Gas condensate reservoirs exhibiting classic "dew point" or retrograde condensate dropout behavior 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.
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. A typical 'lean' or dry gas reservoir is illustrated in
Figure 1 as 'depletion line 1'. In this situation, it can be seen that the reservoir pressure can be decreased from the initial value to the abandonment pressure without the pressure depletion path at the bottomhole temperature ever encountering the two-phase region. This type of reservoir sometimes produces limited amounts of condensate liquids at surface (due to cooling of the gas in the production tubing which induces liquid dropout). For a typical "dry" gas, generally liquid hydrocarbon phase yields are less than 10 bbl of condensate liquids per MMscf of gas (GOR in excess of 100,000 scf/bbl or about 15,000-20,000 m3/m3). This type of reservoir does not exhibit bottomhole liquid dropout problems and, hence, is not the topic for consideration in this paper.
As the original gas composition becomes richer (increased fraction of heavy ends), the original compositional condition moves to the left in Figure 1 (as illustrated by 'depletion line 2'). It can be seen that, at reservoir temperature conditions, as pressure is reduced, the depletion line now intersects the two-phase region. The first point of intersection is referred to as the "dew point" pressure for the gas and represents the maximum pressure at which the reservoir gas system remains single phase. At pressures below this, increasing amounts of liquid condensate retrograde from solution in the gas and appear as an equilibrium liquid phase within the pore system.