Keeping Shale Formation Stability by Optimizing Drilling Fluids, in Yangta Oil Field, Western China
Authors: Liu Xiangjun , Luo Pinya , Liu Hong , Liang DaChuan , Luo Faqian 
Published: May 2016

Abstract
In this paper, the effects of drilling fluids on shale strength have been tackled, and a case study has been presented to study the instability mechanism and the stability methodology in the shale formation. A program has been undertaken to develop a better understanding of the effects of the using polymer drilling fluid on the shale mechanical properties and how to modify its performance.
Over 20 different drilling fluids were examined for their effects on shale strength. Emphasis here was on the mechanical properties of shale samples after exposed to different drilling fluids for 24 hours. All shale samples selected from similar depth and all tri-axial rock mechanic tests were performed under the conditions of 45MPa confining pressure. The mud which maintains high compressive strength of the shale is the optimum selection. The all shale samples exposed to using polymer drilling fluid exhibit the lowest strength, from the stress-strain curve, almost lost all strength. The use of KCl brine in drilling fluids is based on its shale-inhibition characteristics as it is thought that the k+ ion provides additional attraction among the clay platelets due to its size and charge. However, in Quka shale, the shale samples also lose most part of strength when exposed to the modified using polymer drilling muds only adding 5% KCl and 10% KCl respectively. The one of main possible reasons is Kaolinite in the shale. The shale samples will keep more original strength when exposed to the modified using polymer drilling muds with chemical agents with plugging ability to pore throat and micro-fracture. The all shale samples exposed to using polymer drilling fluid with 10%KCl and 3%A and B% exhibit the highest strength, then it was used to be a new system in drilling 2 new wells. The results show that the drilling fluid density has been decreased largely, and the wellbore stability has been improved remarkably while shale formation is stable.

Introduction
Wellbore instability is a serious drilling problem. It is reported that shale account for 75% of all formations1 drilled by the oil and gas industry, and 90% of wellbore stability problems1,2 occur in shale formations. The main factors that influence stability of shale formation are related to mechanical properties and physicochemical factors. Shale hydration is the main factor that causes instability problem3. All drilling fluids were found to cause instability problems due to water phase. Gas drilling fluid, oil based drilling fluid or all-oil drilling fluid can minimize the instability problem which is caused by shale hydration. However, we must be sure that there won't meet water zone in gas drilling process, and gas drilling isn't suitable to brittle shale formation, as well as the gas drilling equipment is expensive, so massive compatible analysis should be made in the drilling process. It is no doubt that oil based drilling fluid and all-oil drilling fluid will minimize the instability in shale formation. But oil based drilling fluid can't replace water based drilling fluid due to the environmental restrictions and expensive disposal costs4–6.