Parametric Study of Gas-Condensate Reservoir Behavior During Depletion: A Guide for Development Planning

Abstract

The results of systematic compositional simulations of gas-condensate depletion in a radial and cross-sectional reservoir model are presented. The effects on productivity, recoveries and fluid distribution are analyzed and quantified when changing various parameters, such as the role of non-Darcy flow, the permeability, the relative permeabilities (kr) in relation with the interfacial tension (IFT), the nature of the fluid. Below the dew point pressure, the drop in gas productivity is linked to the development of a condensate ring around the wellbore. Gas and condensate kr and their dependence on IFT control the time or pressure at which condensate buildup becomes significant and gas productivity drops. Tight reservoirs with lean gas-condensates may pose early and drastic productivity problems. Far from wellbore, condensate may segregate under the low-IFT conditions met during depletion, especially if the reservoir permeability is high and the fluid is rich. In a depletion process, condensate recovery is all the lower as the condensate content of the fluid is higher. The results of this study can be used as a guide during the development planning of a gas-condensate field (1) to estimate the quantitative role played by key rock-fluid parameters during future exploitation, (2) to define the most relevant core-fluid measurements and simulation studies for reducing uncertainties on field performance predictions. INTRODUCTION The industrial importance of gas-condensate fields is considerably increasing in the present years because many such fields have been discovered and are now under development or awaiting development. In 1988, Hill1 mentioned that only 3 out of the 35 to 40 discovered gas-condensate fields of the United Kingdom sector of the North Sea were to be developed at that time, and that proven and probable condensate reserves could be of one billion barrels in that area. A recent review of upcoming field developments2 indicates that the start-up of more than 15 gas-condensate fields in the United Kingdom sector of the North Sea and of more than 10 in the Norwegian sector is anticipated before the end of this decade. The profitability of the development of gas-condensate fields depends on both gas and condensate production profiles. According to the condensate content, the field environment, and technical or commercial factors, priority is given to a maximum and early production of condensate and/or to an optimal gas production with condensate as a by-product. A good exampe of the interrelated role of these factors is shown with the development of Brae fields complex.3 Hence, two major reservoir engineering problems have to be considered during development studies,the possible drastic drop in the gas productivity of wells below the dew point pressure,the loss of condensate trapped throughout the reservoir at the end of exploitation.

Performance Prediction In Gas Condensate Reservoirs

“…The condensate will drain in the parts of the reservoir where N B is greater than I x 10-5 in general (12) and segregated flow will take place especially if the permeability is high and the fluid is rich (13). This is indicated as region B in Figure 2.…”

“…However, as in the case of any gravity segregation process, these low values of condensate relative permeability control the drainage of condensate and impact eventually the distribution of phases(l4). The understanding and prediction of reservoir performance in terms of condensate recovery requires taking into account the phenomena occuring in the heart of the reservoir where the capillary and gravity forces control the condensate flow (13).…”

“…Very high flow rates encountered in this region in gas wells may lead to non-Darcy flow behaviour. Therefore, the dependence of gas condensate relative permeabilities on flow conditions must also account for inertial effects (13,14). The net effect of the various phenomena occuring in the near wellbore region is a balance of those effects that reduce well productivity (inertial effects, perme-WET DRY…”

“…Such a model should take into account the dependence of relative permeabilities on gravity, viscous and capillary forces as well as non-Darcy flow rather than using a single relative permeability curve otherwise results are likely to be unrealistic. For instance, it has been shown that the well impairment may be overestimated if the dependence of relative permeability on capillary number is ignored (13,17,20). The condensate saturation decreases monotonically away from the ability reduction) and those effects that increase well productivity (capillary desaturation, mist and filament flow, viscous stripping)(l).…”

Simulation Study of Re-Entry Drilling for Gas/Condensate Reservoir Development

“…The loss of productivity in gas-condensate reservoirs due to near wellbore condensate dropout is well discussed in the literature. [1,2,3,4] Reentering wells to gain additional production is not new. Since the mid-1950's, oil companies have reentered old wells and drilled sidetracks to bypass formation damage or wellbore mechanical problems, and to exploit new zones, saving the cost of drilling entirely new wells.…”