PREDICTION OF THE DEW-POINT PRESSURE OF A GAS CONDENSATE RESERVOIR.
1.1 Background of the study
Gas Condensate Reservoir is a reservoir having a low-density mixture of liquid hydrocarbons that are present as gaseous components in the subsurface (in the reservoir). It is important to recognize that some gas condensate reservoirs show condensate dropping out within reservoirs, as well as condensate production at the surface due to pressure falling below the dew-point during production. This condensate accumulation in the reservoir initially remains immobile due to interfacial forces between it and connate water within the pores of the formation until its saturation level reaches a threshold value and becomes mobile.
Initially, the gas condensate is totally gas in the reservoir. As reservoir pressure decreases, the gas condensate exhibits a dew-point. The dew-point of a gas condensate fluid occurs when a gas mixture containing heavy hydrocarbon is depressurized until liquid is formed, that is, a substantial amount of gas-phase exists in equilibrium with an infinitesimal amount of liquid phase. Pressure is reduced; liquid condenses from the gas to form free liquid in the reservoir. Normally, there is no effective permeability to this liquid phase and it is not produced. If the pressure continues to decrease, a second dew-point will be reached and the liquid can be re-vaporized. This lower dew-point pressure is usually well below the reservoir abandonment pressure; thus it would be of no interest in reservoir performance.
By definition, dew-point pressure is simply the pressure at which an infinitesimal amount of liquid is in equilibrium with a large quantity of gas. the pressure below which liquid condense out of the gaseous phase.
A phase behaviour can be defined as the characteristics (changes in phase) exhibited by the gas when subjected to different temperature-pressure conditions.
During production, the hydrocarbon molecules undergo various phase and some property change, altering intermediate stages which are crucial in designing and operating the processes efficiently and optimally.
Unlike a pure substance that has both bubble-point and dew-point the same at a particular temperature and pressure, a gas condensate reservoir is a multi-component system thus at a particular temperature and pressure, each component present, exhibit their different characteristics. This is because the natural gas reservoir is not an ideal mixture. This can be seen clearly in fig 1.1.
Figure 1.1, Phase behaviour of Gas condensate reservoir by Li Fan, College Station, Texas, USA.
1.2 Problem statement
Condensate liquid saturation usually builds up near a wellbore area because of drawdown below the dew-point pressure, ultimately restricting the flow of gas. The near-well choking can reduce the productivity of a well by a factor of two or more. The phenomenon called condensate blockage or condensate banking results from a combination of factors, including fluid phase properties, formation flow characteristics and pressures in the formation and in the wellbore. If these factors are not understood at the beginning of field development, sooner or later production performance can suffer. This condensate blockage is a major challenge in the oil and gas sector since the production rate is reduced. Therefore, the dew-point pressure at which this formation occurs needs to be accurately predicted in order to reduce reservoir damage caused by condensate blockage and thus increase the production rate.
1.3 Aim and objectives
To improve the prediction of the dew-point pressure of a gas condensate reservoir.
Generation of gas condensate data Generation of a new mathematical correlation to accurately predicted dew-point pressure
1.4 Significance of work
Every day, the petroleum industries are producing fluid from the condensate reservoirs in order to satisfy human and industrial needs. It is of great concern on the path of production and reservoir engineers to make sure the dew-point pressure of a gas condensate reservoir is accurately known.
As described earlier, the condensate reservoirs when produced below dew-point pressure tends to release liquid which reduces formation permeability and thus reduce production rate.
For example, well productivity in the Arun field, in North Sumatra, Indonesia, declined significantly about 10 years after production began. Well studies, including pressure transient testing, indicated the loss was caused by the accumulation of condensate near the wellbore.
Therefore, this article focuses on the prediction of the dew-point pressure of a gas condensate reservoir so that production and reservoir engineers can be aware and produce gas condensate reservoirs optimally.
1.5 Scope of work
The scope of this project is limited to developing a mathematical correlation that would be used to accurately predict the dew-point pressure of a gas condensate reservoir using data from the literature.
The ‘accurate’ prediction of dew-point pressure for gas condensate reservoirs is very important to maintain the reservoir pressure close to the dew-point ensuring maximum gas production and minimum condensate blockage. For this reason, a new correlation has been developed to predict dew-point pressure for retrograded gas condensate reservoirs. The new model is developed from a large data bank that is comprised of a larger range of pressures, temperatures, and various gas condensate compositions. The new empirical correlation has the following features:
1.Validity of the model has been studied using measured data. The new model shows acceptable agreement between forecasted dew-point pressure values and the measured ones.
2.Precision of the correlation is compared to previously published correlations. The newly proposed performed better than all published correlations. It has the highest correlation coefficient and the lowest errors (AARE and RMSE).
3.The Proposed dew-point pressure correlation is simple, easy to apply and accurate for condensate gas production schemes.
4.The new model can be used for further correlation development due to its simplicity and also predict the dew-point pressure for retrograde gas condensate reservoirs when the measured one is unavailable.
ARE average relative error
AARE average absolute relative error
R coefficient of correlation
RMSE root mean square error.