Research and Application of Underbalanced Drilling Technology for Deep Shale Gas Horizontal Wells in Southern Sichuan
Authors: Fu Li , Yang Henglin , Ye Xinqun , Wang Yuan , Chen Gang , Yuan Guangjie , Zhang Heng , Wang Zixin 
Published: September 2023

ABSTRACT
Deep shale gas has become an important alternative resource for shale gas development in China, and the number of horizontal wells drilled is increasing year by year. However, the rate of penetration, bottom hole temperature and leakage in horizontal section are seriously restricted by the high mud weight. Firstly, an underbalanced drilling technology strategy and mud weight reduction implementation process in shale reservoir are proposed. Secondly, based on the identification of abnormal high pressure mechanism, the pore pressure prediction model is optimized, and the fluid-solid-thermal coupling collapse pressure prediction model of fractured shale reservoir is established according to the characteristics of shale bedding. Finally, combined with the three-dimensional geological model, the safety window distribution of mud weight is clarified, and the favorable area of underbalanced drilling operation is divided. The underbalanced drilling technology has been tested in 10 horizontal wells. Compared with the conventional drilling wells, the rate of penetration, temperature control and leakage control is obviously improved. When the mud weight is reduced by 0.2 ∼ 0.45g / cm3, the bottom hole circulation temperature is reduced by 3 ∼ 8 °C, the average rate of penetration is increased by 17 %, and the average leakage and loss time of single well are reduced by 239m3 and 207h respectively.

INTRODUCTION
The upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is the main strata in the favorable area of marine shale gas in southern Sichuan, China. The working area below 4500 m is 2.1 × 104 km2, and the shale gas resources are more than 10 × 1012 m3. Among them, the shale gas resources buried in 3500-4500 m are more than 8 × 1012 m3, accounting for 80 % nearly (He et al. 2021). Compared with the shallow shale gas buried below 3500 m, the deep shale gas buried in 3500-4500 m has undergone significant changes in reservoir temperature, pressure, ground stress and other aspects. The drilling and completion technology is in the exploratory stage. 94 horizontal wells were drilled in the deep shale gas area of southern Sichuan in 2020, of which the average drilling cycle of 215.9 mm well section was 56.7 days, the average rate of penetration (ROP)was 5.53 m/h, and the average number of trips was 10.6. One of the important factors that affect low rate of penetration and long drilling cycle is the high density of drilling fluid used in shale reservoir drilling. According to the geological design requirements, the density of oil-based drilling fluid in shale reservoir is usually in the range of 2.05-2.25 g/cm3. The high density of drilling fluid will induce serious pressure holding effect that can result in slow ROP of horizontal section. High density of drilling fluid will aggravate the increase of bottom hole circulation temperature during drilling (Keith et al., 2011). The bottom hole circulation temperature of horizontal section for deep shale gas well is between 135 °C and 155 °C, while conventional rotary steering tools and instruments cannot work normally for a long time under the temperature higher than 135 °C. The high density of drilling fluid will also aggravate the risk of well leakage. In 2020, 14 wells had lost circulation in the horizontal section that the cumulative leakage of oil-based drilling fluid is 981 m3 and the loss time is 149 days.