A rock mechanical model developed for a Coal Seam Well

Drilling operation in order to produce from Coalbed methane (CBM) is prone to various geomechanics related problems not only within the coal seam but also across the overburden layers. Wellbore instability in the form of shear failure (breakout) and washout in one hand and mud loss and fracturing in other hand are examples of failures which a wellbore may experience if a proper mud weight is not used for drilling. In order to conduct such an analysis the input data required includes mechanical properties of formations as well as the magnitude and direction of in-situ stresses and pore pressure. It is well known that mechanical properties of formations are related to their physical characteristics. For example, the formation Young’s Modulus or strength is expected to be higher in formations with larger sonic velocities or lesser porosities. Petrophysical logs reflect various rock physical properties from which continuous curves of rock mechanical properties could be estimated using several correlations developed in similar fields. Similarly, continuous logs of in-situ stresses (i.e. vertical as well as minimum and maximum horizontal stresses) could be estimated, for example from poroelastic formulae, in conjunction with rock physical properties. The estimated logs could be calibrated against lab tests on cores and field test data. For example, performing triaxial tests in the lab on cores obtained at different depths, the elastic and strength properties such as Young’s Modulus, Poisson’s ratio and uniaxial compressive strength (UCS) could be measured and this is used to correct the corresponding estimated logs. Similarly, the minimum horizontal stress log could be calibrated against any existing leak-off-test data whereas pore pressure curve can be calibrated if any MDT data is available.The direction of horizontal stress can be estimated from the image logs, for example FMI. The combination of continuous curves of formation mechanical properties and magnitude of in-situ stresses together with stress directions is referred to as rock mechanical model (RMM). The RMM is constructed for a drilled well and then it is used for prediction of events in a new planned well in a nearby area. The RMM includes the input data for any geomechanics study such as wellbore instability analysis, fracturing design or sanding prediction. In this study the RMM was constructed for data corresponding to Well Ridgwood 2 drilled in Surat basin in Queensland, Australia. The results indicate how the mechanical properties are changing across the coal seam comparing to other intervals and that the stress magnitudes experience significant changes accordingly. The results are used to predict the fraccability of the CBM for stimulation purposes using a hydraulic fracturing operation. Other applications of the constructed RMM will be discussed and the results interpreted

Gravitational Collapse of a Homogeneous Scalar Field in Deformed Phase Space

We study the gravitational collapse of a homogeneous scalar field, minimally coupled to gravity, in the presence of a particular type of dynamical deformation between the canonical momenta of the scale factor and of the scalar field. In the absence of such a deformation, a class of solutions can be found in the literature [R. Goswami and P. S. Joshi, arXiv:gr-qc/0410144], %\cite{JG04}, whereby a curvature singularity occurs at the collapse end state, which can be either hidden behind a horizon or be visible to external observers. However, when the phase-space is deformed, as implemented herein this paper, we find that the singularity may be either removed or instead, attained faster. More precisely, for negative values of the deformation parameter, we identify the emergence of a negative pressure term, which slows down the collapse so that the singularity is replaced with a bounce. In this respect, the formation of a dynamical horizon can be avoided depending on the suitable choice of the boundary surface of the star. Whereas for positive values, the pressure that originates from the deformation effects assists the collapse toward the singularity formation. In this case, since the collapse speed is unbounded, the condition on the horizon formation is always satisfied and furthermore the dynamical horizon develops earlier than when the phase-space deformations are absent. These results are obtained by means of a thoroughly numerical discussion.Comment: 17 pages, 17 figure

True Triaxial Strength Testing of Sandstones

Laboratory rock mechanical tests allow estimation of rock strength and deformation behaviour under stress states similar to the in-situ conditions. In general, the in-situ stresses are described by three principal stresses, the vertical, maximum and minimum horizontal stresses. However, most of rock mechanical properties are obtained using only two different stresses, as in conventional triaxial tests where an axial load and an isotropic confining pressure are applied on a cylindrical rock sample. Also the most commonly used failure criterion, the Mohr-Coulomb criterion, is usually applied using only the maximum and minimum applied stresses and thus ignores the effect of the intermediate stress. Experimental and theoretical studies of rocks under true triaxial stress conditions have proved that describing their mechanical properties while ignoring the effect of σ, cannot reflect the rock behaviour under true stress states. In this paper the lab results of an on-going study on deformation behaviour of synthetic sandstones in a true triaxial cell are presented. The effect of both σ and σ has been examined by conducting compressional tests in different stress levels and σ /σ ratios. The results show the impact of changing stress magnitudes and anisotropy on rock strength and deformation behaviour

An experimental study of permeability determination in the lab

Understanding the flow characteristics in laminar and turbulent flow regime is important for different aspects of reservoir and production engineering. One of the most important parameters in fluid flow is the permeability of the porous media. It is common practice in the industry to use Darcy and Forchheimers equations for characterising the fluid flow in the porous media at laminar and turbulent regimes, respectively. Core flooding experiments were performed with 1.5-inch diameter size core samples from limestone and sandstone formations. The permeability of the samples was measured in the laminar regime at basis flow rate. Then, the flow rate was increased in different steps and permeability was measured, accordingly. The plot of permeability versus flow rate was used to track the evolution of the flow regimes in the core porous media. There are different challenges to monitor the transition between laminar and turbulent regime through core flooding experiments. These challenges are discussed in this paper through both literature review and also experimental results. The results indicated that the core sample preparation, experiment control parameters and also test profiles are important aspects when measuring permeability in the lab. © 2012 WIT Press

Brittleness of gas shale reservoirs: A case study from the north Perth basin, Australia

Shale reservoirs have gained the attention of many in recent years due to their potential as a major gas resource. Production from this kind of formation, however, requires an accurate estimation of brittleness and employments of hydraulic fracturing. There have been many studies as to how brittleness can be estimated, but few research works were carried out so far indicating how brittleness indices vary in gas shale formations. The aim of this paper is to evaluate the variation of brittleness in one of the gas shale reservoirs located in the north Perth Basin of Australia. The results obtained indicated that the lower part of the Carynginia shale should be selected for a hydraulic fracturing job due to a high brittleness index, although a careful analysis of Total Organic Content (TOC) might be required before initiating any plans. The mineralogical report and interpretations revealed that the space created by cross-plotting the elastic parameters is able to identify dominant minerals contributing into brittleness. Performing a series of true triaxial tests, which are capable of simulating the real field condition by applying three independent principal stresses, implied that as the stress anisotropy increases, a transition takes place from brittle towards the ductile behaviours. However, when this anisotropy becomes significant, samples regain their strength. This study, therefore, recommends more studies to get a practical conclusion on brittleness under true triaxial conditions

A methodology for wellbore stability analysis of drilling into presalt formations: A case study from southern Iran

Drilling into presalt formations has been a long-standing issue due to the rapid changes in the diameter of the borehole during drilling operations either because of creep or wash-out dissolution. There have been many studies on characterization of salts, with many mathematical models being presented to estimate the pressure induced due to the squeezing salt sheets. However, the results of none of these models have been fully validated against real field data and some recommendations have been made based on numerical simulations. In this study, attempts were made to introduce a methodology based on damage mechanics for wellbore stability analysis of a wells drilled in the southern part of Iran. The results obtained indicated that the presence of a thick salt layer in the well has resulted in significant wellbore closure in the intervals above the reservoir section. It was also found that the salt exhibits viscoelastic behaviour during drilling due to the homogeneous temperature which has not reached the threshold limit of viscoplastic boundary. A complicated change in the stress regime was also observed which could be linked to the existence of the thick salt layer or presences of a fault crossing the well. Therefore, it is recommended to further validate this model in other wells using the methodology presented

The impact of poor cementing casing damage: A numerical simulation study

A good knowledge of the parameters causing casing damage is critically important due to vital role of casing during the life of a well. Cement sheath, which fills in the gap between the casing and wellbore wall, has a profound effect on the resistance of the casing against applied loads. Most of the empirical equations proposed to estimate the collapse resistance of casing ignore the effects of the cement sheath on collapse resistance and rather assume uniform loading on the casing. This paper aims to use numerical modeling to show how a bad cementing job may lead to casing damage. Two separate cases were simulated where the differences between good and bad cementation on casing resistance were studied. In both cases, the same values of stresses were applied at the outer boundary of the models. The results revealed that a good cementing job can provide a perfect sheath against the tangential stress induced by far-field stresses and reduce the chance of casing to be damaged

Interlaminar modelling to predict composite coiled tube failure

In a field operation that uses coiled tubing in its applications, fibre-reinforced polymer matrix composite tubing is seldom used. Fibre-composite coiled tubes offer advantages, compared to steel material, through a reduction in weight and improvement in fatigue life. The stiffness of composite material degrades progressively when increasing the number of cyclic loading. The fatigue damage and failure criteria of fibre-reinforced composite coiled tubes are more complex than that of steel; hence, failure predictions are somewhat unreliable. Among the defects in composite materials, interlaminar delamination is the foremost problem in fibre-reinforced composite material, and it leads to a reduction in strength and stiffness especially in cyclic-load conditions. Delamination causes a redistribution of the load path along the composite structure, which is unpredictable; therefore, delamination in a composite coiled tube in an oil and gas field eventually leads to final failure, which could be catastrophic. A-ply-by-ply mathematical modelling and numerical simulation method was developed to predict interlaminar delamination of filament-wound composite coiled tubes under a combination of different loading scenarios with consideration to low-cycle fatigue. The objective of this paper is to explain interlaminar delamination as an initial crack and source of stress concentration in composite coiled tubes in the framework of meso-cracking progression of matrix damage modelling of composite laminates. The paper focuses on delamination failure because the largest span of the composite lifecycle is at the crack propagation phase, which manifests itself in the form of delamination. The analysis shows that the crack front tip is not uniform, and also shows that carbon fibre possesses higher stiffness values compared to glass fibre. The paper confirms that 2D modelling cannot express the real release strain energy rate at the crack front tip. Mode-I testing, however, showed that the double cantilever beam (DCB) only represents the normal stress from the release strain energy rate. The results also indicated that there were other sources contributing to the strain energy release rate, such as inter-layer frictions and normal stress in the end notched flexure (ENF) testing mode

Structural growth in iron oxide clusters: Rings, towers, and hollow drums

It is shown that the transition from an elementary FeO molecule to the bulk rock-salt FeO proceeds via hollow rings, towers, and drums. Our first-principles electronic structure calculations carried out within a gradient-corrected density functional framework show that small FenOn (n=2,3,4,5) clusters form single, highly stable rings. Starting at Fe6O6, these elementary rings begin to assemble into nano columnar structures to form stable Fe6O6, Fe7O7, Fe8O8, Fe9O9, Fe10O10, and Fe12O12 towers. The rings and the empty towers can be further stabilized by capping O atoms at the ends, leading to FenOn+1 and FenOn+2 sequences. The theoretical results provide insight into the progression of mass intensities in the experimental mass spectra and account for the observed peaks in the negative ion photodetachment spectra of iron oxide clusters