Addressing Structural Complexity in Asmari Fractured Reservoirs with Borehole Imaging Technique and VSP

Movahed Z and Safarkhanlou Z

Published on: 2024-01-09

Abstract

The exploration of carbonate reservoirs in structurally intricate regions, compounded by the presence of an extensive layer of Gachsaran formation evaporates overlaying the reservoir, presents a formidable challenge in structural delineation. The absence of adequate structural geology information, particularly in naturally fractured reservoirs, often hinders drilling operations. In Iran, a dearth of structural data related to these fractured reservoirs has significantly impeded drilling activities. This study addresses the prevalent issues by demonstrating the impact of insufficient structural information on drilling operations in complex naturally fractured reservoirs and highlights the utility of borehole imaging tools in overcoming these challenges. The primary focus of this research is to establish a precise and comprehensive structural model for the Asmari reservoir in the Gachsaran field. Analysis based on well site geologist reports, utilizing formation cuttings, indicates that the well failed to penetrate the lower Asmari until a true vertical depth (TVD) slightly deeper than 2800m, while the predicted lower contact of Asmari with the Pabdeh formation was at 2588m. To investigate this discrepancy, Oil-Based Mud Micro Imager– Ultrasonic Borehole Imager (OBMI-UBI) tools were deployed in the interval from 2117m to 2772m.Analysis of bedding dip from OBMI-UBI data, combined with the well trajectory, reveals an initial northeast (NE) deviation in the upper part of the logged interval. Subsequently, the trajectory gradually shifted southwest (SW) and maintained that direction with a maximum deviation of 36 degrees until reaching the total depth (TD). Plotting Cap rock and bedding dips alongside the well trajectory on the N65E-S65W plane, at the same vertical and horizontal scales, disclosed that the well remained parallel to the dip of the middle Asmari beds/layers in the lower half of the logged interval. Continuing drilling with the same deviation would have been futile, as the well would not have been able to exit the Middle Asmari. Therefore, the OBMI/UBI images in the studied well played a crucial role in resolving structural complexity, providing the exact location of the well in the Asmari Reservoir, which otherwise would not have reached the lower contact of Asmari.