A method and apparatus detects dew precipitation and determines dew precipitation onset pressure in a sample of formation fluid located downhole in an oilfield reservoir. In a preferred embodiment, the method includes (a) isolating a sample of formation fluid downhole; (b) illuminating the sample downhole with fluorescence excitation light; (c) measuring at least one characteristic of fluorescence short from the sample; (d) reducing pressure on the sample; (e) repeating steps (b) to (d); (f) detecting dew precipitation when a change is detected in a parameter that is a function of the at least one characteristic of fluorescence emission; and (g) setting dew precipitation onset pressure equal to pressure on the sample when the change in the parameter is detected. The parameter preferably is a function of fluorescence intensity and fluorescence red shift, and the change is an increase in fluorescence intensity and detection of fluorescence red shift. Also, the function includes a ratio of a measured intensity at a first wavelength to a reference intensity, and the reference intensity is a function of a measured intensity at a second wavelength.
Methods and apparatus for investigating a hydrocarbon bearing geological formation traversed by a borehole are disclosed. An example method to characterize a fluid associated with an underground geological formation obtains a sample of the fluid associated with the underground geological formation. The example method measures, in a borehole associated with the underground geological formation, a chemical composition and a thermophysical property of the sample of the fluid. The example method selects a mathematical model to represent the sample of the fluid based on at least one of the chemical composition or the thermophysical property and adjusts a parameter of the mathematical model based on at least one of the chemical composition or the thermophysical property to generate an adjusted mathematical model. The example method then determines a property of the fluid associated with the underground geological formation based on the adjusted mathematical model.
