Aqueous and hydrocarbon phase traps can occur in porous media when water- or oil-based fluids come into contact with a formation which exhibits a "subirreducible" initial liquid saturation of the phase of interest. This commonly occurs with water-based fluids in many low permeability desiccated gas bearing formations and in depleted conditions in rich gas retrograde condensate reservoirs. This paper documents how phase traps are induced by direct displacement, counter current imbibition or depletion effects, and presents techniques for diagnosing whether a reservoir is a candidate for an aqueous phase trapping problem. Techniques to minimize problems with aqueous and hydrocarbon phase trapping are reviewed, followed by discussion of methods to reduce or remove the effect of existing phase traps, such as increased drawdown, alteration of 1FT, alteration of pore geometry or direct removal methods. A brief discussion of laboratory techniques used to screen the optimum process for selection are also presented..
The phenomena of permanent entrainment of extraneous or insitu generated aqueous or hydrocarbon based liquids in porous media has been documented in the literature as a mechanism for significant permeability impairment in low permeability intercrystalline sandstone and carbonate formations 1–5.
Phase traps normally occur when a water or hydrocarbon based fluid is either forced or imbibed into porous media with a subirreducible water or hydrocarbon saturation (ie. at a saturation less than the irreducible liquid saturation given the geometry, wettability and capillary mechanics of the system under consideration). Subirreducible hydrocarbon saturations are common in rich gas retrograde systems existing in a sub-dew point condition, or in mature gas fields which may have migrated into previously oil saturated strata. Subirreducible water saturations in low permeability gas reservoirs are also quite common. The mechanism for the establishment of a sub-irreducible water saturation in a low permeability gas-bearing reservoir is the subject of some controversy. The dominant mechanism is thought to be desiccation motivated by a large regional migration of gas under conditions of increasing temperature and pressure through a given reservoir area over a long period of geological time. The basic mechanism of a phase trap is created by the relative permeability effect associated with an increase in the immobile water or hydrocarbon saturation. This phenomena is illustrated on a pore scale in Figure I and from a mechanistic point of view as Figure 2.
The severity of the phase trap is strongly influenced by: The magnitude of the difference between the "initial" and final trapped "irreducible" liquid saturation which exists in the porous media. The greater this difference, the larger the adverse relative permeability effect and the greater the potential reduction in permeability.
The configuration of the gas or oil phase relativ permeability curves at low liquid saturation levels. The more adverse the configuration of these curves (ie. the more convex the relative permeability curve), the more significant the reduction in permeability for a given increase in trapped liquid saturation.
The depth of invasion of the trapped phase. The greate the volume of fluid lost and deeper the invasion, the more difficult and slow the mobilization of this fluid becomes.