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Abstract: Improvement on the Evaluation of Thinly Bedded Reservoirs Using Borehole Images in Horizontal Wells

Carolina Coll, Helena Gamero, Tito Lozada, Fernando Chacartegui, Ovidio Suarez

Conventional downhole resistivity log taken in a vertical well penetrating a thinly bedded formation is adversely affected by the limited vertical resolution of this tool. This type of log allows the identification of thick beds, but fails the detection of thin beds. The vertical formation, an isotropy that affects the resistivity curves in a vertical well, is increased in a horizontal well by the extra dimension that is measured by the tools during the navigation through a layer.

In horizontal wells the problem of detecting thin beds of low true resistivity and low bed contrast is magnified by the shoulder bed effect in two dimensions (horizontal and vertical). Additionally, the often-stated concept that the invasion process is shallow just after drilling was proved to be wrong in this reservoir. A proposed methodology is presented for enhancing the petrophysical evaluation. Conventional petrophysical analysis was compared with borehole images and the 3D geological model to understand the heterogeneities. The cuttings from the horizontal well were described in detail and correlated with both the petrophysical evaluation and the borehole images. An additional sedimentary perspective was gained from the horizontal images, thereby increasing the understanding of he depositional model and providing a new dimension to sand body architecture. This information was used to update the 3D geological model used in the simulation study. Borehole images provide a significant definition of the low resistivities of thin sands and proved to be a method to enhance the results of the petrophysical evaluation of this type of reservoir.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela