Today although various methods have been developed for effective over pressure detection in sand/shale formations, pore pressure detection in carbonates is still a difficult problem. As the topic of this paper, we studied the structurally complex, fractured, gas-bearing carbonate formations in East Sichuan, China, to develop both pore pressure detection and wellbore stability evaluation methods to address current drilling problems. A pore pressure detection method based on relationships between the ratio of compressional and shear wave velocity (Vp/Vs) and vertical effective stress, Poisson's ratio and vertical effective stress was developed. The interpretation results obtained from this method, which are also an important parameter for wellbore stability evaluation, are good in comparison to measured pore pressure. As for wellbor estability evaluation, the methodologies implemented in a commercial software(ROXAN*) were applied to the log data of three wells in the field. Field mechanical data and rock mechanical properties test results on cores were used to develop correlations adapted to the specific situations of East Sichuan and to build a mechanical earth model. Rock strengths and geostresses were calibrated with laboratory measurements and FMI images to establish realistic mechanical parameters for the 3D linear poroelastic model. The comparison between the predicted shear/tensile failures under the mud weight used in the three wells and the borehole failure indications from FMI images provided reasonable calibration factors for rock strengths. Together with other wellbore stability managing techniques and tools, the stable mud weight ranges derived from the wellbore stability planning for a new well can optimize drilling efficiency greatly.
Introduction
East Sichuan Field. Sichuan Basin is a giant oil and gas field in South west China. The Sichuan Petroleum Administration {SPA} has been exploring and developing this field since the beginning of the fifties.
East Sichuan field is located in the eastern part of Sichuan Basin, acompressive and distorted basin. Narrow, long, steep and northeast trending anticlines are well developed in this field. The general features of the structures are: shallow structures are intact, the middle part is a compressive belt with well developed folds and reverse faults, the deep zone is characterized by gentle structures (Figure 1). Marine facies carbonates are the dominant lithologies in this field, permeable natural fractures and vugs arevery developed here. Many separated gas pools with varied reserves are located in East Sichuan.
Due to the complex geological features in East Sichuan, It is difficult to drill safely and economically here, the difficulties include:*Structures are steep with high formation dip, the highest formation dip can reach 85 degrees.*Gas pools are buried very deep, the main pay zones in the Carboniferous are buried from 4km to 6km in general.*The upper and middle part of the formations are very loose, with developed fractures and vugs. Lost circulation and wellbore collapse often happen during drilling.*Rocks are hard with low drill ability.*Complex pore pressure systems that are related with many pay zones.*High H2S content in the gas.
Wellbore Instability Problems. The complex structures and fractured gas bearing carbonates in East Sichuan are difficult to drill and have resulted in very expensive wells. In order to reach reservoirs at depths between 4 and 6km, wells must cross reverse faults, natural fracture/vug zones and sometimes very high dipping formations. Multi-phase tectonic events, compressive deformation and diagenesis resulted in these complexities, and the presence of these complexities has led to many of the drilling problems. There are threemain types of drilling problems in East Sichuan: blowout, lost circulation and stuck pipe.