A Mechanical Model of Shale Instability Problems Offshore Abu Dhabi

Overcoming wellbore drilling instability in fractured-rock formations always presents one of the major challenges to drilling engineers. Due to different fluid-diffusion rates and compressibility degrees between the rock's matrix and fracture network, there are two distinct pore-pressure fields and subsequently different effective stresses developed in a saturated fractured formation. It is recognized that wellbore instabilities such as collapses are caused by excessive effective stress concentrations at or near the borehole wall, and mud losses are due to fracturing the rock formation. Because both the mud/rock fluidpressure differential and fractures' deformation modify the effective stresses in the wellbore wall, such intricate responses require a wellbore solution that accounts for the fracture network, intact rock matrix mechanics, and the balance of drilling mud density. In this work, the effects on real-time collapse and fracture gradients that dictate the safe mud weight window are evaluated analytically, incorporating the effects of natural fracture network via a simulated field case. Analyses reveal that neglecting the naturally fractured nature of the formation falls short in simulating the wellbore instability since it predicted a narrower mud weight window for the drilling operation.

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Section: Introductionmentioning

confidence: 72%

Real-Time Wellbore-Drilling Instability in Naturally Fractured Rock Formations with Field Applications

Nguyen

Abousleiman

2010

IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition

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“…As such, it is evident that time-delayed failures are typical in these formations as widely reported in the field (Yamamoto et al 2002) and require a time-dependent poroelastic analysis (Cui et al 1997). In chemically active shale, additional osmotic effect is generated from physicochemical interactions among pore fluid components with the invading drilling fluid and the solid matrix that result in the membrane behavior, i.e., only transport of certain pore fluid species is allowed.…”

Section: Introductionmentioning

confidence: 91%

Geomechanical Coupled Poromechanics Solutions While Drilling in Naturally Fractured Shale Formations with Field Case Applications

Nguyen

Abousleiman

Hemphill

2009

SPE Annual Technical Conference and Exhibition

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Numerous wellbore instability problems have been reported when drilling through chemically active naturally fractured shale. It is usually accepted that wellbore collapses are caused by excessive effective stress concentrations at or near the borehole wall, and excessive mud losses are due to fracturing the rock formation. Since both the pore pressure differential and mud/shale chemical activity modify the effective stresses in the wellbore wall, the cures for such intricate instabilities are therefore in a wellbore solution that accounts for fractures, intact matrix mechanics, and the balance of mud density and salinity. In this work, the effects on collapse and fracture gradient window due to mud chemistry and densities are evaluated through the salinity, membrane efficiency, and the existing natural fracture network. The coupled poromechanics analytical solutions of fracture and matrix dual pressures and stresses predict large variations in the collapse and fracture gradient when compared to wellbore stability in non-fractured shale formations. The physical and mathematical solutions follow the coupled poroelasticity Biot theory through modeling fracture and matrix as dual-porosity and dual-permeability porous media. The solution is helpful in the prediction and visualization of time-dependent alteration of near-wellbore effective stress concentration and the effects on the mud-weight window when compared to non-fractured shale. Specifically, the solution is applied to assess wellbore stability for a simulated downhole drilling condition. Analyses that neglect the naturally fractured nature of the shale fall short in simulating wellbore instability since they predicted a narrower mud-weight window for the drilling operation, while the chemical and mud salinity effects can be utilized as a stabilizing factor.

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“…Failure at the Wellbore Wall Yamamoto et al (2002) described the failure of highly-fractured shales in the Arabian Gulf area. Here the Nahr Umr formation is notorious for presenting instability problems to drillers.…”

Section: Description Of Unstable Shalesmentioning

confidence: 99%

Wellbore Pressure Management in Unstable Shales: It's Not Just About Rocks

Hemphill

2012

SPE Annual Technical Conference and Exhibition

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Much of the drilling in unconventional resource plays occurs in unstable shales, which are usually fractured and can be easily destabilized. Drilling through them successfully can be difficult at best, and many high-angled holes in these plays are often lost due to mechanical instability. This paper examines the problems of shale gas drilling from the theoretical perspective of Wellbore Pressure Management (WPM) and keys in on the effects of equivalent circulating density (ECD) while drilling and on the effects of equivalent static density (ESD) when there is no circulation.In this paper the following questions pertaining to drilling a typical fractured shale or highly-laminated weak zone are addressed from the WPM perspective: What mud density do I need to drill a fractured shale?  Why can a typical shale gas play well be drilled with no drilling problems, yet becomes very unstable on the last trip out of the hole before wireline logging or running casing?  Why are drilling problems especially acute in laminated shales or similar weak zones?  Why is the wellbore unstable while the drilling density is within the range demarcated by the Safe Drilling Window?  Why does shale instability often not improve significantly when drilling fluid density levels are increased?  Which tools in the driller's toolbox are often used that actually make the wellbore stability issue more problematic?By using a Wellbore Pressure Management approach to understanding instability in fractured shales, the reader can readily see how to best deal with the problem in the field and hopefully improve stability in future wells.

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A Mechanical Model of Shale Instability Problems Offshore Abu Dhabi

Cited by 21 publications

References 6 publications

Real-Time Wellbore-Drilling Instability in Naturally Fractured Rock Formations with Field Applications

Real-Time Wellbore-Drilling Instability in Naturally Fractured Rock Formations with Field Applications

Geomechanical Coupled Poromechanics Solutions While Drilling in Naturally Fractured Shale Formations with Field Case Applications

Wellbore Pressure Management in Unstable Shales: It's Not Just About Rocks

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