May-June 1999
Volume 40 No. 3

Thomas D. Barber, Tracy Broussard, Gerald N. Minerbo, Zlatko Sijercic, and David Murgatroyd

 
ABSTRACT: A new processing algorithm for multiarray induction tools has been tailored for highly deviated wells. This algorithm provides the same interpretation for invasion that has been available in the past only in vertical wells.
The new algorithm is based on maximum-entropy inversion of the raw, borehole-corrected array data through a fast 1D forward model. It allows interpretation of multiarray induction logs even in the presence of invasion. Tests on a wide range of invasion profiles computed with a 3D induction forward model code at relative dip angles as high as 85° show that the new inversion allows determination of Rxo, Rt, and the invasion profile with similar accuracy to that determined in vertical wells. In theory the process works to 90°; however, the current parameterization requires that the wellbore cut all beds of interest.
Studies of sensitivity to incorrect dip angle show that in most cases the relative dip must be known to ±5° to satisfy reasonable petrophysical requirements. Sensitivity studies to other sources of error, including coherent borehole noise, show that the new process has a sensitivity to these effects similar to the current field processing. Because the nonlinear response of induction arrays to conductivity is handled explicitly, and without approximation, the new process also handles large shoulder-bed contrasts without horns and overshoots at all apparent dip angles.
Application to field logs at a variety of relative dip angles confirms that the results predicted from modeled data transfer to the real world, producing Rt estimates that are fully corrected for dip effect.
 

G. M. Hamada

ABSTRACT: One of the most controversial problems in formation evaluation in the Nubian Sandstone of the Zeit Bay Field, Gulf of Suez, is the shale effect in the reservoir rocks. An accurate determination of formation porosity and fluid saturation in shaly sand is subject to many uncertain parameters. It is necessary to integrate information from several different log responses using various interpretation models and local knowledge in order to accurately estimate the desired formation properties. This note illustrates that a conventional approach for handling the problem of shaly sand provides accurate values of shale volume from different shale indicator tools and thereafter a reliable effective porosity. Hydrocarbon saturation profiles have been calculated using a laminated shale model. The validity of petrophysical parameter values estimated by this integrated approach is confirmed by a comparison with petrophysical properties measured on core samples collected from shaly sand sections in the same wells.