Over the last two decades downhole fluid analysis (DFA) using visible and near-infrared spectrometry has proven to be one of the most effective means for obtaining accurate and detailed reservoir fluid property information during formation tester operations. In a previous publication (SPE 166464) a methodology was introduced for estimating fluid properties, such as fluid type, hydrocarbon composition (C1, C2, C3, C4, C5, and C6+), carbon dioxide content, and gas/oil ratio (GOR), from downhole optical spectrometer data acquired during sampling operations. We have extended the methodology introduced in the previous publication to the real-time estimation of asphaltene content of black oils.

The equation derived for quantifying the asphaltene content of crude oils uses optical densities (OD), the absorption coefficients of asphaltene and resin, stock tank oil (STO) density, resin content and formation volume factor (FVF). In the process of deriving the asphaltene content a new method was devised for estimating FVF from optical data. The unknown parameters in the equation and the uncertainty in the estimate of asphaltene content are calibrated against a database that contains asphaltene content data of various crude oils and the corresponding optical spectra. Using the derived equation, a maximum likelihood estimate of asphaltene content of crude oil and its associated uncertainty can be obtained.

The accuracy of the method for estimating FVF and asphaltene content was verified and validated using laboratory crude oil data. The method was also applied to downhole optical spectral data acquired during a sampling-while-drilling (SWD) operation. It was found that the estimated asphaltene content and FVF obtained from the measured downhole spectral data showed very good agreement with the results of laboratory pressure/volume/temperature (PVT) analysis performed on captured fluid samples.