Extra Deep Azimuthal Resistivity Application for Efficient Well Placement in a Long Horizontal Drain: Mitigating Challenges Due to Heterogeneity in Thin Target Carbonate Reservoirs Associated with Water Encroachment in a Mature Field – A Case History from Abu Dhabi Offshore
Authors: Khemissa Hocine , Al Mutwali Omar Imad , Al Safi Alwai Yaseen , Muhammad Ashraf , Ibrahim Alabed Sundos , Alfelasi Ali Saeed , Alkhoori Aysha , Mumtaz Asim , Potshangbam Sanathoi , Hameed Mahmoud , Valerio Romie , Saravanakumar Krishnan , Wharton Paul , Rawy Mohammed 
Published: October 2022

Abstract
This abstract demonstrates the application of an advance Extra Deep Azimuthal Resistivity tool to efficiently maximize reservoir exposure and map the reservoir architecture in an extended reach well of a mature field. The study shows how to maximise benefit from using advanced geosteering technology to successfully place the horizontal drain in an oil-saturated layer, map the water encroachment and design lower completion accordingly.
The reservoir is composed mainly of Grainstone to Packstone, with rock analysis showing highly heterogeneous limestone of good to excellent porosity and permeability. Total thickness is about 30 to 40 ft. with the target zone delineated on the upper part of the layer, within 6 ft of the top.
The challenges identified during the pre-well stage are: (1) Oil columns getting thinner as fields become more mature; (2) OWC exhibits less clear-cut boundaries and commonly displayed as transition zone profiles; (3) Resistivities within the transition zone vary along the lateral due either to proximity to other producing wells or to porosity variations within the geology; (4) Low resistivity environment; and (5) The increased uncertainty associated with conventional ‘deep’ resistivity tools to differentiate multiple resistivity layers.
The reservoir is under water injection and consequently, water movement is irregular due to the presence of high permeability streaks. There is high risk of water encroachment, which the Conventional ‘Deep’ Resistivity tools with limited depth of detection would not be able to differentiate.
Utilizing an Extra Deep Azimuthal Resistivity tool offered a solution to efficiently geosteer the drain within 6 ft. from the top boundary, and map the water saturation around it.
A feasibility inversion model incorporating the geological setting of the area and the water encroaching scenario predicted that the Extra Deep Azimuthal Resistivity tool would map the denser top of the target zone, the transition and water encroachment intervals with high confidence at a remote distance from the wellbore. This reduced the uncertainty around well placement so the tool was included in a Triple combo BHA to enable informed and proactive geosteering decision-making process.
The long horizontal drain was successfully placed along the porous oil-saturated zone and the water risk was mapped continuously below the wellbore.
The Extra Deep Azimuthal Resistivity technology mapped multiple resistivity layers leading to better understanding of the structural model and fluid distribution. The delineation of the resistive dense layer above the wellbore also facilitated good well placement at the required stand-off from the roof. Furthermore, the tool demonstrated its ability to map the top of the transition zone and the resistivity below, which is valuable in identifying intervals where the transition zone was swept with water requiring isolation in the completion. As a direct result, the completion design was optimized to reduce water risk.