Brian Ronald Crawford

A constitutive law for low‐temperature creep of water‐saturated sandstones

An accurate predictive model for the long-term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems. While laboratory experiments are consistent with Mohr-Coulomb brittle faulting in the cold, upper continental crust, there is increasing evidence that time-dependent processes may also be important in these rocks, even at low temperature. However, there is some ambiguity as to the exact form of the constitutive law for describing time-dependent behavior of upper crustal rocks. Here we present results of room temperature creep experiments on a suite of water-saturated sandstones spanning a range of petrophysical and rheological properties and underlying deformation mechanisms. On physical and microstructural grounds our analysis suggests that a modified power law creep, of the form ˙ A(d f) , where d is the differential stress and f is the long-term failure (fundamental) strength, provides a more complete description of the experimental data. In particular, the parameters can be used to differentiate between sandstone types, with A, f, and varying systematically with cementation state, rock rheology, and confining pressure. The fundamental strength (f) for time-dependent deformation varies much more than the other parameters of the distribution, making it a potentially sensitive indicator of underlying creep mechanisms. Further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three-parameter model is statistically better in the general case.

Predicting Rock Mechanical Properties from Wireline Porosities

The rock mechanical behaviour of reservoir rocks is important in the design and implementation of drilling and production programmes. Traditionally rock mechanical properties are obtained from direct measurement on core samples or from mechanical calculations on acoustic wireline log measurements. This paper reports the rock mechanical properties of many different reservoir rocks of different porosities. This has led to the development of a new method of predicting rock mechanical properties directly from porosity. The paper discusses the measurement of experimentally derived porosity, elastic moduli and fracture strength parameters and the interpretation of these mechanical properties results into direct correlations with porosity. The application of these results to obtain continuous rock mechanical property plots of the reservoir from wireline derived porosity is discussed. The practical use of these rock mechanical property profiles in drilling, production and enhanced reservoir simulation is also emphasised. Porosity (Φ), modulus of elasticity (E), Poissons Ratio (ν), uniaxial compressive strength (UCS), cohesion (τ 0 ), angle of internal friction (ψ), and triaxial stress factor (k), were measured on samples from a wide range of North Sea reservoirs using a conventional triaxial testing machine. This paper describes the procedure used and presents the correlations obtained from plotting each of the rock mechanical properties against porosity. The derivation of wireline porosities along with empirical corrections are presented and the results of applying the correlations to these wireline derived porosities to produce continuous rock mechanical property plots are discussed. Logs were calibrated to core-measured values to reveal realistic elastic and inelastic moduli profiles. The continuous property logs provide a reasonable estimate of the possible behaviour at discrete points throughout the reservoir interval, but they are limited in their description of the behaviour of individual beds as coherent bodies. A technique has been developed to pick out these individual beds and assess how they will perform as Rock Mechanical Coherent Units, i.e. sets of beds that perform in a similar or dissimilar manner to adjacent layers. Finally a discussion on how the results are used to aid production and generate enhanced reservoir simulation will be presented.

Strength characteristics and shear acoustic anisotropy of rock core subjected to true triaxial compression

Abstract Results are presented from an initial experimental programme aimed towards evaluating the capabilities of a new true triaxial cell, designed to apply independent and unequal principal stresses to the curved surfaces of cylindrical core plugs. A series of discrete failure tests on dry specimens from two sandstone lithologies exhibiting different deformation, strength and poroperm characteristics, were conducted under azimuthal stress anisotropy ( σ 2 > σ 3 ) with σ 1 being applied axially. The true triaxial cell consistently orientates induced brittle shear fractures so that they strike parallel to the direction of σ 2 , and slip against the direction of least confinement, σ 3 . Both peak (fracture) and residual (friction) strengths are shown to be strongly dependent on the magnitude of the applied σ 2 , as well as on that of σ 3 . Results from multi-failure state testing using the conventional “triaxial” compression configuration are contrasted with discrete failure tests conducted in the true triaxial cell, by means of the familiar von Mises and extended 3-D Griffith criteria. Digitised records of shear-waves obtained at 40, 60 and 80% of peak failure strength during true triaxial testing, show clear evidence of progressively increasing stress-induced “splitting” or birefringence between the arrival of the faster S1(∥ σ 2 ) and the slower S2(∥ σ 3 ) shear-wave. Microseismic data and macroscopic observations from discrete failure tests performed within the true triaxial cell, are thus supportive of a brittle deformation mechanism involving stress-induced dilatant microcracks extending parallel to σ 2 and opening against σ 3 , progressively coalescing with increasing σ 1 to form a pervasive fault also oriented by the applied 3-D stress field.

A rock test cell with true triaxial capability

Conventional so-called triaxial test cells apply the axial stress to a cylindrical sample using steel platens, with the confining stress developed via an annulus of hydraulic fluid retained by a liner in a pressure cell. This does not allow differentiation between the two principal stresses around the core and inhibits the realism with which the rocks can be tested, for example in determining the effect of the intermediate principal stress on the strength of the sample.This paper describes the development and application of a new test cell – believed to be the first in the world – which enables truly triaxial stresses to be applied to cylindrical core samples, opening up the possibility to test rocks routinely in a more realistic manner. An array of 24 trapped tubes replace the single annulus which usually generates the uniform radial stress. Selective pressurization of the tubes enables differential radial stresses to be generated, while axial stresses are applied as before through steel platens. The first results of multi-state failure and permeability stress sensitivity of samples tested in the cell are presented. These demonstrate the influence of the intermediate principal stress on measured rock properties and the orientation of induced fracture planes.