Qu, Hongyan (China University of Petroleum) | Zhou, Fujian (China University of Petroleum) | Jiang, Zhenxue (China University of Petroleum) | Pan, Zhejun (CSIRO Earth Science and Resource Engineering) | Peng, Yan (University of Western Australia)
Matrix permeability, a key factor controlling gas production in shale gas reservoir, is difficult to measure at laboratory and affected by the geochemistry and geology settings including the Total Organic Carbon (TOC) contents, mineral compositions, pore structure, and deposit environment. In this work, the pressure decay method was modified before applied to measure the matrix permeability of the continental shale with extensive TOC variation from Yanchang formation, Ordos Basin, China. Results indicate that matrix permeability varies significantly with both TOC and formation depth, while TOC is the predominant factor controlling the matrix permeability in Yanchang formation shale. Even at the same depth, porosity and permeability are different due to the variation of TOC resulted from geological anisotropy.
Yanchang formation shale in the Ordos basin is the largest and typical continental shale formation with high gas content in China. However, the shale gas production remains very low even after hydraulic fracturing in recent years, impeding the economical industrial development. Among all the factors controlling the long-term gas productivity, matrix permeability is very critical but seldom studied (Chalmers et al. 2012). Unlike the enormous acknowledgement of the marine shales in the USA (Ghanizadeh et al. 2013, 2014), the micro gas flow mechanism in the continental shale is still poorly understood, and there have been few attempts to characterize the petrophysical properties of Yanchang formation shale.
There are multiple methods for conventional laboratory permeability measurement (Clarkson et al. 2012, Handwerger et al. 2011, Sinha et al. 2012, Suarez-Rivera et al. 2012), based on different physical principles and samples at different scales are utilized. However, only the non-steady-state gas flow method is feasible for the unconventional gas reservoir, due to the low porosity and permeability (Cui et al. 2009, Luffel et al. 1993, Tinni et al. 2012). The permeability measurements with pressure decay profile, pulse-decay and pressure-decay methods are primarily applied for the tight gas shale (Ghanizadeh et al. 2015).
In this work, laboratory study was carried out to investigate the controlling factors of the fluid transport properties in the matrix system of Yanchang formation shale. The pressure-decay method with crushed samples was modified before applied to measure the shale matrix permeability, and the effect of different geological and geochemical factors including TOC and porosity on matrix permeability was analyzed. The experimental data, combined with the Back Scattered Electron Microscopy (BSEM) observation extend our understanding of this continental shale reservoir in China and provide some basic insights of the micro gas flow mechanism behind the low gas production in Yanchang formation shale.
Grombacher, Denys (Stanford University) | Knight, Rosemary (Stanford University) | Parsekian, Andrew (University of Wyoming) | Flinchum, Brady (University of Wyoming) | Munday, Timothy (CSIRO Earth Science and Resource Engineering) | Davis, Aaron (CSIRO Earth Science and Resource Engineering) | Cahill, Kevin (CSIRO Earth Science and Resource Engineering) | Hatch, Michael (University of Adelaide)
Summary Communities in the Anangu Pitjantjatjara Yankunytjatjara (APY) Lands of South Australia live in a remote and extrememly arid environment. To ensure continued access to sustainable groundwater resources, which these communities rely upon, we will conduct a geophysical survey consisting of complementary surface Nuclear Magnetic Resonance (NMR) and Time-Domain Electromagnetic (TEM) measurements to map local aquifers, quantify groundwater resources, and locate optimal sites for potential future wells. By pairing surface NMR and TEM measurements we take advantage of the unique ability of the NMR measurement to give unambiguous water detection, while exploiting the fast TEM measurements to map aquifer geometry over a large region entirely non-invasively. The project, funded through the Geoscientists without Borders Program of the Society of Exploration Geophysicists, aims to use geophysical tools to help address a critical water security problem facing several remote and underprivileged communities. Introduction The remote communities of the Anangu Pitjantjatjara Yankunytjatjara (APY) Lands, located in the northern desert lands of South Australia rely on groundwater resources for access to potable water.
Piane, Claudio Delle (CSIRO Earth Science and Resource Engineering) | Almqvist, Bjarne (Uppsala University Geocentrum) | MacRae, Colin (CSIRO Microbeam Laboratory) | Torpy, Aaron (CSIRO Microbeam Laboratory) | Mory, Arthur J. (Geological Survey of Western Australia) | Dewhurst, David N. (CSIRO Earth Science and Resource Engineering)
Microstructural and textural measurements from two Ordovician shale units (Goldwyer and Bongabinni formations) within the Palaeozoic–Mesozoic Canning Basin indicate that the former unit was affected by mechanical compaction and clay mineral transformation whereas the latter preserves an early fabric due to syn-depositional precipitation of authigenic dolomite and anhydrite. Conventional petrographic analysis coupled with quantitative mineralogy, electron micro probe analyses, X-ray Texture goniometry (XTG) and cathodoluminescence (CL) spectroscopy were used to decipher the post-depositional evolution of marine and supratidal facies in the Goldwyer and Bongabinni formations. Differences in diagenesis are strongly reflected in the orientation of clay minerals as quantified by XTG: in both cases the c-axes of illite diffract strongest normal to the bedding plane but the measurements clearly illustrate that shale in the Goldwyer Formation has a stronger preferred orientation relative to the Bongabinni Formation, with multiple of random distribution (m.r.d.) values of 5.77 and 2.54, respectively. Laboratory measurements conducted at 10 MPa effective stress also indicate distinct rock physics signatures: the Bongabinni Formation shows low anisotropy, whereas the Goldwyer Formation displays a higher degree of elastic anisotropy in terms of both P- and S-waves. The crystallographic preferred orientation of illite, highlighted by the XTG, is likely to contribute to the significant difference in elastic anisotropy observed in the two units. Therefore, the Bongabinni Formation is mechanically stronger and stiffer than the Goldwyer Formation, due to the early dolomite and anhydrite cementation of the former providing a rigid microstructure framework.
Karekal, Shivakumar (CSIRO Earth Science and Resource Engineering) | Subramanian, Srikrishnan Siva (Central Institute of Mining and Fuel Research) | Porathur, John Loui (Central Institute of Mining and Fuel Research)
Highwall mining operation involves driving a series of parallel unsupported, unmanned and unventilated excavations into a coal seam exposed at the open pit Highwall using a remotely operated continuous miner with attached conveying system. These parallel excavations are separated by web pillars of pre-designed width which are critical to the Highwall mining operations. The Highwall slope must remain stable during Highwall mining operation to ensure safety of workers and machinery. In this paper, Highwall slope stability is investigated with respect to different Highwall mining parameters using FLAC3D numerical modeling software. The parameters included in the study are: (i) single seam and multiple seams Highwall mining excavations with different width to height ratios; (ii) different Slope angles; (iii) different excavation heights; and (iv) different cover depths. A narrow strip of rock mass is considered by taking a plane of symmetry. The modeling results reveal that stability of open pit slopes have profound influence on the Highwall mining parameters, and the web pillar design can affect the stability of Highwall slopes. In designing Highwall slopes for an open pit, the design must include Highwall mining excavations, otherwise, near critical failure slopes could become critical and fail with Highwall excavations. In authors’ knowledge, this work is the first attempt at exploring the effect of Highwall mining parameters on overall slope stability.
Josh, Matthew (CSIRO Earth Science and Resource Engineering)
Shales from a borehole (NAB 10-25) located in Schlattingen, Switzerland were investigated. Dielectric analysis was performed on preserved samples using brine coupling and cling-film coupling to distinguish conduction and polarization phenomena. Paste samples made from intact rock fragments were also analyzed using an endloaded transmission line dielectric probe. The samples had identical burial history, but a very diverse mineralogy containing samples with 7 to 42% quartz; 2 to 64% clays; and 2 to 95% carbonates. An equally wide distribution of specific surface area (SSA) and cation exchange capacity (CEC) was observed from approximately SSA = 11.9 m2/g and CEC = 2.9 cmol/kg for the most carbonate-rich samples, up to SSA = 110.1 m2/g and CEC = 18.3 cmol/kg for the most clay-rich samples. Excellent correlations exist between low-frequency (10 MHz) dielectric response, CEC, and SSA using paste samples, because the specific surface area determines the amount of hydratable cations that can participate in surface polarization. Surprisingly, this may also apply to the P-wave velocity, which would normally be attributed to rock texture, but is found to correlate strongly with low-frequency dielectric response determined from paste. The correlation between the water content and the dielectric permittivity of the Schlattingen shales lose correlation very quickly as the frequency is increased and this is quite unusual compared with worldwide shales. It is therefore impossible to infer that the high-frequency dielectric permittivity of shales is simply linked to moisture as is universally accepted, but other complicating factors occur.
Hydraulic fracturing has been an essential technique for enhancing production in oil, gas and geothermal reservoirs for decades. Constructive interactions between induced fractures by hydraulic stimulation and pre-existing ones are critical to the success of stimulation treatment and understanding this complex process remains a challenging task. A coupled hydro-mechanical (HM) process has been implemented based on the existing fracture mechanics modeling code FRACOD (Fracture Propagation Code) where the Displacement Discontinuity Method (DDM) is used to simulate rock fracturing processes. In the paper, we presented theoretical foundation of the code with the focus on the implementation of HM coupling. Two verification cases were provided in which the FRACOD results show a very good agreement with the analytical ones from Kirsch solution and the hydraulic fracturing process in intact rock has been successfully modelled. Finally, two demonstration examples were conducted to simulate the hydraulic fracturing in fractured rock mass to account for the interaction between induced and natural fractures.
Bunger, A.P. (University of Pittsburgh) | Kear, J. (CSIRO Earth Science and Resource Engineering) | Dyskin, A.V. (The University of Western Australia) | Pasternak, E. (The University of Western Australia)
This paper presents data from laboratory scale hydraulic fracturing experimentation with acoustic emission (AE) monitoring. The motivation is firstly to confirm existence of the post-injection surge in the AE rate observed by other investigators. Secondly, unlike previous investigations, a re-pressurization stage is included to test whether AE is driven by the interaction of the two hydraulic fracture surfaces as they contact one another during the closure period that follows an injection-shut in cycle. Our results show that the AE rate indeed increases when the pressure is relieved after the first injection/propagation stage. However, re-opening the fracture through a second pressurization and allowing it to close again proved unsuccessful in causing a second increase in the AE rate. Instead, the AE rate is observed to decay hyperbolically with the time from the first moment of pressure relief with no impact from the second injection/closure stage. This hyperbolic decay in AE rate is in accordance with Omori’s law, that is, it is statistically similar to earthquake aftershocks. The AE in our laboratory experiments were therefore apparently not associated with closure but rather to the somewhat surprising propensity of the rock to produce AE aftershocks from the vicinity of a hydraulic fracture under zero-loading conditions during the hours to days following its creation.
The propagation of a single fluid-filled fracture from the surface of a semi-infinite isotropic elastic solid, subjected to both a transient temperature field and a constant source fluid pressure that is less than the confining stress, is studied using a boundary element method. Fluid flow in fractures is described by the lubrication equation, while the local pressure is determined by the strong coupling between elastic deformation, heat conduction and fluid pressure. Numerical results show that the combination of cooling-induced tensile stress and the source pressure can enhance the propagation speed. Parametric studies are carried out for identifying speed regimes and show the importance of the initial fracture aperture. Three speed regimes are found to exist. If the fluid penetration into the fracture is heavily restricted, the fracture length grows exponentially at early time, and then it suddenly reaches a large speed and progressively decelerates in a finite transition time as fluid diffusion speed varies, but eventually it follows the exponential fracture growth curve at a higher index for stable fluid flow in high-permeability fractures. The time-dependent crack growth behavior does not show any signs of unstable growth, even in the high-speed transition regime. The predictions of crack growth kinetics show a good agreement with some published experimental results and highlight the stabilizing effect of fluid transport on crack growth.
Jeffrey, R.G. (CSIRO Earth Science and Resource Engineering) | Chen, Z.R. (CSIRO Earth Science and Resource Engineering) | Zhang, X. (CSIRO Earth Science and Resource Engineering) | Bunger, A.P. (University of Pittsburgh,) | Mills, K.W. (SCT Operations Pty. Ltd.)
Hydraulic fracture breakdown and reorientation data collected from two test instrumented borehole sites have been analyzed to assess the effect of the initiation type on the treating pressure. Vertical boreholes were drilled and fractures were placed in a conglomerate at depths of 140 to 180 meters into a far-field stress field that favored horizontal fracture growth. Axial initiation resulted in high injection pressures, as a result of near-borehole tortuosity generated as the hydraulic fracture reoriented to align with the far-field stresses. A fracture initiation analysis determined that initiation at the abrasively cut slots should occur before axial initiation. Acoustic scanner logging of the boreholes after fracturing demonstrated that, in many cases, axial initiation occurred and when this was the case, treating pressures were high and consistent with near-borehole tortuous fracture paths. Transverse initiation from the vertical boreholes at pre-cut slots lowered the injection pressures by up to 12 MPa for water injected at approximately 500 liters per minute.
Esteban, Lionel (CSIRO Earth Science and Resource Engineering) | Sarout, Joel (CSIRO Earth Science and Resource Engineering) | Josh, Matthew (CSIRO Earth Science and Resource Engineering) | Clennell, Ben (CSIRO Earth Science and Resource Engineering) | Dewhurst, David (CSIRO Earth Science and Resource Engineering) | Marschall, Paul (NAGRA Switzerland) | Raven, Mark (CSIRO Land and Water)
Summary Preserved shale samples of widely varying clay and carbonate content were recovered from a deep geothermal well near the village of Schlattingen in the Molasse Basin of northern Switzerland. The cored borehole section comprised various stratigraphic sequences from Upper Triassic to Lower Jurassic at depths between 725 and 989 m below surface. A laboratory multi-physics assessment was conducted to characterize their mineralogy, texture, porosity distribution between clay-bound water and weakbound water, broad frequency electrical response, and their geomechanical properties. Despite some mineralogical variations within the sample set, the porosity and rock-fluid water interactions depend on the pore size distribution while the geomechanical and electrical responses are mostly controlled by the clay/carbonate content. The borehole is intended for producing geothermal energy for horticulture with low CO2 emissions.