Optimizing Completion Design through Fracture Evaluation in Oil-Based Mud using High-Resolution LWD Ultrasonic Images

Abstract

Identification and interpretation of fractures, bed boundaries, and borehole breakout from high-resolution images plays a crucial role in optimizing completion design. In low-angle wells drilled with oil-based mud (OBM), images may be acquired using wireline. However, using wireline has been a challenge when inclinations exceed 45°, making logging-while-drilling (LWD) acquisition preferable. This paper presents the first use of a 4¾-in. ultrasonic LWD service to provide high-resolution images to assess fractures in the Marrat formation in North Kuwait. This paper presents LWD log data and high-resolution acoustic amplitude images used to evaluate carbonates within the Middle and Lower Marrat formations and describes their input into the design of the completion program. The 4¾-in. ultrasonic imaging tool was placed within a complex bottomhole assembly (BHA) composed of density and neutron porosity, acoustic, and nuclear magnetic resonance (NMR) sensors. The methodology used to create high-resolution images for both drilling and wipe run data sets using the logging speed and tool rotation is detailed, along with a description of how the image interpretation was used to optimize the completion design. The 6-in. borehole sections of the Middle and Lower Marrat formations are known to have prominent open fractures. During drilling, significant mud losses were encountered which required a reduction of mud weight to stabilize the well. From the memory data, 256-sector acoustic amplitude images were interpreted to provide an initial assessment of fractures and geological features. It was observed that an interval of log where mud losses were believed to have occurred corresponded with a large fracture and borehole breakout. In addition, multiple sections of borehole breakout at the top and bottom of the borehole were observed, with bed dip interpretation supporting the known field structure. Further post-well processing of the acoustic amplitude data was performed which created enhanced-resolution images. The processing method takes all of the raw impedance measurements–up to 2000 acquisitions per second–and re-sectors the data based upon the logging speed and tool rotation. The resulting images (540 sector for the drilled section and 360 sector for the wiped section) enabled identification of 255 features over the logged interval. The interpretation of fractures, their location, and dip and strike directions were used to optimize the completion design. The ability to acquire high-resolution LWD images in OBM applications within high-angle 6-in. hole sections to identify a wide range of features, including fractures, bed boundaries, and borehole breakout, represents a first in Kuwait. Removing the need to use wireline logging technologies in high-angle wells with wellbore stability concerns helps to reduce well time and logging risk. Deliverables from the 4¾-in. ultrasonic imaging service provide direct input into completion design, helping to optimize production.