ABSTRACT

The quantitative interpretation of mud logging and gas data is still challenging due to the uncertainties associated with data acquisition and lack of data quality control. Nonetheless, Beda & Tiwary (2011) have developed a method that provides quantification, on reservoir quality and water saturation (SW) using gas and mud logging data.

The approach used in this work is based on the Beda & Tiwary equation (B&T), a technique employed to estimate water saturation while drilling. This equation bears similarities to the known Archie equation, which also determines water saturation using resistivity and porosity as input parameters. However, the B&T equation includes normalized gas data (gas readings that exclude external interferences) and Perforability Index (PI) as inputs for predicting water saturation. Another factor, the Gas Water Baseline (GW), serves as a reference for 100% water saturation. This corresponds to the normalized gas count over a water saturated zone and can be compared to Ro (100% water saturated reservoir resistivity) in the Archie formula (Archie, 1942) or the baseline in a pickett plot (Pickett, 1973). The primary modification in this methodology lies in the adjustment of the PI equation, to accommodate PDC drill bits used in the studied well. The number of bit nozzles affects the bits pressure, hence, the formula needed to be adjusted to overcome this effect. Additionally, the control of the rate of penetration (ROP) was included to enhance chromatography quality control. In terms of porosity assessment, this methodology involves two steps: normalizing ROP by Normalized C1 and, subsequently, calculating the Perforability Index (PI). The porosity will be estimated by using the regression formula derived from cross-plot PI x PHIT in a correlation well.

The Cabeças Formation, in the studied well (Fig. 1), located at the Parnaíba Basin, Brazil, has gas bearing sandstone and heterolytic reservoirs deposited in a deltaic environment under tidal influence. The facies associations of this formation include channel filling sandstones and tidal bars. The comparison of water saturation (Sw) values between the proposed method and the petrophysical analysis using traditional Archie equation (a=1, m=n=2) resulted on a R2 of 0.83 and for porosity compared to gas corrected density-neutron, an R2 of 0.63 was obtained. The results were used to support the definition of the Gas-Water contact (GWC), confirmed with pressure data and petrophysical evaluation. Although, the lower R2 obtained for the porosity, the average values are quite similar. The trend and magnitude of the predicted porosity are well aligned with the porosity derived from log data. In conclusion, this methodology finds applications in predicting both water saturation (Sw) and total porosity (PHIT) before conducting wireline log measurements, and by that, adding value to the already existing mud logging data by performing an early formation evaluation. This can support decision making and avoid lack of information in case of operational restriction for a later wireline log operation.

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