Fracture Detection and Structural Dip Analysis From Oil-Base Microresistivity Image Logs in a Horizontal Well: A Case Study From the Longmaxi Shale

Abstract

A large amount of mud loss and the cluster design of hydraulic fracture stimulation in an oil-based mud horizontal well motivated running oil-base microresistivity image logs in the horizontal section of 1600 m in the Silurian Longmaxi shale. The image log interpretation reveals the distribution of natural fractures in the horizontal section, and indicates that the horizontal section cuts through the structural fault identified from structural dip analysis. The mud loss of total 342 m³, however, occurred nearby the natural fractures in the horizontal section. It suggests that the fractures could cause the large amount of the mud loss. The attributes of the fractures and fault were used as important inputs for the cluster design of hydraulic fracture stimulation in the horizontal well.

A total of 381 fractures in the horizontal section were recognized from the oil-base microresistivity image logs. In the target layer of the pilot hole of the horizontal well, 15 cemented fractures were interpreted from electrical image logs. Core observations from vertical offset wells indicate that the fractures in the Longmaxi shale are partially cemented fractures. The core and electrical image logs suggest that the 381 fractures observed on the oil-base microresistivity images could be partially cemented fractures.

Fractures and minor faults are very common because of complex structural fault system in the Silurian Longmaxi shale. However, it is impossible to detect the fractures and minor faults from conventional open hole logs and seismis data because of resoultion limitations. And water-base electrical image logs, such as wireline image logs and LWD image logs, can’t be used to run in an oil-based mud borehole well. Although the oil-base microresistivity image logs was a good solution for fracture and fault detection in the Longmaxi shale.

On the other hand, the case study presents that the attributes of fractures are important not only for the cluster design of hydraulic fracture stimulation, but also for better understanding the distribution of fractures in the Longmaxi shale. The horizontal section drills through more fractures than the pilot hole, because the horizontal section trajectory is perpendicular to the high-angle fracture strike. Regarding further deploy methods, the new generation oil-base microresistivity image logs will be used to the distinction between open fracture and cemented fracture by inversion processing.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018