Physical and chemical properties of oil and gas in high pressure and high temperature are essential for the development of oilfield. These data have been determined by PVT apparatus with mercury for a long time. Although many mercury-free PVT instrument have been developed, mercury has to be used in sampling and transferring of sample. As well known, mercury vapor has serious toxicity, hence it is very harmful to the health of operators and to environment. This paper launches a low melting point alloy without toxicity and corrosion that can replace mercury as a pressure transferring and heat-transferring medium in PVT apparatus. Firstly, physical and chemical properties of the substitute are tested. Melting point, specific gravity, boiling point, viscosity, surface tension, vapor pressure, temperature coefficient, pressure coefficient, conductivity, specific heat and etc. are respectively -14.80 °C, 6.58(20 °C), 2170 °C, 2.30(20 °C)Pa ·S, 0.36(20 °C)N ·m, 2.96 ×10]−23 (100 °C)Pa,2.97 ×10−5/K, 8.99 ×10−4/MPa, 0.36(20 °C)J/cm ·S · °C and 0.30(20 °C)J/g · °C. Secondly, the solubility of pure substances and mixtures such as N2, CO2, H2, H2S, oil and natural gas in mercury and the substitute are determined, respectively. Z-factor of these systems are also evaluated using the substitute and compared with those obtained using mercury. Thirdly, the phase behavior experiments of oil and gas including bubble point, dew point, constant composition expansion, differential liberation, vapor-liquid equilibrium, constant volume depletion and multistage separation are determined with mercury and the substitute, respectively. It is shown that the substitute performs as well as or better than mercury when used as heat-transferring and pressure-transferring medium in PVT system, and possesses toxinlessness and corrossionlessness.
PVT phase behavior apparatus is applied to the basic test of high-pressure physical properties in the oil and gas development. For a long time, these apparatuses rely on import and more than 20 sets have been imported from American Ruska Corp.; Corelab; Canadian DBR and Hycal corporations and some corporations of France, Denmark etc.. Each apparatus adopts mercury as its pressure transfer medium. Mercury has the advantages of small chemical and physical effect on the samples, small compressibility, good stirring effect at the large mechanical stirring angle which is helpful to phase equilibrium, and high test accuracy. At meanwhile, it also has some defects, when adopted as transfer medium under normal experimental conditions, each set of PVT apparatus needs preparing several kilograms of mercury. It is all known that mercury steam is very toxic under high temperature and high pressure, it is especially harmful to lab workers, and causes pollution to the surroundings. So it is extremely important to produce a PVT apparatus without mercury.
There are two ways at present to solve the problem of toxicity of the mercury in PVT apparatus. One is to apply piston-style PVT apparatus, the other is to find substitute for mercury. Since 1986, the type of mercury-free PVT apparatuses have been produced at home and abroad. They all have a common defect. The former is that they all adopt piston to transfer pressure, so the dead "volume" will be caused by the piston contacting inside the PVT cell, by which some high-pressure physical properties can not be measured.