In a reservoir where a thin oil column lies between a large gas zone and a water zone, production can be improved considerably if wells are perforated below the oil-water contact rather than opposite the oil-bearing section. By coning oil down to the water zone perforations, that is, by reverse coning, continued production would increase the oil saturation and all relative permeability around the wellbore, thereby reducing the amount of free gas produced along with the oil.
This paper describes a technical approach used by Husky engineers to investigate the merit of reverse coning to increase oil recovery from thin oil banks in vertical hydrocarbon miscible floods in the Rainbow Field. The results of this study concluded that a properly designed reverse coning scheme could improve oil recovery by reducing oil sandwich loss. The degree of success depends on the initial oil bank thickness, reservoir Kv/Kh ratio, pressure drawdown near the wellbore, well completion configuration, aquifer strength and solvent-oil miscibility. Each of these key factors were investigated, and from the results, graphical correlations were developed as a guide to help practising engineers identify reverse coning potential and implement reverse coning in the field.
This paper also presents an innovative multi-zone completion technique which utilizes controlled autolift and continuous monitoring of zonal bottom hole pressures at the surface for reverse coning.
Husky Oil Operations Ltd. operates eight vertical hydrocarbon miscible floods in the Rainbow Field (Figure 1). Most of these floods are mature, and each pool has an oil zone sandwiched between an injected gas zone and a water zone. As production continues and the oil bank diminishes, gas and water coning at the producers become more and more compares reverse coning performance for water completion depths up to the maximum completion depth for a reservoir Kv/Kh ratio of 0.1. In this example, the key differences are not in oil production, but in the relative volumes of water/gas production and makeup gas requirements. This suggests the placement of water completion interval within the maximum allowed depth is determined by the economics of reverse coning production. Key economic factors such as time to form the reverse oil cone (start of oil revenue), fluid handling and re-injection costs, makeup gas costs and government fiscal incentives must be considered by the operator in ultimately deciding where to locate the water production interval for reverse oil coning.
The economic evaluation of reverse coning was performed using Husky's operating costs in the Rainbow Area and price forecasts provided by a reputable engineering consulting firm. The results are summarized in Figure 12 as net present values discounted at 12% before tax for each water zone perforation depth and reservoir Kv/K,\h ratio. For this particular evaluation, the economics would suggest locating the water zone perforations 0 to 3 metres below the oil-water contact for maximumnet present value.
