Water Coning Suppression
Authors: Rajan Varagur S.V. , Luhning Richard W. 
Published: March 2010

Many oil reservoirs in Alberta overlay active water zones which provide well maintained high pressure drives for primary oil recovery. However, such high pressure aquifers create water coning which is the entry of the bottom water into the production well during the primary oil production. Thus, a well can produce a substantial amount of water along with oil at the expense of oil production. Cold, non-condensable gas injection into an oil reservoir with bottom water has been observed as an effective method for water coning suppression thereby extending the economic life of a production well. A project was initiated to investigate the effect of gas injection in suppressing water coning during oil production from an oil reservoir overlaying an active aquifer. The research program included partially scaled, high pressure physical model experiments in sand packs supported by numerical simulations to understand the mechanisms by which injection of gases (such as carbon dioxide and methane) suppresses water coning. Tests with different non-condensable gases showed consistent reduction in water production or reduction in water-oil ratio of the produced fluids.

This paper indicates a number of possible mechanisms for the anti-coning effect of gas injection. It is inferred that the injected gas migrates toward the production well along the oil-water interface as a blanket thereby increasing the free-gas saturation. The presence of even a small amount of free-gas creates a three-phase region of oil, water and gas which results in reducing both the relative permeability for water flow and the residual oil saturation. Thus, water production is reduced. The injected gas (if soluble) also dissolves in the oil and reduces its viscosity. When the oil is less viscous and more mobile, the injected gas sweeps a greater volume of oil. Any swelling caused by gas dissolution would also help in more oil displacement. The preceding mechanisms based on gas dissolution may not be significant in reservoirs with ‘live’ oil containing significant amount of dissolved gas. Another minor mechanism is the plugging of pores by the possible in situ formation of viscous water-in-oil emulsion with acidic gas injection.

A key result of this research study is the first time demonstration of the anti-coning effect of cold gas injection under controlled laboratory conditions and the investigation also provided a strong support for the development of Alberta Oil Sands Technology and Research Authority's (AOSTRA) commercial Anti-Water Coning Technology (AWACT).

Background

Water coning is a term given to the phenomenon of the entry of bottom water into a production well. Under static conditions, water being denser than the formation hydrocarbon fluids, remains below the production well. When oil is produced at high rates however, the interface between the oil zone and the underlying water zone rises and imbibes into the oil depleted region near the production perforations as a ‘water cone’ (Fig. 1). The water cone may extend into the wellbore when the upward pressure gradient associated with the flow of formation fluids into the production wellbore balances the hydrostatic head of the resulting elevated water column.