Phase trapping refers to the temporary or permanent trapping of oil or water based fluids introduced into a porous media during drilling or completion operations which result in a reduction in the effective permeability to the desired producing or injected phase. This problem has emerged in recent years as a major mechanism of formation damage, most notably in low permeability gas producing formations. This paper reviews the current state of the art with respect to new technology which is being used to identify formations that are particularly susceptible to this mode of formation damage, as well as describing some of the most current technology being used to avoid and remove these problems. This includes the use of various types of new non-alcohol based surface tension reducing agents, foamed fluids and other surfactants, transient wettability modifying agents, oil based fluids, as well as CO2 or nitrogen based fracturing technology. The potential advantages of some of these new technologies and where they have the greatest degree of potential application are discussed.
Phase trapping is a common mechanism of formation damage that can occur in a variety of oil, gas or water bearing formations and can create severe reductions in productivity or injectivity. The problem has been documented in numerous publications in the literature (1–12) as well as in previous publications by the authors (13–18). The basic mechanism of a phase trap refers to either the transient (sometimes referred to as a ‘phase load’) or permanent (phase trap or block) retention of water, oil or sometimes gas based fluids which are generally introduced from an outside source into the formation during drilling and completion operations. There are also situations, such as retrograde condensate gas reservoirs or the depletion of black oil reservoirs, where the creation of potentially permeability reducing near wellbore phase traps may occur through the normal depletion process of the reservoir.
The most common types of phase traps can be summarized as follows;
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Water based phase trapping (caused by water based drilling, completion or kill fluids)
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ydrocarbon based phase trapping (caused by hydrocarbon based drilling, completion or kill fluids) or by retrograde condensate liquid dropout in rich gas producing situations
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Water based phase trapping (caused by water based drilling, completion or kill fluids)
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Gas based phase trapping (caused by gas liberation by sub-bubblepoint production)
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Hydrocarbon based phase trapping (caused by hydrocarbon based drilling, completion or kill fluids)
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Gas based phase trapping (caused by free gas injection) Gas Injection Wells
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Water based phase trapping (water of condensation)
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Oil based phase trapping (compressor lubricant carryover)
Phase trapping is caused by an interaction of capillary pressure and relative permeability phenomena in porous media. A simplified formulation of the capillary pressure equation in porous media can be expressed as; (Equations (1))(Available in full paper)