Phase states of water-hydrocarbon fluid systems at elevated and high temperatures and pressures: Evidence from experimental data

Abstract

As it is known, liquid and gaseous hydrocarbons (HC) in the Earth's crust are usually associated with aqueous solutions. This relationship mainly results from the similarity of their aggregate state and the same hydroand gasdynamic laws of migration in the Earth's crust. Saline aqueous solutions (L1), liquid hydrocarbons (L2), and gases (G) form relatively spar� ingly soluble mechanical mixtures under nearsurface conditions, as well as at shallow and medium depths at a temperature of 50-200 °C and a pressure of ~5- 20 MPa (3). However, the increase of P-T parameters results in significant change of the situation, and even at a temperature of 250-400°C and a pressure of >40 MPa, the solubility of oil (especially its light frac� tions) and hydrocarbon gases increases by orders of magnitude (3, 5). This should be necessarily reflected in the phase state of water-hydrocarbon fluid systems and, thus, influences the forms of HC migration in the Earth's crust. Direct observations of phase states and the behavior of hydrous-hydrocarbonic fluids in the Earth's interior at elevated and high temperatures and pressures are practically impossible. However, they may be established on the basis of experimental studies. Experiments with use of fluid inclusions in natural minerals and in synthetic crystals are the most con� vincing in this regard. Quartz is chosen as host crystal because of its mechanical and chemical stabilities in presence of aqueous solutions and crude or gasfree oil at high P—T conditions. At the same time under selected conditions of experiments the aqueous- hydrocarbon inclusions in crystals are actively formed. Subsequent study of individual inclusions by the methods of common and IRmicrospectroscopy and gas-liquid chromatography together with microther� mometry allows establishing the phase states of fluids in situ in a wide range of temperatures and pressures (1. 4, 6). In this paper we present the results of such investigations. Quartz crystals with aqueous-hydrocarbon inclu� sions were synthesized in autoclaves with a volume of 50 ml in alkalescent (5 and 10 wt % NaHCO 3 ) and alkaline (3, 5, and 7.5 wt % Na2CO3) solutions in the presence of crude oil. Its portion in aqueous-oil mix� tures varied from 0.01 to 50 vol %. Experiments were performed at temperatures of 260/311, 330/350, 388/454, and 490/500°C (the slash separates the tem� peratures of the upper and lower parts of autoclave) and pressures of 30, 70, 74, and 90 MPa, respectively. The duration of runs was 15-30 days.