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Typical ultrasonic laboratory measurements of rock physical properties are conducted with ultrasonic transducers that are relatively large compared to the rock sample. We have determined the impact of the transducer size on the resulting velocity estimations. To improve data interpretation, we explore two complementary avenues: (1) explicitly account for the finite size of the transducers as part of the data interpretation/correction workflow, rather than assuming point sources, and (2) reducing the effective size of the transducers at the hardware design level. Both approaches as well as their combination have been tested successfully on multiple data sets including artificial homogeneous and isotropic samples as well as natural anisotropic rocks such as shales.

In-situ nuclear magnetic resonance (NMR) core flooding system has enabled researchers to monitor several rock properties such as porosity, pore size distribution, and fluid saturation along the tested samples with high resolutions and under reservoir conditions. However, spatially resolved rock strength/mechanical property alteration coupled to fluid migration/substitution remains poorly characterized. To this end, Fiber Bragg Grating (FBG) multiplex sensors were integrated with NMR core flooding system to monitor rock strength changes, or generally speaking, to observe hydro-mechanical-chemical coupling mechanisms during core flooding tests. In this study, we present a novel approach on how to conduct core flooding experiments, while simultaneously monitoring NMR and FBG strain response of the tested limestone plug. The NMR cell was modified to integrate FBG technology without impeding the NMR signal and core flooding high pressure/temperature capacity. A high spatial resolution optical fiber was attached onto the sample radial surface. The results show the successful association of NMR and FBG sensors to track any change at each stage of brine injection. The FBG is capable of measuring the rock strain variations induced by rock-fluid interactions during brine injection, allowing it to capture the fluid front location along with the sample and at a faster rate than the NMR.

Overburden shales that overlie and seal hydrocarbon reservoirs usually exhibit polar anisotropy, also called Vertical Transverse Isotropy (VTI). This anisotropy is important for correct seismic inversion, seismic-to-well ties as well as having geomechanical implications. P-wave anisotropy cannot usually be determined from a vertical well unless a walkaway vertical seismic profile (VSP) has been obtained, however, such measurements are still rare. S-wave anisotropy though can be estimated from logs if the speed of sound in mud and the Stoneley wave velocity in the shale are known. Then, the P-wave anisotropy can be computed using theoretical models or empirical trends. The Stoneley wave velocity is nowadays routinely measured by sonic tools and, if a reliable mud velocity is known, the horizontal shear wave velocity (parallel to and polarised in the bedding plane) can be estimated. Thomsen’s gamma parameter for S-wave anisotropy can then be calculated. If mud velocity is not known, the horizontal shear wave velocity can be obtained using calibration in an isotropic interval. Using this method, we analyse the VTI anisotropy in the Torosa-6 well in the Caswell Sub-basin of the Browse Basin, Australia. Torosa-6 drilled through the Jamieson and Echuca Shoals shaly formations where Vclay reaches ~75%. Elastic anisotropy of the shaly Jamieson and Echuca Shoals Formations has been analysed. Thomsen’s gamma shows a good correlation with the clay fraction in each of these formations. However for the same clay fraction, anisotropy is about 20% higher in the Jamieson Formation compared to the Echuca Shoals. This Jamieson Formation contains up to 15% of smectite, and we are investigating how this may lead to higher levels of VTI anisotropy compared with illitic clays predominant in the Echuca Shoals Formation.

A new platen was designed and developed for a nuclear magnetic resonance (NMR) core flooding cell for the integratation of fibre Bragg grating (FBG) strain sensors. The platen is made from carbon fibre material with a special channel to insert the FBG inside the cell, and it is compatible with an NMR overburden system (i.e. no metal, and also resistant to pressure, temperature and reactive chemicals) to allow, for example, advanced chemical experiments. This development has allowed us to acquire both fluid dynamics behaviour, from NMR, and deformation/elastic properties, from FBG sensors, during NMR core flooding experiments for advanced hydro-mechanical-chemical coupling characterisation. Generally, pore fluid redistribution occurs while changing the surrounding stress and/or temperature conditions of the tested material, and will generate elastic and geomechanical responses. The impact of rock–fluid interactions during NMR core flooding has not been evaluated until now, mainly due to technical limitations. Fortunately, the FBG sensing technique is a localised/discreet micro-strain gauge (8 mm long and 125 micro thick), and it is applicable to the monitoring of pseudo-tomography during core flooding experiments. We recently managed to attach eight FBG sensors on a carbonate rock plug to monitor strain alteration during brine imbibition under 800 psi confining pressure. The results validate the ability of the FBG sensors to track mechanical strength alterations due to pore pressure changes and also to detect waterfront velocity during injection. The integration of FBG sensors into the NMR core flooding system has added a new capability to the NMR overburden system, enabling it to simultaneously monitor material deformation with FBG sensors during pressure/temperature alterations as a first basic application.

There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yield phase or group velocities. The main problem here is that the size of the transducers used in the laboratory for ultrasonic measurements is comparable with the size of the rock sample or propagation distance. As a result, it is not clear how to interpret the measured traveltimes of ultrasonic waves. The main question is how to define an appropriate effective travel path, its length, and orientation. To answer this question, we look in detail at the full wavefront generated by a finite-size transducer using a specifically designed experimental setup, and a synthetic material (phenolic grade CE). We devise an algorithm for the interpretation of traveltimes measured in the laboratory with finite-size transducers arbitrarily oriented with respect to the symmetry planes of the material. To illustrate/validate the proposed algorithm, we designed a special testing program. The results of the experiments are presented and turn out to be in very good agreement with the analytical predictions on which we base our interpretation algorithm.

We have developed a new numerical algorithm for inversion of ultrasonic data in transversely isotropic media. This algorithm is able to determine from the measured P-wave velocities the orientation of the symmetry axis of a rock sample and the Thomsen’s parameters, only assuming transverse isotropy. The inversion of ultrasonic data acquired on natural and potentially heterogeneous shale samples produced reasonable results. In addition, the algorithm was successfully tested on ultrasonic data acquired on synthetic samples with predefined orientations of the symmetry axis. An additional outcome of the algorithm is a simple approximation of Thomsen’s formulation, which can be effectively used for interpretation of seismic data in transversely isotropic media.

Oscillatory disc cutting (ODC) technology has been analyzed. It has been shown that previously observed reduction of the cutting force can be fully attributed to the disc kinematics. The paper presents a simple analytical model, which is in very good agreement with available experimental data. Also it has been demonstrated that although the ODC technology is able to reduce reaction forces in excavation machines, it does not reduce the power required to destroy rock. Finally, the paper provides a very simple analytical expression for estimation of reaction forces, which is very useful for design of an ODC excavator.

In search for the root cause of stick–slip, a mode of torsional vibrations of a drilling assembly, a linear stability analysis of coupled axial–torsional vibrations has been carried out. It has been shown that in a rotary drilling system with axial and torsional degree of freedom two distinct modes of self-excited vibrations are present: axial and torsional. These axial (torsional) modes of vibrations are due to resonance between the cutting forces acting at the bit and the axial (torsional) natural modes of drillstring vibrations. It has been demonstrated that although axial and torsional modes of vibrations do affect each other the underlying mechanisms driving these modes of vibrations are completely different. In particular, the only driving mechanism of the axial vibrations is the regenerative effect, while there are two distinct mechanisms that drive the torsional vibrations: (i) the cutting action of the bit, and (ii) the wearflat/rock interaction. Moreover, in the case of the torsional vibrations the regenerative effect plays only a secondary role. The results of the present study indicate that the axial compliance can play a stabilizing role. In particular, the stabilizing role of the axial compliance increases as the ratio of the torsional to the axial natural frequency of the drillstring vibrations decreases.

A simple analytical model of bit whirl is presented. In contrast to the previous works, which consider bottom hole assembly mass imbalance to model bit whirl, the model here takes into account the history-dependent boundary conditions at the bit–rock interface as well as the bit geometry. In particular, an analytical expression for the bit–rock interaction is derived. It is shown that the bit geometry affects the bit–rock interaction only through three dimensionless parameters. Depending on the value of these parameters the system can be stable or undergo forward or backward whirl.

This article reexamines the classical PKN model of hydraulic fracture Perkins and Kern (J. Pet. Tech. Trans. AIME, 222:937–949 (1961)) and Nordgren (J. Pet. Tech. 253:306–314 (1972)) using novel approaches, which have recently been developed to tackle this class of problems that are characterized by a moving boundary and strong non-linearities in the governing equations. First, we demonstrate, using scaling arguments only, that a PKN hydraulic fracture has two limiting time asymptotic behaviors: storage-dominated at small time, and leak-off-dominated at large time. Next, we investigate the multiscale nature of the tip asymptotics and its implication for the construction of a robust and efficient numerical algorithm. In particular, we show that in the storage-dominated regime the tip aperture w behaves according to w ~ x 1/3 (where x is the distance from the tip), and in the leak-off-dominated regime according to w ~ x 3/8. However, the solution in the leak-off-dominated regime has a boundary layer structure at the tip, with w ~ x 3/8 acting as an intermediate asymptote that matches an inner solution with a w ~ x 1/3 asymptote to the outer (global) solution. Finally, we describe an efficient numerical algorithm with the multiscale tip asymptotics embedded in the tip element, which is used to compute the transient solution that connects the small- and large-time asymptotes.

Hydraulic fracturing is commonplace in the geo-industry, whether designed or unintended; e.g., stimulation of hydrocarbons reservoirs [1, 2], disposal of waste water [3], waterflooding operations [4], enhanced oil recovery by injection of CO2 [5], and preconditioning of rock mass in the mining industry [6, 7]. Nonetheless, modeling of hydraulic fracturing usually relies on oversimplified assumptions [1, 8]; in particular, fluid leak-off is often studied within Carter’s model [9] that assumes that the transport of the filtrate and the porous fluid through the porous medium is one-dimensional. While this assumption is quite reasonable in the case of short treatments such as hydraulic fracturing of a hydrocarbons reservoir [2], it is unlikely to be applicable for injection operations over long periods of time.

A simple analytical model of bit whirl is presented. In contrast to the previous works, which consider BHA mass imbalance to model bit whirl, the model here takes into account the history-dependent boundary conditions at the bit-rock interface. Within this model, the geometry of the bit is characterized by three dimensionless parameters. The results have shown that, depending on the value of these parameters, the system can be stable or undergo forward or backward whirl. As a result, the model has potential application as a tool that assists bit design.