Designing Drilling Mud for Stability in Shale Reservoirs: Summary & Review of ‘Mud Design for Shale Stability: More Than Just Mud Weight’ by Jovie Yunara

     This is a quick summary and review of a LinkedIn post by Jovie Yunara, a drilling operations and engineering professional. It pertains to drilling mud design for shale reservoirs to optimize wellbore stability during drilling. I remember spending a lot of time at well sites during the early part of drilling horizontal wells in shale reservoirs and the problems encountered with wellbore stability, especially with water-based muds. Oil-based mud systems are used in most shale plays and offer better well stability. However, some plays utilize new and improved water-based muds.

     A 2024 paper in Unconventional Resources by Chang Hong Gao notes that oil-based mud (OBM):

“…provides effective shale stability, excellent lubricity, and high rate of penetration (ROP). As a result, more than 70 % of shale gas wells have been drilled with OBM with very few reported cases of wellbore instability. WBM (water-based mud) is made of water and necessary chemical additives. WBM is less costly and more environment-friendly than OBM, however some shale wells drilled with WBM reported severe instability issues. Nevertheless, recent innovations in WBM lead to successes in drilling major shale plays. WBM has great potential in shale drilling and deserves more research and improvements.

     Water-based mud is more environmentally friendly and can be cheaper. I would assume that Yunara’s LinkedIn post is addressing shale drilling via OBM.

     Yunara argues in his post that mud weight alone has been overused as a solution to wellbore stability in shales. While he acknowledges the importance of optimizing mud weight, he also stresses the importance of two other parameters: mudcake integrity and chemical compatibility.  

     An effective mudcake provides a barrier to the invasion of drilling fluids into the formation, which compromises wellbore stability. As mud pressure penetrates into the shale, the possibility of shear failure and borehole instability increase due to near-wellbore pore pressure increases.

     Shale contains reactive clay minerals, which create risks for hydration and swelling of the rock. These can make the shale less cohesive, increasing the risk of failure and wellbore instability.

     Increasing mud weight has been the traditional way to increase wellbore stability. However, if the mudcake has already been compromised, then increasing the mud weight may also increase penetration into the shale. Thus, he concludes that higher mud weight does not always mean improved wellbore stability. Of course, in some formations with high bottomhole pressure, it may not be possible to increase the mud weight without effectively hydraulically fracturing the rock on a small scale, which creates even more severe well stability problems.

“The objective should be to identify a mud-weight window that maintains mechanical stability while minimizing pressure invasion and chemical weakening.”

     The figure below illustrates Yunara’s arguments about the three parameters affecting wellbore stability in shales.

References:

 

Mud Design for Shale Stability: More Than Just Mud Weight. Jovie Yunara. LinkedIn. October 27, 2026. Post | LinkedIn

Drilling fluids for shale fields: Case studies and lessons learnt. Chang Hong Gao. Sci Engine. Unconventional Resources. Volume 4. December 1, 2023. Drilling fluids for shale fields: Case studies and lessons learnt