Rathnaweera, Tharaka D. (Monash University /Nanyang Technological University) | Gamage, Ranjith P. (Monash University) | Wei, Wu (Nanyang Technological University) | Perera, Samintha A. (The University of Melbourne) | Haque, Asadul (Monash University) | Wanniarachchi, Ayal M. (Monash University) | Bandara, Adheesha K. (Monash University)
Over the last several decades, many studies have generated a large amount of proppant performance data, but these studies have only focused on proppant conductivity, with no attention to how proppant mechanical properties vary under loading conditions. The impact of mechanical behaviour on proppant performance can only be fully understood by the combined investigation of micro-structural and mechanical changes with increasing loading. Therefore, this study aims to identify such micro-structural behaviour, and in particular the impact on proppant mechanical properties. Proppant samples were tested under one-dimensional compression loading using high-resolution X-ray CT scanning technology. The reconstructed images taken at different load stages were analysed to capture the micro-structural behaviour and finally correlated with the mechanical behaviour of the proppant.
According to the results, there are significant micro-pore voids inside the proppant mass. When the proppant has a higher degree of porosity, there is a considerable reduction of the compressive strength which is not favourable for hydro-fracturing treatment designs. Moreover, it is clear that the brittleness of the proppant decreases with increasing porosity, as its Young’s modulus reduces with increasing pore voids. Therefore, it is important to have high manufacturing standards to achieve effective proppant performance at great depths. The micro-structural behaviour under increasing loading was investigated by performing comprehensive CT image analysis using Drishti software. According to the results, under compressive loading, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages.
Unconventional oil/gas production has recently attracted the research community due to the uncontrollable increasing demand for primary energy sources (Perera et al., 2016; Wu et al., 2017). Since this method provides a good solution to energy scarcity, over the last several decades, the industry has tried to enhance the production rate, mainly focusing on production enhancement techniques which can be effectively used in the energy extraction from sub-surface geological formations. Of the various options, hydraulic fracturing is one of the best ways to enhance oil/gas extraction, as it increases the formation’s permeability, allowing easy movement of the extracted oil/gas towards the production well (Rutledge and Scott, 2003; Orangi et al., 2011; Vengosh et al., 2014; Wanniarachchi et al., 2015). However, this process may be jeopardised due to the high stress levels acting on the formation at great depths (both vertical overburden and confining pressures). One possible consequence is re-closure of the fracture network under downhole stress conditions, which severely affects the post-fracturing production. Such issues can negate the use of proppant as a hydraulic fracture treatment method where proppants injected with the fracturing fluid prop the fractures, withstanding the fracture-closure stress (Wanniarachchi et al., 2015). Although the proppant gives a reliable solution to overcome this issue (propping the fracture network), sufficient closure stress can cause mechanical failure of the proppant, changing the fracture conductivity, causing re-closure of the fracture network, and altering the bulk properties of the proppant pack, which can negatively influence oil/gas extraction. Therefore, it is important to understand the mechanical behaviour of proppants under downhole stress conditions before injecting proppant with the hydro-fracturing fluid.
The dynamics of nonlinear water waves and extreme waves in particular can be accurately investigated numerically using a numerical wave tank (NWT), implemented by the higher-order spectral method (HOS). In fact, a number of laboratory studies validated the proposed numerical approach, while a very high validation accuracy is reached. We present a numerical study based on a HOS-NWT scheme in the investigation of the onedimensional propagation of extreme events in JONSWAP seas, having their origin from exact breather solutions of the nonlinear Schrödinger equation. Indeed, breathers are known to model extreme waves, when the wave field's spectrum is assumed to be narrow-banded. On the other hand recent experimental studies confirmed that these localized structures can also evolve, when wind or significant perturbations are at play. We will investigate the validity of the proposed numerical scheme by comparing the simulation results to laboratory data. The dynamics of both wave fields show indeed a very good agreement.
The accurate dynamics description and prediction of ocean extreme waves is crucial from the view point of ocean engineering applications. Large amplitude waves are called extreme, freak or rogue waves, in oceanographic terms, when the ratio between their height and the significant wave height exceeds two, see Kharif et al. (2009). This latter ratio is also called the abnormality index (AI) of the extreme wave. Indeed, there is number of mechanisms that may be responsible for the formation of rogue waves Kharif and Pelinovsky (2003). Due to the nonlinear feature of water waves Stokes (1847), the modulation instability (MI) of Stokes waves Benjamin and Feir (1967) provides a reasonable and realistic mechanism for the extreme wave focusing on the water surface, as described in Kharif et al. (2009), Osborne (2010). The MI can be also modeled within the context of the nonlinear Schrödinger equation (NLS), a wave envelope evolution equation that describes the motion of weakly nonlinear water wave packets in time and space Zakharov (1968). In fact, the NLS admits exact analytical models that describe the modulation instability Dysthe and Trulsen (1999). The classical MI problem, consisting of periodic perturbation of Stokes waves that are followed by the expected exponential growth rate of the corresponding excited side-bands, can be discussed within the family of Akhmediev breathers Akhmediev et al. (1985). One particular case is when the modulation period is assumed to be infinite. In this case the Akhmediev solution converges to the so-called Peregrine breather Peregrine (1983). This solution is localized in time and space and can be regarded as model for rogue waves, since besides its double localization it significantly increases the wave focusing amplitude Shrira and Geogjaev (2010).
ABSTRACT: The growing demand and depletion of deep-earth mineral resources has increased the potential of considering mineral recovery in low-grade ore-bodies. In-situ leaching is an effective method of recovering resources such as uranium, and copper from low-grade ore by injecting a leaching solution to the deposit to dissolve minerals. Most common sedimentary ore-deposits and oilfields, however, needs a well-developed fracture network for enhanced recovery. Fracturing the ore-deposit for enhanced recovery requires sound knowledge of strength properties of sedimentary deposits. This study presents experimental results of mechanical properties of sandstone saturated in water, oil, and NaCl brines to represent strength characteristics of sandstone oilfields and saline, mineral-bearing sandstone formations. Unconfined compressive strength tests were conducted on sandstone samples and the stress-strain characteristics were captured through a non-contact digital image correlation system. Acoustic velocities and acoustic energy release of samples during failure were measured using a PCI-2 based AE system. Compared to water saturated samples, brine saturated ones showed further strength deterioration at lower brine concentrations and this influence diminished with increasing salinity. A significant strength retention in sandstone saturated in oil was observed compared to water and brine saturated samples. The acoustic velocities measured can be correlated with strength variation pattern of tested specimens.
With the depletion of high-grade ore deposits and the environmental concerns arising with conventional mining methods, in-situ leaching (ISL) of minerals has emerged as a new technology making mining low-grade ore deposits more economical (Sinclair and Thompson, 2015). ISL involves drilling holes into the ore deposit followed by pumping a leaching solution into an ore body, dissolving the metals in the leach solution. The solution is then pumped to the surface and processed to recover the dissolved minerals. At present, ISL is limited to highly permeable ore bodies and fracturing technologies may be used to improve the performance of ISL by creating open pathways in the deposit for leaching solution to penetrate. However, fracturing of ore deposits, require a sound knowledge of strength properties of the target rock to minimize the nucleation and propagation of unwanted cracks which could lead to groundwater contamination in some cases (De Silva et al., 2016). Most sedimentary oilfields and ore deposits are usually are in saturated conditions and the strength variation in the rock under different saturation conditions is an important aspect that must be investigated before any field application of ISL in a water sensitive areas.
ABSTRACT: Water-based fracking fluids are currently used to enhance the gas production from unconventional gas reservoirs, while the formation damage due to the interaction between rock matrix and residual water significantly reduce the pore space available for gas movement. In this paper, the effect of different fracturing fluids on fracture permeability is investigated using the same fractured siltstone sample in five rounds of cycling test (CO2-CO2-CO2-water-CO2). The experimental results reveal that the maximum confining pressure dominated fracture permeability. The fracture permeability for CO2 at 10MPa confining pressure decreased by around 75% after applying 40MPa confining pressure. After water flooding in the fourth round, the fracture permeability for CO2 was considerably reduced by around 70-85% compared with that in the second cycling test, and the influence of water flooding was much stronger at lower CO2 injection pressure. The experimental results showed that the fracture permeability for gas reduced significantly after water flooding due to formation damage. Thus, liquid CO2 is much more suitable than water as fracturing fluid for clay-abundant unconventional gas reservoirs.
As an efficient and greener source of energy, shale gases currently attracting more interest (Conti et al., 2014), however, the ultralow permeability (<0.001mD) of shale matrix is the major barrier to shale gas commercial production (Daigle and Screaton, 2015, Kumar et al., 2015). The development of hydraulic fracturing and horizontal drilling has been therefore used to enhance the shale formation productivity, and water-based fracturing fluids has been commonly used (Johnson and Johnson, 2012, Nicot and Scanlon, 2012, Boz and PE, 2014, Ghahremani and Clapp, 2014).
While the interaction between water and rock minerals, especially clay minerals, can lead to severe formation damage and long clean time (Arnold, 1998, Lal and Amoco, 1999). Many experimental investigations have been carried out on the effect of water saturation on the mechanical behavior of reservoir rock (Shukla et al., 2013, Rathnaweera et al., 2014, Dyke and Dobereiner, 1991, Erguler and Ulusay, 2009). For example, according to Shukla et al., (2013) and Rathnaweera et al., (2014), water sensitivity or softening of rock samples due to the saturation effect can greatly reduce the mechanical strength of dry reservoir rock. While the soften effect can induce a decrease of propped fracture conductivity because the deformation squeezes the fracture width, and strong absorption of rock matrix to water will block the gas flow channel in fractures, so the presence of water in formation can greatly reduce the gas production, especially for water sensitive formations (Cheng, 2012).
Rostami, J. (Pennsylvania State University) | Kahraman, S. (Hacettepe University) | Yu, X. (China University of Mining and Technology) | Copur, H. (Istanbul Technical University) | Balci, C. (Istanbul Technical University) | Bamford, W. (The University of Melbourne) | Asbury, B. (Colorado School of Mines)
ABSTRACT: Uniaxial compressive strength (UCS) and the Brazilian tensile strength (BTS) are the most commonly used rock properties. The UCS/BTS ratio is used as an indicator of rock brittleness and to estimate UCS from BTS (or vice versa). However, the relation between UCS and BTS has not been investigated in detail. This paper focuses on UCS/BTS ratio using the raw data pertaining to 1842 rock types from different research institutes. Regression analysis indicated that power law or exponential relations offers a better best fit correlation between UCS and BTS for each data set from different institute than linear function which has been typically offered in the past. Power function was also found to offer a better relationship between UCS and BTS for the all data under evaluation. The correlation coefficient of power function (r = 0.79) was higher than that of the linear function (r = 0.64).
Rock engineers widely use the uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) of rock in designing surface and underground structures, rock excavation projects, drilling and blasting projects etc. Numerous researchers have investigated the relation between UCS and BTS and suggested linear correlations between two parameters. However, some researchers (Altindag & Guney 2010, Arioglu & Tokgoz 2011, SeshaPhanietal. 2013, Nazir etal. 2013) recently found non-linear relations between UCS and BTS. In this study, the raw data from laboratory testing on 1842 different rock types were analysed to investigate the best formulas for describing the relation between UCS and BTS.
The literature (Farmer 1983, Budavari 1983, Jumikis 1983) indicates the tendency of some researchers to assume rock compressive strength that is approximately 10 times tensile strength for most rocks. However, it has been indicated by others that the UCS/BTS ratio may change from 1.9 to 176.6 (Table 1).
Many researchers presented linear correlations between UCS and BTS. Limited number of recent studies has suggested exponential or power functions to define the relationship between UCS and BTS. The study of SeshaPhani et al. (2013) is on concrete and the studies of Nazir et al. (2013) and Arioglu & Tokgoz (2011) are limited specific rock types, including limestone and rocks from the North Anatolian fault zone, respectively. However, the study of Altindag & Guney (2010) comprises data set containing 143 rock sample compiled from previous studies, ranging from weak rock to very strong rocks, including different types from different areas.
3 THE DATA USED IN THE STUDY
Results of UCS and BTS measurement on 1842 different rock types which were tested over years at different institutes such as Colorado School of Mines (CSM), Pennsylvania State University (PSU), Istanbul Technical University (ITU), University of Melbourne (UM), and Nigde University (NU) were collated and used to evaluate the relation between UCS and BTS. It must be noted that the data was collected from different countries and geographical locations and includes different rock types, covering all rock classes such as sedimentary, igneous, and metamorphic. After receiving of the data from various laboratories, an initial evaluation was performed to eliminate the test results related from structural failure of the samples based on the recorded type of failure. But the classification of the failure as Structural Failure could be subjective and simply based on the observation of the existing planes of discontinuity in the sample prior to testing. If no such defects were observed or reported, a judgment could not be made as to the validity of data and absence of such defects. This could lead to scatter in the data set and presence of some outliers in the data that was inevitable and will be discussed in following sections. CSM data set consists of the UCS and BTS values of 182 different rock types, mainly welded tuff, granite, sandstone, limestone, and argillite. The data have very wide strength range. The UCS values ranges from 1.3 to 468.5 MPa. But, the strength of most samples is less than 150 MPa.
Numerous measurement and characterization methods have been developed over the past decades to quantify the roughness of rock joints. The characterization methods traditionally consider a 2D linear trace and are often limited to the laboratory scale. In this research, 3D point clouds of joint surfaces obtained with Terrestrial Laser Scanning (TLS) have been analyzed to obtain 3D representations of joint roughness. While TLS enables remote acquisition of large scale in-situ joint surfaces, the resulting data include inherent range noise which generally results in significant overestimation of joint roughness. To denoise the TLS data, two wavelet transforms are applied in combination with different thresholds. The denoising procedure is successfully tested by comparing TLS data obtained on a 20×30 cm joint sample at a range of 10 m, to reference data for the same joint surface obtained using the Advanced TOpometric Sensor (ATOS).
Rock joint roughness can be measured and parameterized according to several different methods. In-situ measurements are traditionally made using a contour gauge or a simple profilograph with a straight edge. Results are generally sparse and limited to 2D profiles measurements. Therefore, many existing roughness parameters are based on 2D profiles including the Joint Roughness Coefficient (JRC), maximum asperity angle (Patton 1966) and different statistical parameters, e.g. roughness profile index or average angle (Papaliangas 1995).
Remote sensing has recently been applied to joint characterization, with measurement techniques including total station, photogrammetry, the Advanced TOpometric Sensor (ATOS) and Terrestrial Laser Scanning (TLS), as described in (e.g. Feng 2003, Haneberg 2007, Grasselli et al. 2002, Khoshelham et al. 2011). Remote sensing techniques can provide 3D digital surface data, allowing new roughness parameterization approaches such as fractals (e.g. Fardin et al. 2004) or the angular threshold method (Grasselli 2001).
In our research the efficacy of TLS as a means for in-situ characterization of joint surfaces is investigated. TLS enables fast, accurate and detailed acquisition of distant, inaccessible, large scale surfaces. However, the data resolution and accuracy are limited by laser spot size and range noise, respectively. Laser point density (resolution) determines the smallest observable roughness scale, and data accuracy influences roughness amplitude. Sturzenegger & Stead (2009) extracted first-order roughness from TLS data, however the question remains whether reliable estimation of second-order roughness is also possible. Therefore, the objective of this research is to define a method, which can efficiently remove the TLS range noise while preserving important details of the surface roughness.
Aoki, T. (The University of Melbourne, Australia & Taisei Corporation, Yokohama) | Tan, C.P. (CSIRO Division of Petroleum Resources) | Cox, R.H.T. (CSIRO Division of Petroleum Resources) | Bamford, W.E. (The University of Melbourne)