Water Based Mud High-Resolution Resistivity Images, Innovated Operational Practices to Enhance Log Quality in Lateral Boreholes Drilled with Multiple Fluid Systems.
Authors: Mohamed Ahmed , Woldeamanuel Mandefro B , Gouda Mohamed , Rashad Hesham 
Published: January 2023

Understanding how natural fractures are behaving is providing a growing knowledge about reservoir's structures and stresses, hence it becomes important to deploy high-resolution resistivity imaging service, especially in case of severe drilling fluid losses. In lateral boreholes, deployment of such service comes with its known additional operational challenges. In certain cases, drilling of these laterals starts with oil-based mud (OBM) to achieve certain objectives, then displacement with water-based mud (WBM) takes place when mud losses encountered at certain depth while drilling. Recording these sensitive measurements in such environment requires special planning for the string configuration and logging parameters optimization, and this article is explaining innovative operational practices followed to acquire high-quality image logs, and it will go through different scenarios while highlighting the potential constraints and limitations.

Firstly, it is important to identify how long the borehole interval is drilled with OBM before displacement operations, as this controls the borehole conditioning process after TD and before pull-out-of-hole (POOH) for pipe-conveyed logging (PCL). It depends, so the situation is being assessed and discussed to decide the feasibility of logging operations. Afterwards, decision comes to either cancel the service request if it is confirmed that quality of data is expected to be highly degraded, or to proceed with logging operations if log quality is anticipated to be acceptable and meeting the deployment objective(s). With the latter option, clear plan of borehole conditioning procedures will be prepared, while considering an optimized tool's acquisition parameters in such mixed and challenging borehole environment.

Different scenarios of complex mud environments were tested, including one case with variable WBM salinities across the logged MD interval without having any interference from any OBM during the drilling process. General conclusion is that less interval drilled with OBM pours into the benefit of obtaining higher data quality from WBM resistivity imager after less borehole conditioning time. In certain cases when longer interval was drilled with OBM, different treatment and conditioning processes resulted in accepted WBM resistivity imagers data quality. These practices of real tested cases are not granting accepted images quality all the way because of different variables like mixed fluids additives and salinities as well as borehole wall exposure time to such conditions, but they are providing sufficient awareness about how and when to expect an accepted image quality. To illustrate this conclusion, this article will also present a case where performed treatments and borehole conditioning processes were insufficient and had never produced any accepted image quality at all, which gives an insight that anticipated images quality in such scenario is considerably poor, and where to set valid cutoffs for the whole service request depending on how mixed borehole environmental conditions are.

Instead of completely omitting resistivity imaging service in mixed borehole environments, these practices can beat the challenges, reduce the service limitations, and experimentally deliver accepted high-resolution resistivity images quality with clearly mapped natural fractures.