Ever since Muskat presented his approximate theory on water coning in 1935, the primary research focus has been on design and performance optimization. Subjects such as critical production rates, time for the cone to break-through and, production after break-through, have been studied with physical, analog, analytical and numerical simulation models. Guidelines have been developed on optimal placement of wells in a stratified system and, in situations involving simultaneous coning of water and gas.
For most practical situations. ultimate oil recovery was determined to be virtually independent of producing rates. This gave rise to the concepts of infilling wells on a close spacing, producing wells at very high fluid rates and, periodic recompletion of wells to minimize coning/ maximizing reserves. Associated problems such as handling of large amounts of high water-cut effluent and, treatment ofe.ff1.uentfor their reinjection into the reservoir also received technological attention.
The advent of horizontal wells In 1980's resulted in a significant increase in 'clean' oil rates and reserves (without coning). As a result, many otherwise marginal or, uneconomic pools became viable. Consequently, the research efforts were redoubled not only to optimize operations but also, 10 find supporting technology 10 mitigate coning. Two fundamental problems remain with conventional (vertical) as well as, horizontal wells:
1.Sustaining oil rates at economic levels and, 2. Handling and Disposal of the effluent. Several ideas have been developed and some have been tried with varying degrees of success. These include:
Placing barriers to movement of water and gas around the well. These involve introducing water, oil, gas or. chemical such as gels, foams, polymers, cementing agents to preferentially reduce permeability or relative permeability to water or gas; injecting oil into the water or gas zone; injecting gas into the water zone; and injecting foam into gas zone to reduce relative permeability to gas.
Modifying flowing pressure distribution around wells by i) producing water or, gas via separate completion intervals in the same well; or it) reverse coning of oil into water or gas zone;
Minimizing operating costs via sub-surface water separation and disposal of the separated water in the same well.
This paper presents a brief review of the current state of the art.
In many situations, recovery of oil is limited by excessive production of water or. gas. After a period of low (or no) water and gas production. Water-oil ratios and/or, gas-oil ratios begin LO increase substantially. Under these conditions. production rates may have to be restricted due to equipment, operating, regulatory, or environmental constraints. This results in not only reduced oil rates, but also in increased cost of handling (separation, processing, and disposal) of effluents and ultimately, in a rather premature shutting-in of the wells. Due to significant economic implications, the phenomenon of water and gas coning has received some attention in the literature.
Over geological times, the reservoir fluids (oil, water, and gas) exist under Static pressure (gravity-capillary) equilibrium. An attempt to withdraw oil from the pool would result in a pressure draw-down around the well