Nouri A. Berruin, Zoltan Barlai
Faculty of Petroleum and Mining Engineering Dept.
Al-Fateh University, Tripoli, Libya
ABSTRACT
The determination of initial oil in place and consequently recoverable reserves concentrates largely on the determination of net pay. While the oil column in a formation may be continuous down to the water-oil contact, presence of shale may affect the productivity of sections of this oil column severely. Therefore, such non–productive sections should be neglected when calculating net pay for reserve estimates.
One approach is to use permeability and/or water saturation cut-offs. Sometimes these cut-off values may be determined solely on economic basis. More frequently, shale indicator logs such as the SP and GR logs are used to provide a quantitative estimate of non-pay fractions of the formations.
In the present work movable oil index, f (Sxo - Sw), and shale content, Vsh as indicated from GR logs are taken into consideration to determine the productivity of an oil bearing sandstone reservoir. Flowmeter surveys were used to establish the effect of shaliness on movable oil index and consequently a productive/non-productive “dividing line” was determined. Initially, four
wells from a shaly but prolific sandstone reservoir in Libya were examined and a movable oil index vs. shale content–correlation was established. Low productivity was observed even for shale contents as high as 6.5% and for movable oil index as low as 9%.
Further improvements of predicting the “dividing line” between productive net pay zones and non–permeable shaly zones can be expected by introducing the lithologic influence factor, C, as the independent variable of the self-learning pattern-recognition schemes. Along the ordinate the MOI is traced as the regular combined quantity, f (Sxo - Sw), or even a more effective version,
of it is suggested.
f((Sxo – Sw) {(K/(f)) / ((K/f)MAX))} 1/4