Drilling Through Highly Fractured Formations: A Problem, a Model, and a Cure

Abstract

It is usually accepted that wellbore instabilities are caused by either or both an excessive stress concentration at the borehole wall and the chemical reactivity of the formation. Assuming good hole cleaning, common cures for such instabilities are therefore mud density increase and/or change of the mud system. However, even though these methods have proved highly successfull when drilling through intact formations, the same may not stand when it comes to highly fractured rocks. The purpose of this paper is to describe the studies performed in order to allow the safe drilling of a particularly troublesome, highly fractured formation which had led previously to several successive side tracks.The efficiency of the different attempts at solving the problem are analysed in the light of the various side tracks. The core taken during one of these side tracks was analysed, showed that the formation was highly fractured and chemically inert, and provided many parameters for the modelling of the problem which was performed by a discrete element programme. Further modelling was performed on the mud hydraulics. Both theoretical and field data show that density increase has, in such a case, a negative role on borehole stability while filtrate reduction and mud rheology enhancement are highly positive.

Drilling in South America : A Wellbore Stability Approach for Complex Geologic Conditions

“…Where this occurs, the fractured blocks are no longer subject to the mud overbalance pressure, and the destabilised (or 'floating') blocks can cave into the wellbore as a result of swabbing the formation when tripping. Other authors have proposed a similar mechanism to explain instabilities in fractured shales 12 .…”

Stressed Shale Drilling Strategy--Water Activity Design Improves Drilling Performance

“…Even at the increased formation depths, the water activity of the formation is higher than the water activity of the drilling fluids by a factor of 0.12 a w . Therefore, in stressed shale formations, the mud weight increase would have negative consequence on wellbore stability [6] . By using Equation (13), the osmotic pressure can be calculated.…”

Shale Stability: Drilling Fluid Interaction and Shale Strength

“…Several papers have been written on this phenomenon. 9 In Norway Valhall field, this phenomenon is suspected to be one of the major causes of shale instability. Preventive measures include use of effective sealing agents for fractures, e.g.…”