An approach for estimating formation factor parameter from transit time data in clean sand formation

One of the major tasks in quantitative log interpretation is to accurately estimate formation factor (F), a variable used in the Archie water saturati on equation, since such parameter varies with lithology and porosity. This paper is mainly addressed to introduce an equation to accurately determine this parameter utilizing sonic logs with known matrix, specifically where n o other porosity tools such as neutron and density are available. This esti mate can be achieved by merging the time-average equation, after reformulat ing it with an equation binding the porosity to transit time recorder in a porous media, with the well-known Archie equation. This equation, F = a[(Delta (t) - Delta (tma))(Delta (1/x)(t) - Delta (1/x)(tma))/Delta (1 /x)(t)(Delta (tf) - Delta (tma))](-m/2) takes into account the effect of variable matrix (Delta (tma)), fluid natur e (Delta (tf)), cementation exponent (m) and tortuosity factor (a), and is apparently applicable to porous media. The results of applying our approach to real field data taken from the Gulf of Mexico reflect the validity and accuracy of the proposed equation with clean lithology and explain why vari able matrix with single-mineral lithology, has to be accounted for in order to correctly derive porosity from the time-average equation. Finally, in the case of clay content, the application of the equation shoul d include the same correction procedures of the sonic log regarding those a pplied in usual interpretation. (C) 2001 Elsevier Science B.V. All rights r eserved.