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The impact of clay fraction on strength and stress ratio (K (sub 0) ) in Gulf of Mexico mudrock and quartz silt mixtures; implications for borehole stability and fracture gradient

Mark Zablocki, John T. Germaine, Richard Plumb and Peter B. Flemings
The impact of clay fraction on strength and stress ratio (K (sub 0) ) in Gulf of Mexico mudrock and quartz silt mixtures; implications for borehole stability and fracture gradient
Petroleum Geoscience (February 2022) 28 (2)

Abstract

Mudrock strength parameters are required to improve prediction of fracture gradient during drilling in poorly consolidated formations. Understanding the influence that mineral composition and consolidation stress have on the mechanical properties of mudrocks, aids in safe well design. Small changes in mudrock clay content are shown to have significant effects on two important engineering parameters, the lateral stress ratio at rest, K (sub 0) , and critical state effective friction angle, Phi ' (sub cs) . A laboratory testing program using quartz silt and a Gulf of Mexico clay mixtures, investigates the effects of mudrock clay content on K (sub 0) and Phi ' (sub cs) at two stress levels, 1 MPa and 10 MPa. Values of K (sub 0) and Phi ' (sub cs) were obtained by subjecting composition-controlled clay-silt specimens to a drained uniaxial vertical strain (K (sub 0) consolidation) phase followed by an undrained compression triaxial shear phase. Triaxial tests illustrate consistent and systematic increases of K (sub 0) and decreases of Phi ' (sub cs) with increases in clay content at both stress levels. Stress dependence is observed through increases of K (sub 0) and decreases of the Phi ' (sub cs) with increases in stress level. Stress dependent behavior is shown to be more pronounced in mudrocks of high clay contents. A predictive model is presented for K (sub 0) in normally consolidated mudrocks, as a function of clay content and effective stress. The model is used to calculate the fracture gradient (least horizontal stress) profile at the International Ocean Drilling Program (IODP) expedition 308 site 1324. The estimated fracture gradient illustrates the effects of clay content and consolidation stress on safe mud weight design. Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure


ISSN: 1354-0793
Serial Title: Petroleum Geoscience
Serial Volume: 28
Serial Issue: 2
Title: The impact of clay fraction on strength and stress ratio (K (sub 0) ) in Gulf of Mexico mudrock and quartz silt mixtures; implications for borehole stability and fracture gradient
Affiliation: Tufts University, Advanced Geomaterials Laboratory, Medford, MA, United States
Published: 20220214
Text Language: English
Publisher: Geological Society Publishing House for EAGE (European Association of Geoscientists & Engineers), London, United Kingdom
References: 45
Accession Number: 2022-010039
Categories: Engineering geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Part of a thematic collection entitled Geopressure
Illustration Description: illus.
N28°04'60" - N28°04'60", W89°07'60" - W89°07'60"
Secondary Affiliation: University of Texas at Austin, USA, United States
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 202209
Program Name: IODPIntegrated Ocean Drilling Program
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