Overview of the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project

The Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) is one of the projects of the National Science and Technology Supporting Program. It aims to drill five boreholes along the May 12, 2008 Wenchuan earthquake zone in Sichuan province, China, more specifically along the Beichuan-Yingxiu fault belt and the Anxian-Guanxian fault belt, at the front of Longmenshan Range (with depths of 500 m, 1200 m, 2000 m and 3000 m). The sub-surface data will allow scientists to better understand the mechanism of the Wenchuan earthquake through scientific drilling. Long-term earthquake observation stations will be set up and the earthquake detection instruments will be installed in the boreholes to provide the critical and basic data for earthquake monitoring, forecasting and warning. At present, two boreholes have been completed, two more are being drilled, and the 3000 m-deep hole should be drilled in 2011. The drilling technical scheme has been conceived according to the Longmenshan fault zone's formation condition , which is highly fractured and the fault gouge very thick because of the repeated earthquakes. Based on the technical and economical evaluation on the existing coring methods, we have selected the top-drive and wireline coring method as the main drilling and coring method of the project. We have developed an hydraulic top-drive deep drill rig and a set of large diameter wireline coring tool. The drill rig has an electro-hydraulic proportional control, is easy to operate, and has a long feeding stroke. It can be used in coring, back and forth reaming. Its depth capacity is 3000 m for the 150 mm final diameter. We have used split barrel coring technology to solve the problem of coring in fractured formation. The core barrel is 4.5 m-long and the core diameter ranges from 85 to 100 mm. Over 92% core recovery is achieved with the cores being in good original state. Some extreme technical difficulties, such as hole gushing water, fractured formation, borehole collapse and tight hole were encountered in the drilling operation. To solve the tight hole problem in the fault gouge section, we have developed special coring and cementing procedures suitable for this formation's condition, and we succeeded in small annulus cementing with a long borehole section. We used a mud system with high density, low water loss and good lubrication properties to avoid clay expansion and counterbalance tight hole resulting from geostatic stress in the fault gouge section.