Analysis of downhole OBM-contaminated formation fluid

A method for validating initiation of sample capture of borehole fluid, using a borehole tool having a pump, a flowline, an optical analyzer, and means for capturing a sample, the method comprising the steps of:

• a) pumping borehole fluid through the analyzer;
• b) measuring OD of borehole fluid to produce a series of OD values at intervals of time;
• c) testing for scattering;
• and
• d) initiating sample capture if scattering is less than a predetermined value.

A method for validating initiation of sample capture of borehole fluid, using a borehole tool having a pump, a flowline, an optical analyzer, and means for capturing a sample, the method comprising the steps of:

• a) pumping borehole fluid through the analyzer;
• b) measuring OD of borehole fluid produce a series of optical density values at intervals of time;
• c) calculating an asymptotic value indicative of optical density of formation fluid from the series of optical density values;
• d) repeating steps a) through c) to produce a series of asymptotic values;
• and
• e) initiating sample capture if asymptotic values are monotonicaly changing at less than a predetermined rate.

A method for predicting OBM filtrate fraction of borehole fluid after a predefined second period of pumping, using a borehole tool having a pump, a flowline, and an optical analyzer, the method comprising the steps of:

• a) pumping borehole fluid through the analyzer;
• b) illuminating the borehole fluid with light in the visible spectrum and with near infra-red (NIR) light at a wavelength associated with gas;
• c) detecting optical absorbance in the visible spectrum to produce a visible spectrum optical density value and NIR absorbance to produce NIR optical density value;
• d) calculating GOR as the ratio of the NIR optical density value to the visible spectrum optical density value;
• e) repeating steps a) to d) during a first period of pumping to produce a series of GOR values at intervals of time;
• and
• f) calculating a GOR asymptotic value indicative of predicted OBM filtrate fraction, including solving a third mathematical function for coefficients by fitting the series of GOR values to the first mathematical function, then using at least one of the coefficients in a fifth mathematical function to determine predicted OBM filtrate fraction.

A computer usable medium having computer readable program code thereon, the medium adapted for use with borehole apparatus, the program code including code structured to

• (a) accept a series of borehole fluid parameter values indicative of OBM filtrate contamination;
• (b) calculate from the parameter values an asymptotic value;
• and
• (c) calculate from the asymptotic value a quality of a downhole fluid.