Gas Condensate Reservoir Behaviour: Productivity and Recovery Reduction Due to Condensation

Abstract

Abstract The depletion of gas condensate reservoirs to pressures below the dew point has been studied by reservoir engineers for many years. Pressure decline below the dew point pressure causes condensation to occur which creates a hydrocarbon liquid saturation in the reservoir. This process reduces liquid recovery and may reduce gas productivity and gas recovery. Exxon experience, particularly in low-productivity, high-yield gas condensate fields, suggests that liquid condensate formation can result in severe loss of well deliverability and therefore of gas recovery. This study was undertaken to evaluate the historical frequency and severity of productivity impairment due to near-wellbore condensate buildup and to identify reservoir parameters associated with severe productivity and recovery reduction. This study of gas condensate reservoirs included a survey of Exxon and published industry experience, a review of published laboratory data, and simulations with single well flow models. Data from 17 fields are included in this paper to demonstrate that severe loss of gas recovery occurs primarily in low productivity reservoirs. Production data from two wells were history matched with simple radial models to evaluate the potential range of the critical condensate saturation (the minimum mobile condensate saturation) and its impact on gas recovery. Published laboratory data for gas-condensate relative permeability were used as a starting point for these simulations. The primary conclusion from this study is that productivity impairment results in reductions in gas recovery for wells with a permeability-thickness below 1000 md-ft. The history matched simulations support a range of critical condensate saturations from 10% to 30%, in good agreement with published laboratory values. Introduction The depletion of rich, gas condensate reservoirs to pressures significantly below the dew point is a topic of increasing interest as deeper, hotter hydrocarbon reservoirs are exploited. The cost and risk to develop reservoirs under these extreme conditions highlights the need to be able to confidently predict the recovery of gas and liquids from these reservoirs. In particular, there is a need to better understand the factors controlling the decline of well productivity due to hydrocarbon liquid saturation developing in the near-wellbore region of the reservoir as the flowing pressure declines below the dew point pressure. Reservoir engineers have been concerned about the impact of condensate blocking on productivity for many years. Several examples of severe productivity decline are available in the literature. At Exxon, several fields have been identified in which productivity loss below the dew point has significantly reduced gas recovery by pressure depletion. One example of poor performance is shown in figure 1. This is a moderately rich gas condensate field with an initial condensate-gas ratio of 73 bbl/Mscf. The well produced at initial rates over 1 Mscfd. When the flowing bottom-hole pressure reached the dew point, gas production declined rapidly and the well died. Pressure surveys indicated that the well was full of liquid hydrocarbons. Attempts to swab the well were unsuccessful, even though data from surrounding wells indicated the average reservoir pressure was still over 2000 psi above the dew point pressure. The well appears to have 'locked up' and ceased production shortly alter the flowing bottom-hole pressure passed below the dew point pressure. Eventually the well was successfully fracture stimulated, returning the well to initial production rates. P. 677