Overview of Phase Behavior in Oil and Gas Production
Authors: Katz Donald L. 
Published: December 2007

A review is made of the growth of in knowledge of phase behavior to permit the prediction of phase compositions in oil and gas reservoirs and for surface processing. Subjects treated include recovery of retrograde condensate, gas hydrates, surface tension, viscosity, and dense phases. phases. Introduction

Early scientific inquiry into physical behavior of fluids under pressure was under way a century ago with basic work on pure substances and simple mixtures. Current knowledge was gleaned during three periods: the basic work on pure substances and simple mixtures during 1870–1910, the study of multicomponent oil and gas mixtures in the laboratory or field with empirical correlations during 1930–55, and the currently accepted theories with equations of state (EOS's) to predict behavior of complex systems from 1956 to the present. The total effort on phase behavior has mushroomed in scope, and this paper concentrates on applications in oil and gas production familiar to the author.

Early Work

A summary of early work was prepared in 1946 by abstracting and reproducing papers published before 1910. Here, reference is made to three researchers whose investigations were fundamental to our knowledge of oil and gas behavior. From 1885 to 1910 Young used pressurized glass equipment to measure the vapor pressure, phase densities, and critical points for 30 pure substances, including 8 petroleum hydrocarbons. pure substances, including 8 petroleum hydrocarbons. Kuenen in 1892 deduced from van der Waals' theory that liquid phases could be vaporized by gas pressure and that liquid forms on pressure reduction of a vapor, and demonstrated it with a CO2/methyl chloride system. He coined the term "retrograde condensation." (Katz and Kurata later clarified the nomenclature by using hydrocarbon systems.) The third investigator, Villard, did early work in the 1880's on gas hydrates to 4,000 psia (27 MPa) and demonstrated that solids and liquids psia (27 MPa) and demonstrated that solids and liquids may be vaporized by gas pressure.

Such studies laid the foundation for later investigations. Unfortunately, much of the early work was buried in the literature and not included in textbooks as of the 1930 period, when the interest in oil and gas mixtures arose.

Studies of Petroleum Mixtures

With an API grant, Sage, Lacey, et al. made extensive measurements of hydrocarbon mixtures at pressures to 3,000 psia (20.7 MPa) in the 1930's and to 10,000 psia (69 MPa) later. In 1934, their methane/propane system gave insight into natural gas behavior. Their work did not convey the physical appearance of the phase changes.

Katz and coworkers used glass-windowed equipment (Fig. 1). They measured the quantities of gas and liquid phases for a natural-gas/natural-gasoline mixture and its phases for a natural-gas/natural-gasoline mixture and its critical point. By holding the glass-windowed cell at constant temperature, they determined isotherms showing the two-phase boundary and volume percent liquid. Fig. 2 shows the isotherms, including the 169 deg. F (76 deg. C) critical isotherm, that initiated at 50% of each phase.

JPT

P. 1205