The Cretaceous Bashijiqike formation is the main target of the tight gas production of Tarim Basin. Low porosity and low permeability were validated by core data. However, mud loss of 46.6 m3 occurred near high angle open fractures, and gas production and oil production has been tested in high angle open fracture section. It suggests that high angle open fractures can cause mud loss and they also have a positive effect on tight gas reservoirs.
Open fractures are the key point to the tight gas reservoirs of Tarim Basin. However, it was impossible to identify open fracture and cemented fracture through conventional logs and seismic data. Ultrasonic image logs and oil-based electrical image logs make it possible to oil-based mud well. The new-generation oil-based microresistivity image logs provide high resolution images with high borehole coverage. It improves the image quality of the previous oil-based microimager logs (OBI) in oil-based mud well.
A total of 39 open fractures and 33 cemented fractures in a vertical well were recognized from the new-generation oil-based microresistivity image logs. Core observation for this vertical well also validated the presence of both cemented fracture and open fracture. In addition, ultrasonic image logs were acquired in this vertical well. According to the ultrasonic image features, it only responds to large open fracture with bigger aperture and has limitation to relatively small-scale open fractures with smaller aperture. The new-generation oil-based microresistivity image logs complement the limitation of the ultrasonic logs.
The case study presents how to identify different kind of fractures by the high-resolution oil-based microresistivity image logs in oil-based mud well, especially open fracture detection. As the main gas migration pathway, the open fractures were important to tight sandstone gas reservoir quality in the Bashijiqike formation of the Tarim Basin. Better understanding of the fracture attributes was instructive for fracturing stimulation design of tight gas reservoir as well.
