The paper presents a methodology and results of a study to derive the gas-oil ratio (GOR) of oil in situ based on downhole ultrasonic sound speed formation tester measurements. Authors provide several cases as a reference for in-situ fluid acoustic measurement values, and cleanup profiles in various borehole and downhole settings, to illustrate their benefits for formation testing as well as other geoscience applications.
Formation tester dynamic fluid measurements of analysis of 47 samples from 25 wells were assessed. Reference GOR measurements were collected from different sources. The relationship between in-situ sound speeds corresponding to reference GOR was assessed and showed strong correlation within the investigated range. To enhance estimation accuracy, another two methods of deriving GOR using fluid slowness, downhole condition parameters, and computed pseudo-density are presented.
The assessment of formation tester fluid slowness data showed its consistent association with live oil GOR within the studied range (~20-1,500 scf/bbl). Further increase in GOR showed change in the relationship trend. Several methodologies of GOR estimation are discussed including multiple linear regression using fluid slowness, in-situ pressure, temperature, and linear regression with estimated pseudo-density of a fluid. The multiple regression output showed slightly better match with laboratory data. Results of other data modelling approaches are also discussed.
The proposed GOR evaluation methodology is based on acoustic surveys made downhole without phase separation. The results are available at the time of formation testing cleanup and fluid sample capturing stages. The additional benefit of the method is to aid in assuring fluids are maintained in single phase for collection of representative samples.