Abstract

As still deeper formations are drilled, gas condensates containing high carbon-number hydrocarbons which may precipitate during production and in the reservoir, are encountered. An increasing focus of attention on its phase behavior has been made worldwide in recent years. The main purpose of this paper is to investigate experimentally the characteristics of phase behavior of this new type of gas condensate systems with high wax content. Therefore, a series of experiments are carefully made for different GOR fluids, including constant composition expansions and the determination of wax deposition points. The separator gas and liquid are collected from the same producer and recombined in the laboratory to cover the ranges of GOR encountered in the field.

The results show that the quality lines of the highest GOR fluid in two-phase region are parallel to each other and terminated at the appearance of wax deposition. Thus the fluid has no convergence point, or critical point. It is also found that the biphase regions expand with the increase in GOR. In other words, gas injection may highly increases the saturation pressure locus contrary to the usual belief that the two-phase region will become smaller as more gas injected into the reservoir leads to more leanness of the reservoir fluid. Another interesting finding is that the more amounts the heavy hydrocarbons, the lower dew- & bubble- point pressures of the gas condensate systems used in this study. This is also contrary to the previous literature results that increasing amounts of heavy hydrocarbons increase the retrograde dew point pressure. Thus the deduced conclusion28 that a compositional gradient in the segregated fluids results in higher observed dew point pressures at the bottom of the fluid column, may not be made to the highly waxy gas condensate systems used in this study.

Additionally we also found the wax deposition temperature increased with decreasing GOR.

Introduction

A thorough understanding of pore-scale flow mechanism1, relative permeability2 and phase behavior3 is very important to predict the performance in gas condensate reservoir.

The search for hydrocarbon accumulations in deep strata has lead to numerous discoveries at extremely high-pressure high temperature conditions, for instance, up to about 110MPa and 200°C, which impose specific technical difficulties in the development4–14. Under these underground conditions, the reservoir fluids may contain significantly high molecular weight hydrocarbons. These components may precipitate at the temperatures and pressures encountered during production and transport. Therefore, the wax deposition from gas condensate production facilities and pipelines is very undesirable, as it often leads to malfunction of equipment and a large economic loss. It has been reported in an Eni Agip offshore gas and condensate field15 that the surface processing system may be plugged by wax deposition and the interruption of the production lead to a loss in terms of delayed production of the order of 1.25 million US$.

While wax precipitation from crudes has been extensively studied, precipitation from gas condensates is not generally well recognized14. A better understanding of the overall phase behavior, including the popular gas-liquid equilibria and wax crystallization characteristic, would help to analyze the reservoir performance and design the processing facilities. Recently there is a growing body of worldwide interests4,5,9–22 in the phase behavior of gas condensate with high wax content, or precisely the heavy normal alkanes with high melting temperature. One may encounter wax precipitation at temperatures as high as 65.55°C13,14.

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