Methods and apparatus for optically measuring fluid compressibility downhole

A method for determining the compressibility of hydrocarbon fluid in a formation, comprising:

• a) obtaining a sample of said hydrocarbon fluid from said formation;
• b) conducting a first spectral analysis of said sample at a first pressure to obtain first spectral results;
• c) exposing said sample or an equivalent sample of hydrocarbon fluid to a second pressure different than said first pressure;
• d) conducting a second spectral analysis of said sample or said equivalent sample at said second pressure to obtain second spectral results;
• e) determining an indication of said compressibility of said hydrocarbon fluid from an indication of said first pressure, an indication of said second pressure, and said first and second spectral results.

A borehole apparatus for measuring downhole the compressibility of hydrocarbon fluid in a formation, comprising:

• a) a testing region;
• b) a conduit for directing the formation fluid into said testing region;
• c) a sensor for measuring pressure on the fluid in said testing region;
• d) means for changing pressure on the fluid in said testing region;
• e) a light source which emits at least near infrared rays into said testing region;
• f) a spectral detector optically coupled to said testing region;
• and
• g) a processor coupled to said spectral detector and to said sensor, said processor for determining the compressibility of the formation fluid as a function of a change in pressure determined from at least two pressure measurements made by said sensor and as a function of measurements made by said spectral detector.