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3D INDUCTION LOG MODELING AS A PRACTICAL AID TO HIGH- ANGLE

nduction logshavebeenusedsuccessfullyformorethan 40 years to extract resistivity and invasion profile information. However, these tools and most processing schemes-from charts to signal processing-are designed for vertical wells. At high angle, only inversion can separateouttheconductivities ofeachbed.Inhorizontal wells, particularly at wireline time, making simplifying assumptions becomes much more difficult. Indeed, in some situations in horizontal wells, a tool may have more sensitivity to the resistivity in an adjacent bed or to invasion than to the true resistivity (Rt) in the bed it is logging. In these casesiterative forward modeling is often the only way to investigate tool sensitivity and to determine Rt.
Previous work has shown it is possible to accurately model induction response in 3D formations. Recent advances in these 3D codes include building simple interfaces and, in particular, greatly improving speed.Run times of 10 to 20 seconds per logging station have been achieved on fast workstations. These advancesnow allow the use of 3D codes for practical log analysis in high- angle and horizontal wells, using the same “iterative interpretation” approach that was pioneered with ELMOD and similar 2D modeling codes.
As a case study, we modeled multiarray induction response in horizontal and near-horizontal wells in a Middle East reservoir. The need is for an understanding of induction response for quick completion decisions. In the case of massive formations, the deep measurements can be used with some confidence in the center of the formation (invasion may influence only the shallower readings), with little effect of the adjacent beds on the deepest readings. However, in thin beds or those with complex geology leading to resistivity variations, the effect of adjacent beds combined with the invasion creates confusion in interpreting the logs. In this case, the sequence of the multiarray induction measurements with respect to depth of investigation does not make petrophysical senseandisconsideredtobealoganomaly that must be studied before a completion decision is taken.
Current practice is to use 1D forward modeling to illustrate theinfluence oftheadjacentbedsonthedeepest measurements. The effects of invasion are ignored, and this simplistic approach to determining Rt is often incorrect. The 3D casestudy includes modeling the well path traversing facies and lithologies with the effects of invasion added to porosity and resistivity variations at a low angle. By carefully incorporating geological information, we refined the model to untangle confusing curve sequences and obtain more accurate values for Rt in individual facies. In addition, we extracted the geometrical structure of the formation layers and the invasion profile.
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