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

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

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 comparison of very short lived halocarbon (VSLS) and DMS aircraft measurements in the tropical west Pacific from CAST, ATTREX and CONTRAST

We present a comparison of aircraft measurements of halogenated very short lived substances (VSLSs) and dimethyl sulphide (DMS, C₂H₆S) from a co-ordinated campaign in January–February 2014 in the tropical west Pacific. Measurements were made on the NASA Global Hawk, NCAR Gulfstream-V High-performance Instrumented Airborne Platform for Environmental Research (GV HIAPER) and UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 (see Sect. 2.2) using four separate gas chromatography–mass spectrometry (GC-MS) instruments: one operated by the University of Miami (UoM), one from the National Center for Atmospheric Research (NCAR) and two from the University of York (UoY). DMS was measured on the BAe-146 and GV. The instruments were inter-calibrated for halocarbons during the campaign period using two gas standards on separate scales: a National Oceanic and Atmospheric Administration (NOAA) SX-3581 standard representative of clean low-hydrocarbon air, and an Essex canister prepared by UoM, representative of coastal air, which was higher in VSLS and hydrocarbon content. UoY and NCAR use the NOAA scale/standard for VSLS calibration, and UoM uses a scale based on dilutions of primary standards calibrated by GC with FID (flame ionisation detector) and AED (atomic emission detector). Analysis of the NOAA SX-3581 standard resulted in good agreement for CH₂Cl₂, CHCl₃, CHBr₃, CH₂Br₂, CH₂BrCl, CHBrCl₂, CHBr₂Cl, CH₃I, CH₂ICl and CH₂I₂ (average relative standard deviation (RSD)  <  10 %). Agreement was in general slightly poorer for the UoM Essex canister with an RSD of  <  13 %. Analyses of CHBrCl₂ and CHBr₃ in this standard however showed significant variability, most likely due to co-eluting contaminant peaks, and a high concentration of CHBr₃, respectively. These issues highlight the importance of calibration at atmospherically relevant concentrations ( ∼  0.5–5 ppt for VSLSs; see Fig. 5 for individual ranges). The UoY in situ GC-MS measurements on board the BAe-146 compare favourably with ambient data from NCAR and UoM; however the UoY whole-air samples showed a negative bias for some lower-volatility compounds. This systematic bias could be attributed to sample line losses. Considering their large spatial variability, DMS and CH₃I displayed good cross-platform agreement without any sampling bias, likely due to their higher volatility. After a correction was performed based upon the UoY in situ vs. whole-air data, all four instrument datasets show good agreement across a range of VSLSs, with combined mean absolute percentage errors (MAPEs) of the four platforms throughout the vertical profiles ranging between 2.2 (CH₂Br₂) and 15 (CH₃I) % across a large geographic area of the tropical west Pacific. This study shows that the international VSLS calibration scales and instrumental techniques discussed here are in generally good agreement (within ∼  10 % across a range of VSLSs), but that losses in aircraft sampling lines can add a major source of uncertainty. Overall, the measurement uncertainty of bromocarbons during these campaigns is much less than the uncertainty in the quantity of VSLS bromine estimated to reach the stratosphere of between 2 and 8 pptv