An Overview of Miscible Flooding
Authors: Asgarpour S. 
Published: March 2010

Introduction

In most reservoirs, oil recovery can be improved by the implementation of an enhanced oil recovery (EOR) scheme. An EOR scheme can be classified as either a miscible or an immiscible process. In a water flood or an immiscible gas flood, the displacing fluid is not soluble in the displaced oil. The displacement results in a residual oil saturation due to the inter-facial forces between the displacing fluid and the displaced oil. In contrast, miscible fluids are soluble in oil so there will be no inter-facial force between oil and solvent and the theoretical residual oil saturation will be zero. Hydrocarbon miscible floods are the most common tertiary EOR schemes m Western Canada. They can be sub-divided into vertical or horizontal miscible floods.

Vertical Miscible Floods

Vertical miscible floods are usually implemented in pinnacle reefs or reservoirs with a high relief angle. The majority of Canadian vertical floods have been implemented in the Rainbow Lake, Brazeau River, Pembina/West Pembina, and Wizard Lake areas of Alberta. The solvent is injected as a blanket at the top of the reservoir to take advantage of a gravity stabilized displacement. Subsequent chase gas injection drives the solvent blanket downward.

The highest wells in the structure are usually chosen as the injectors to maximize oil displacement and the producing wells are completed in the lowest porous interval above the oil water contract. Production rates are controlled to restrict solvent and water production. Horizontal wells are becoming popular in vertical miscible floods. These wells arc usually drilled as producers near the water/oil contact to reduce water and gas coning problems, and thus increase the production rate and reduce the sandwich loss. Innovative completion techniques such as perforation below or at the oil/water contact have also resulted in reduced sandwich losses.

The expected incremental recovery compared to upward waterflood is in the range of 15–40%. The vertical solvent flood process develops a good volumetric sweep efficiency as a result of gravity stabilization of the interface, and is ideal in homogenous reservoirs. In heterogeneous reservoirs with horizontal shale barriers, a substantial reduction in oil recovery improvement can be expected as a result of poor vertical sweep efficiency (e.g. Golden Spike, Alberta).

Horizontal Miscible Floods

In horizontal miscible floods, solvent and water are injected alternately to mobilize residual oil and push it to the producers. After the injection of solvent and water, chase gas or leaner solvent (which is miscible with the solvent) and water are injected to extend the solvent bank size and complete the displacement process. After injection of 25%–40% hydrocarbon pore volume (HPV) of solvent and chase gas, the process reverts to horizontal water flood to depletion. In other words, in the early stage of the miscible project, oil is replaced by miscible solvent and moved toward producing wells. Later, residual solvent is mobilized by chase gas and moved to where it can contact more residual oil. Through this process, an expensive commodity, residual oil, is replaced by a cheaper commodity, chase gas.