Retrograde Condensate Dropout Phenomena in Rich Gas Reservoirs-Impact on Recoverable Reserves, Permeability, Diagnosis, and Stimulation Techniques

Abstract

Abstract Gas condensate reservoirs exhibiting classic "dew point" or retrograde condensate dropout behaviour exist in many areas in the world. These reservoirs are unique in that, as the reservoir pressure is decreased, a certain volume of the heavy end fraction of the gas is precipitated in liquid form from solution in the gas. This condensate liquid may be temporarily or permanently trapped in the reservoir, causing severe reductions in gas production rates and the permanent loss of a large portion of the volatile and valuable condensate liquids (due to capillary pressure- induced trapping effects in the porous media). This paper reviews the basic theory of gas condensate dropout and describes, in detail, damage problems that may be associated with production of reservoirs of this type. Techniques for mitigating condensate dropout problems on a production basis, as well as stimulation techniques such as repressurization, lean and rich gas injection, surfactant and solvent injection, in situ combustion and water/gas injection, are reviewed, and the advantages and disadvantages of the techniques discussed. Introduction Rich gas or retrograde condensate gas reservoirs are common on a worldwide basis. Figure 1 provides a pressure-composition diagram for a typical hydrocarbon system at a fixed temperature level. The shaded portion of this figure represents an area of two-phase equilibrium at the specified composition and pressure condition. This is generally a region where an immiscible hydrocarbon liquid and gas phase co-exist in thermodynamic equilibrium. Outside this area is a single phase region, where only one continuous and homogeneous uniform phase exists. Potential Problems Associated with Rich Gas Condensate Systems There are two main categories of problems commonly associated with rich gas reservoir systems:Formation damage effects associated with the condensate dropout.Permanent loss of valuable light condensate liquids due to trapping effects in the reservoir. This paper addresses both of these issues and various techniques that can be used to reduce their impact on productivity and recovery. Formation Damage Issues in Gas Condensate Reservoirs The literature is replete with detailed discussions on formation damage(1–8). This paper does not address different types of damage which can occur during conventional drilling, completion and production operations, but concentrates solely on issues associated with condensate liquid dropout. The primary damage mechanism resulting in a reduction in gas production rate in a rich gas-condensate reservoir is generally associated with capillary pressure-motivated phase trapping effects. These effects result in permanent reduction in the effective permeability to gas in the region affected by the condensate dropout surrounding the wellbore. This phenomenon is illustrated pictorially in Figure 2 and using a set of gas-condensate relative permeability curves in Figure 3. As the condensate drops from solution in the gas while still in the porous media, capillary pressure effects are present [due to the generation of a second immiscible phase which has a finite interfacial tension (IFT) between it and the gas phase].