Water And Hydrocarbon Phase Trapping In Porous Media-Diagnosis, Prevention And Treatment

Abstract

Abstract The entrapment of extraneous phases within porous media can occur in a number of different situations during drilling, completion, workover and production operations. The introduction of an additional immiscible phase, or an increase in an existing phase saturation within porous media can cause deleterious relative permeability effects which can substantially impact the permeability and relative permeability to oil or gas. This phenomenon is commonly described as aqueous phase trapping or hydrocarbon phase trapping, depending on the situation under consideration. This paper describes specific conditions required for the establishment of aqueous and hydrocarbon phase traps and provides diagnostic equations to evaluate the potential severity of an aqueous phase trap in a given reservoir situation. Specific procedures are recommended for the prevention of aqueous phase traps during drilling, completion and production operations and, in a situation where phase traps are determined to exist in a reservoir, a variety of treatment techniques are presented to attempt to remove or reduce the severity of the aqueous or hydrocarbon phase trapping phenomenon. Introduction Aqueous phase trapping and hydrocarbon phase trapping represent a significant mechanism of impaired productivity in many oil and gas reservoirs in various locations throughout the world. Bennion et al. (1994)(1) have provided a detailed discussion of the basic mechanisms of aqueous and hydrocarbon phase trapping and associated reductions in permeability. Aqueous phase trapping can occur in both oil and gas reservoirs and may be associated with reservoir situations where the reservoir exhibits a sub-irreducible initial water saturation. Specific additional documentation on subirreducible water saturation reservoirs is documented by Katz et al. (1982)(2) and Masters et al. (1984)(3). Hydrocarbon phase traps may be established in gas reservoir applications where extraneous immiscible hydrocarbon phases are introduced into the reservoir, or in retrograde condensate reservoir applications where the reservoir is produced at some pressure below the dewpoint resulting in the accumulation of liquid retrograde condensate within the pore space. This paper discusses various types of phase trapping and provides criteria for the diagnosis of the potential severity of a reservoir to be susceptible to aqueous phase traps. Additional discussion is then presented with respect to the prevention of aqueous and hydrocarbon phase traps in different reservoir operational situations and potential remediation of oil and gas reservoirs where aqueous or hydrocarbon phase traps have already been established. Mechanisms of Aqueous Phase Trapping Figure 1 provides a schematic illustration of the establishment of an aqueous phase trap within a low permeability gas reservoir application. The initial basis for the establishment of an aqueous phase trap is a reservoir which exists at what is classified as a subirreducible saturation, where the initial water saturation in the reservoir is less than what would be typically quantified as the irreducible water saturation which would exist in the porous media under the prevailing capillary conditions. Sub-irreducible saturations are postulated to be established by a combination of dehydration, desiccation, compaction and diagenetic effects which occur over the life of certain reservoirs.