Understanding and Research on the Drilling Fluid Technology for Shale Gas Horizontal Wells in South Sichuan, China
Authors: Yan Lili , Wang Jianhua , Zhang Jiaqi , Cheng Rongchao , Yang Haijun , Yang Haowei 
Published: January 2020

Recently, shale gas resource has been developed beneficially on a large scale in China. More than 400 shale gas horizontal wells have been drilled in south Sichuan since 2014. However, with the increasing buried depth of target stratum and the extension of horizontal intervals of shale-gas horizontal wells, the development of Chinese shale gas is becoming more and more difficult. Over the past four years, some wellbore instability related issues–stuck pipe and rotary steering system fish downhole occured during drilling the shale gas wells. The reason is particularly analyzed based on the drilling performance data of these horizontal wells, which will offer the establishment of general practice guidelines and recognition of opportunities for improvement in south Sichuan shale drilling.

Oil-based drilling fluid (OBM) is a typical drilling fluid type currently used in the south Sichuan shale play. However, water-based mud (WBM) has also been used in more than 60 wells since 2015. It is analyzed the drilling performances of over four hundred horizontal south Sichuan shale wells drilled by 14 operators from 2014 to 2018, including overall drilling days, well depth, lateral length, as well as fish rotary steering systems. A comparative analysis is also made among different drilling fluid types of different operators to assess their performances and to identify the key challenges when drilling south Sichuan shale. The analyses contained formation characteristics, mud chemistry, mud rheological property, plugging property, solid content, stability, density and chloride and so on. Then, lots of experiments are carried out to solve the problems of borehole stability and hole cleaning.

The analysis shows that the drilled well depth and the horizontal length are getting bigger and bigger. The overall performance of WBMs lags behind that of OBMs in south Sichuan shale drilling. Most fish RSS happened when the operations of back reaming, pick up stands and trip-out were carried out. The lost circulation nearly covered all the well sections. These issues lead to excessive amounts of time spent on costly fishing, sidetracking, plugging operations and the long drilling days. The performances of oil-based drilling fluids of different operating companies were quite different. Lab test results show that shale formation is more complex than before. With the increase of well depth, the clay minerals decreased and brittle minerals raised. The natural fractured formation increases and the drillable "sweet" layer thins. The main mechanics of shale instable is the strong capillarity on amphiphilic shale surface causes the invasion of fluid into formation, then leads the fracturing and de-lamination along the bedding planes and enlargement of natural fractures. It is effective to control the harmful low-density solid for improving the performance of OBM. The suitableφ6 value of OBM and a big delivery capacity will work in hole cleaning while drilling lateral sections.

A nanoscale polymer was developed in lab to improve the plugging effect significantly after adding in OBM. An amphiphobic material was used to transform the wettability of shale, which was favorable for borehole stability. The analyses and results of this study on drilling performance data provide lessons learned and general guidelines for current drilling practices such as drilling fluid selections and mud property control in the south Sichuan shale gas of China.