An Innovative Approach for Estimating the SW and Porosity Using Gas and Mud Logging Data in Real Time

Beda, Giulio ¹; Tiwary, Devendra ¹
(1) Exploration, Repsol, Madrid, Spain.

Interpretation of mud logging and gas data in a quantitative way is still far from simple. In most of the cases the conclusions reached are generally viewed skeptically due to the uncertainties inherent to the acquisition. The reputation of being an unreliable source of data with inconsistent results is quite well known. The aim of this paper is to draw attention to a “new aspect” of interpretation of the mud logging data which has not yet been properly exploited but is capable to provide quantified information about reservoir quality and water saturation. Two methodologies have been developed, consisting in deriving the main petrophysical parameters: SW and Porosity in real time throught new techniques and equations (Beda &Tiwary, in this paper).

The Beda&Tiwary equation is similar to the well known Archie equation for deriving water saturation using resistivity and porosity as input.

B&T equation uses normalised gas data and perforability index as input to predict water saturation. The other main parameter, GW (baseline for 100% water saturation), which is normalised gas count over fully water saturated zone, is analogue to the Ro (resistivity in 100% water saturation zone) of Archie formula. GW is derived using Beda&Tiwary plot which is akin to Picket plot. The B&T equation predicts water saturation, which has similar values to those obtained by petrophysiscal analysis. The methodology has very important application in predicting the SW before the wire line log suite is run.

The other petrophysical parameter (total porosity), is obtained by converting the basic drilling data into a perforability index and then into porosity. Two different methods have been suggested to estimate the total porosity.

The first one is applied when the well log data is available, which is used as calibration well, the second method does not need any calibration well and convert the perforability index into total porosity using the equation developed by authors. The derived porosity has satisfactory correlation with the log derived porosity.

Important and significant results have been achieved by integrating the gas data with the petrophysical evaluation techniques which is going to define the shape of future interpretation of gas while drilling data.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.