Studies on Lost Circulation Risk of Deep Shale Formation Based on Fracture Prediction and Rock Mechanical Properties
Authors: Wen Bo , Wang Yuan , Chen Gang , Zhang Peiyu , Guo Kaijie , Fu Li , Feng Ming , Yang Henglin , Zhong Chengxu 
Published: September 2022

ABSTRACT:
Lost circulation is a technical problem in drilling engineering, resulting in increased drilling cost and extended drilling cycle. Reservoir rocks in Block A of Shale Gas have the characteristics of low porosity and low permeability, strong heterogeneity and fault fracture development. Lost circulation is mainly caused by fracture. Seismic fracture prediction to assess lost circulation risk has limitations. Therefore, it is necessary to study more effective methods of lost circulation risk assessment. In this paper, based on the analysis of lost circulation, seismic prediction on fractures and rock mechanics parameters in block A, a comprehensive evaluation method of lost circulation index is formed by using seismic prediction of the relative degree of fracture development, fracture apparent length and the combination of Young’s modulus and Poisson’s ratio in 3D attribute modeling, and the classification evaluation standard is established. The method is used to evaluate the lost circulation risk of completion drilling in shale gas well of Block A, and the evaluation results are consistent with the well drilling data. Lost circulation index evaluation method can effectively evaluate the risk of lost circulation in shale formation with low porosity and low permeability, strong heterogeneity and fracture development, which can promote the efficient development of shale gas.

1. INTRODUCTION
With reservoir exploration and development, it is more difficult for conventional oil and gas resources to increase reserves and production. Unconventional oil and gas resources are of strategic importance. As an important unconventional energy source, shale gas has become a hotspot in global resource development. Shale gas development can effectively alleviate the energy crisis, reduce the pressure on natural gas gaps, improve the energy structure, and ensure national energy security. In recent years, with the continuous development and improvement of the basic theory and technology development of shale gas in China, Fuling, Sichuan Changning-Weiyuan and Zhaotong national shale gas demonstration areas have been established. Shale gas production in southern Sichuan basin reaches 128×108m3 in 2021. Horizontal well drilling technology is the key to the shale gas resources development. With continuous research and exploration, PetroChina has made remarkable progress in unconventional oil and gas horizontal well drilling and completion technology through the introduction of advanced oil and gas engineering technology from North America. The average length of the horizontal section of shale gas horizontal wells in 2021 has reached 1760 meters, and the average drilling cycle is controlled within 80 days(Jun et al.2018). However, the complex geological conditions of China’s shale gas lead to drilling accident. According to statistics, the average well accident time of shale gas horizontal wells in 2021 is 7.25 days, mainly for lost circulation time. Lost circulation is still the main factor restricting the drilling cycle and drilling efficiency of shale gas horizontal wells.