-miscible gas injection can be used for unconventional oil reservoirs. The target oil reservoirs for this discussion are slightly heavy oil and heavy oil reservoirs. Combining hot waterflooding and water-alternating-miscible gas injection provides a hybrid method that is believed to be suitable for this category of reservoirs. If this hybrid method provides good microscopic efficiency, there is good possibility that all previous discussions of macroscopic scale are applicable to these reservoirs. Of course, additional studies on the reservoir-scale will be essential for this new method.