Mechanical properties of North Sea Tertiary mudrocks: investigations by triaxial testing of side-wall cores

Abstract

In the North Sea Tertiary section, wellbore instability problems are frequently reported in Palaeocene-Early Oligocene smectite-rich mudrocks. Analysis of the mechanical properties of these Tertiary mudrocks is generally hampered by the lack of suitable core material. This study represents an attempt to study the geomechanical behaviour of mudrocks by triaxial tests of side-wall cores obtained from the borehole wall. The tests performed include measuring the changes in pore pressure during shearing and undrained shear strength in specimens initially consolidated to in situ effective stress levels. The coefficients of permeability (kf), estimated from the consolidation time behaviour range from 2.6 x 10-11 to 2.4 x 10-12 m/s. The tested cores behaved like slightly overconsolidated to normally consolidated materials with an initial near constant volume (elastic behaviour) for low deviatoric load followed by an increasingly contractant behaviour approaching failure. Compared with results from onshore analogues, the strength properties of the investigated mudrocks appear to be related to their content of expandable clay minerals. A wellbore stability chart to forecast adequate drilling fluid pressures for future wells has been developed by the use of linear (Mohr-Coulomb) failure criteria based on the peak strength data. It is demonstrated that side-walt cores can provide satisfactory test materials for rock mechanical analysis, and their use may serve to improve our knowledge of the rock mechanical behaviour of typically troublesome mudrocks for which no conventional cores are available.

Empirical strength prediction for preserved shales

“…Marsden et al (1992) noted some correspondence between smectite content, cation exchange capacity, specific surface area and strength in a study of weak mudrocks under high stress levels but noted no such correlation for porosity or velocity. Steiger and Leung (1988), Nakken et al (1989), Ewy et al (1994), Wensaas et al (1998) and Nygard and Gutierrez (2002) also observed a correlation between smectite content and rock strength. Ingram and Urai (1999) developed the following correlation for shale friction angle (f) from specific surface area (SSA) obtained using the dielectric constant method:…”

“…In addition, friction coefficient is dependent on mean stress, which apparent friction at low effective normal stress much higher than friction coefficients measured at higher stresses (e.g. Marsden et al, 1992;Wensaas et al, 1998;Kågeson-Loe et al, 2004). The tests run here avoided the very low effective stress end specifically due to this phenomenon and virtually all the failure envelopes recorded in these tests were close to linear.…”

Kinematics of Polygonal Fault Systems: Observations from the Northern North Sea

“…Rock mechanics predicts slip when the shear stresses at the fault overcome the residual shear strength. Although residual shear strengths of FIGURE 13 | (A) Mohr circles of effective stresses for burial under earth pressure at rest (K 0 ) relative to Coulomb failure envelope based on current mechanical properties of the lower Hordaland Group (Wensaas et al, 1998); (B) Mohr circles of effective stresses for burial under earth pressure at rest (K 0 ) relative to Coulomb failure envelope based on mechanical properties of "weak" clays (Moore and Lockner, 2007), as expected during shallow burial; (C) Mohr circles of effective stresses for burial for active earth pressure (K a ) relative to Coulomb failure envelope with cohesion and friction angle of the lower Hordaland Group (Wensaas et al, 1998); (D) Mohr circles of effective stresses for burial for active earth pressure (K a ) relative to Coulomb failure envelope based on mechanical properties of "weak" clays (Moore and Lockner, 2007), as expected during shallow burial; (E) Mohr circles of effective stresses for burial for earth pressure (K ′ ) relative to Coulomb failure envelope for low coefficients of residual friction (φ r ≈ 9 • ; Bishop et al, 1971).…”

“…The coefficient of earth pressure at rest for normally consolidated clays that have not been pre-compressed or pre-sheared (Terzaghi, 1996), such as the mudstones of the Hordaland Group (Wensaas et al, 1998), can be calculated from the friction angle using Jaky's (1948) equation:…”

“…Fault nucleation is commonly explained by initial failure of the host strata, which can be predicted from the friction angle (φ) of the host strata and the effective horizontal and vertical stress. A friction angle of 17 • for the lower Hordaland Group is determined by Wensaas et al (1998) using triaxial testing of side-wall cores from exploration well 25/7-2 (Figure 2A). While this friction angle describes the current consolidated state of the host strata, the angle was probably lower during fault nucleation, which likely occurred during shallow burial of these sediments, based on the observation that the faults reached the seabed and controlled the accumulation of growth strata.…”