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.

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

“…If the difference between mud pressure and pore pressure exceeds the threshold pressure, the synthetic-based fluids might invade the microfractures, which may reduce the internal friction angle and destabilize the wellbore (Hale et al 1992;Labenski et al 2003;Chen et al 2002;Al-Bazali et al 2005). Therefore, in stressed-shale formations, the mudweight increase would have negative consequences on wellbore stability (Santarelli et al 1992). Meanwhile, very high osmotic pressure was generated between the drilling fluid and the formation.…”

“…However, some field cases in the GOM, Colombia, and Canada have demonstrated that wellbore-instability problems still occurred even when oil-or synthetic-based fluids were used, especially in fractured formations (Santarelli et al 1992;Rojas et al 2006;Gallant et al 2007;Last et al 1998).…”

“…Once the wellbore instability is initiated, it would be difficult to stop (Kelly 1968a;Labenski et al 2003). Santarelli et al (1992) and Labenski et al (2003) discussed how to improve wellbore stability in fractured formations by keeping mechanical balance. In this paper, we will focus on maintaining chemical balance between shale formations and nonaqueous drilling fluids to minimize wellboreinstability problems.…”

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.…”

Analysis of the Formation Mechanism of Crack Network in Shale by CT

“…Obviously, the influence of natural or artificial induced fracture around the wellbore on wellbore stability cannot be ignored. Santarelli [1] (1992)，Maury [2] (1994), Last [3] (1995), Mclellan [4] (1996), Zhang [5] (1999), Labenski [6] (2003), Franck [7] (2003), Chen [8,9,10,11] etc. derived the same conclusion from experiment and analysis of discrete element, Due to the existence of fissure, the seepage of drilling fluid or its filtrate along the microfracture and bedding surface will lubricate the fissure and reduce the angle of internal friction, the gradually reduced strength of formation is the fundamental reason for wellbore instability.…”