ABSTRACT: In geologic CO2 storage, it is important to find a proper barrier material that will avoid or limit acidic fluid migration. Shales that are ductile and have high capillary entry pressure and low permeability can be considered as good candidates for the caprock. Faults may contain high percentage of clay and act as barriers for fluid flow in reservoirs. Experimental techniques have been developed to characterize the behavior of clay-rich materials at elevated pressures. Intact and remolded specimens of Opalinus clay – a Jurassic shale from Switzerland – are brought to the conditions of deep (> 1 km depth) geologic storage and fully saturated with in-situ brine. Poromechanical parameters and failure characteristics are measured in drained and undrained conventional triaxial compression experiments. CO2 breaktrough pressure and permeability of shale are assessed in oedometric tests on thin (12 mm) samples. Experimentally measured parameters are used in numerical simulations to assess fault stability and the shaly caprock integrity for the case of geologic carbon storage, where cooling is likely to occur around injection wells. It is found that clay-rich faults may induce microseismic events, but without leading to CO2 leakage.
Safe long-term carbon dioxide (CO2) geologic storage largely depends on caprock integrity (IEAGHG, 2011). The caprock may be especially altered at the early stages of injection, when the maximum overpressure is reached (Vilarrasa et al., 2010). As CO2 is injected in the storage formation, it tends to advance through the top of it and below the caprock due to buoyancy (Hesse et al., 2008). Furthermore, the injected CO2 will reach the bottom of the well at a colder temperature than that of the storage formation (Paterson et al., 2008). As a result, a cold region is formed around the injection well, but the cooling front advances much behind than the CO2 plume interface (Vilarrasa and Rutqvist, 2017). Thus, the lower boundary of the caprock is in contact with CO2 saturated pore water or pore fluid that consists almost of pure CO2, which may be cold (Figure 1). These thermal and chemical interactions between the pore fluid and the caprock may change its material properties and affect stability.
ABSTRACT: Despite the significant hydrocarbon production from unconventional shale resources in the last decade, hydration induced mechanical responses of shales especially at micro-scale level is not yet fully understood. Such mechanical responses especially to hydraulic fracturing fluids control the leak-off rate and near fracture permeability evolution and therefore is crucial for hydrocarbon production. Therefore, we ran a series of systematic experiments by exposing organic matter rich shale samples to ionic solutions (NaCl, KCl, MgCl2 and CaCl2) with different concentrations (0.2, 1 and 2M) at controlled temperature. Images of the samples at their initial and wet states (exposed to ionic solutions for 7 hrs at 25 °C temperature) were taken using environmental electron microscope. The image processing was performed to register the images and extract the evolved features. Digital Image Correlation was further employed to calculate the strain field developed by change in sample structure in order to investigate the likelihood of micro-fracture development. From the results of this study, it was found that a) the sample exposed to 0.2M NaCl solution experienced the most significant micro-structural changes especially clay hydration while the sample exposed to 0.2M CaCl2 solution almost showed no changes, b) the sample exposed to 0.2M KCl solution on the other hand showed no/little swelling but its pore space was increased, c) surprisingly the micro-structural changes were more pronounced at 1.0M concentration of all ionic solutions especially for those with divalent ions, d) Digital Image Correlation analysis confirmed that micro-fracture development is more likely to occur when the sample is exposed to solutions of monovalent ions in particular NaCl.
Despite long-time research carried out on clay rich sediments, the micro-structural evolution of these types of rocks when exposed to different ionic solutions has not been yet fully understood. Understanding such micro-structural evolution has wide range of applications in many engineering disciplines such as petroleum engineering [1, 2], nuclear waste disposal  and groundwater investigation . This is especially pronounced for hydraulic fracturing operation as one of the key requirements for development of many unconventional resources. Among several examples, the low recovery of hydraulic fracturing fluid in unconventional shale reservoirs and its link to microstructural alteration is of great interest [5, 6].
ABSTRACT: Results from three compressive creep tests on dried crushed salt, focused on identifying the influence of pore pressure during reconsolidation, are presented. All tests were completed under a macroscopic hydrostatic confining pressure of 30 MPa while the internal pore pressure varied from zero to 20 MPa across the tests. All tests were held under constant creep conditions for approximately 24 hours at an elevated temperature of 170°C, and isotropic volumetric plastic deformations were observed as the sample porosities decreased by over 3%. Plastic volumetric strain rates during the creep tests varied from 3 • 10–5 down to 4 • 10– 8 sec –1, and the combined results from these tests indicate the volumetric deformations of a crushed-salt sample under creep conditions is influenced by the application of pore pressure within a bulk sample.
Deformation of crushed salt is of interest to both the mining and waste repository industries because crushed salt could economically be used as a backfill and sealing material in these environments. However, the design for backfilling a drift in a salt mine along with the sealing of a room or borehole of a waste repository all depend upon understanding crushed-salt's constitutive properties, i.e., the relationship between stress and strain over a bulk volume of crushed salt. The constitutive response of crushed salt is nonlinear, as its response varies greatly with bulk porosity and is heavily influence by water content, particle size, and temperature. To date, numerous models have been developed in a continuing effort to accurately describe crushed salt's constitutive properties (Butcher, 1980; Sjaardema and Krieg, 1987; Holcomb and Zeuch, 1990; Zeuch, 1990; Spiers and Brzesowsky, 1993; Callahan et al., 1998; Olivella and Gens, 2002; Kröhn et al., 2015). These existing constitutive models have been developed based on a wide range of approaches that aim to reproduce the characteristics observed during laboratory experiments.
ABSTRACT: We study the effects of a highly permeable sand layer on pore pressure and stresses near a salt diapir by using a forward hydro-mechanical finite-element model. We compare a salt basin entirely composed of mudrocks to the same basin but with a highly permeable sand layer across its bottom. We show that the sand layer causes a significant increase in pore pressure near the diapir by transmitting overpressure from deep sediments far from the diapir. This pressure increase makes it difficult to safely drill wellbores near the diapir. We examine the stress history of mudrocks at the crest of the sand layer and show that the pore pressure increase near diapir causes unloading of sediments, which is a crucial finding for seismic prediction of pore pressure in these sediments. Overall, our results offer an insight into the effects of basin sand layers on pore pressure distribution, seismic pore pressure prediction, and wellbore design near salt diapirs.
Many hydrocarbon reservoirs are trapped against salt diapirs. The stability and safety of wellbores drilled to produce from these reservoirs are controlled by the pore pressure and stress state near diapirs. Inverse, reverse, and forward modeling are approaches that are typically used to predict pore pressure and stresses. The first two methods predict pore pressure and stresses for the present geometry of the salt system. However, they are based on assumptions that may not be valid, particularly near salt diapirs and sand layers in the basin. Forward modeling may not fully honor the present geometry of a system but, because it does not make assumptions, its predictions can be used to evaluate the assumptions made in the first two methods.
Seismic methods are well-known inverse methods for pore pressure prediction in basins. These methods predict the present magnitudes of pore pressure and stresses based on seismic velocity of sediments measured across the basin. The predictions of these methods, however, depend highly on the loading/unloading status of sediments, that is, whether the sediment present stresses are the maximum stresses ever experienced by the sediments. This status is particularly unclear for sediments at the crest of sand layers near a salt diapir. Because the permeability of sand layers is orders of magnitude higher than the permeability of mudrocks, they behave as conduits and transmit significant overpressure generated in deep sediments far from the salt diapir to shallow sediments at the crest of the sand layer near the diapir. The transmitted overpressure can be high enough to cause unloading of the crest sediments. Because seismic methods predict only the present pore pressure and stresses, they are unable to determine and hence assume this status. Furthermore, seismic methods can predict pore pressure only in mudrocks; they are unable to predict pore pressure in embedded lithologies such as sandstone or carbonate layers. Because of their significantly different permeability, considerably different pore pressures can develop in these layers and their adjacent mudrocks over the basin evolution (Dickinson, 1953, Traugott, 1997 and Bowers, 2001). Seismic methods often assume that pore pressure in these lithologies equals pore pressure in adjacent mudrocks.
ABSTRACT: Conventional techniques such as boreholes and test pits have been used for geotechnical site investigation purposes for many years. These conventional techniques are invasive, time-consuming and expensive. To minimize costs and to complement intrusive investigation techniques, geophysical methods are now commonly employed for geotechnical investigations in karst terrain. One relatively new non-invasive geophysical technique is the active multichannel analysis of surface waves (MASW) tool. This paper explores the utility of using the active MASW tool in karst terrain. A 20-pound sledge hammer was used as an acoustic source; a Seistronix Ras-24 channel seismograph was used to record the MASW field data. MASW field data were processed with SurfSeis4 software; the output at each test location was a 10-layer shear-wave velocity profile of the subsurface. Borehole control and 2-D electrical resistivity tomography (ERT) data were used to verify and constrain the interpretations of the output 1-D shear-wave velocity profiles. The results show that MASW can reliably be utilized to map variations in the engineering properties of soil/rock and to estimate depth to top of rock in karst terrain.
Conventional methods such as drilling and trenching have been utilized for subsurface investigations for many years. However, geophysical techniques are now being increasingly used to investigate construction sites in karst terrain. Geophysical surveys typically complement intrusive geotechnical investigations. They are also non-invasive, less expensive and more rapid. Geophysical techniques that have been utilized for investigating karst terrain include electrical resistivity tomography (ERT), ground penetrating radar (GPR), gravity, electromagnetic (EM), and seismic methods (Obi, 2012; Yassin, 2013; Nouioua et al., 2013; Martinez-Moreno et al., 2014; Giorgi and Leucci, 2014). The choice of a particular geophysical method for imaging the subsurface is dependent on the physical characteristics of the target.
Active Multichannel analysis of surface waves (MASW), a geophysical technique for determining the accoustic properties of soil/rock and estimating the top of rock is being increasingly employed to investigate complex karst terrain. This study is aimed at exploring the utility of the geophysical technique in karst terrain. The active MASW tool consists of a seismograph, an active acoustic source (e.g sledge hammer) and an array of receivers (geophones). The active MASW tool records acoustic energy generated by the active source and calculates surface wave phase velocities. The phase velocity data are analyzed; the output at each test location is a 10-layered 1-D shear-wave velocity profile that typically extends to a depth of about 100 feet (30 m). Shear-wave velocity is a function of soil and/or rock rigidity.
ABSTRACT: In this study, 3D finite element models are developed to investigate thermally induced stress fields during cryogenic thermal stimulation using liquid nitrogen (LN2). Laboratory tests using LN2 as a fracturing fluid were carried out on concrete, sandstone, and shale samples under confined and unconfined conditions. These tests indicated different fracturing patterns. 3D finite element modeling of the laboratory tests was conducted to predict and analyze the stress behaviors around the wellbore. The combination of laboratory experiments and the 3D finite element modeling provided insight into the potential for cryogenic thermal fracturing in unconventional reservoirs. Three different types of specimen blocks were modeled. These concrete, sandstone, and multi-layer shale blocks were subjected to cryogenic thermal treatments to obtain the temperature and stress profiles and how they are influenced by the formation stiffness. Results show that model developed was successful in simulating the experimental outcomes and observations indicating distribution of high tensile stresses in tangential and longitudinal directions around the wellbore at −321° F. The results of this paper help in understanding the mechanisms of complex fractures created by thermal shock around the wellbore in reservoirs settings.
Cryogenic fracturing is the act of creating fractures by introducing very low temperature liquids into a warmer formation rock under reservoir conditions. This sudden heat transfer will cause the face of the rock to shrink, which will eventually cause the rock to fail in tension (Wang et. al, 2016 and Cha et al, 2014).
King (1983) examined the use of gelled liquid carbon dioxide to stimulate dry gas sand formations. King notes that carbon dioxide returns to the gaseous phase at formation conditions and does not cause sloughing or swelling effects in low permeable water-sensitive formations. In another study, Grundmann et al. (1998) treated a Devonian shale well with cryogenic nitrogen and observed an initial production rate 8% higher than the rate in an offset well that had been stimulated with nitrogen gas. Although the increased initial production rate in this research suggests the efficacy of cryogenic fracturing, there could be a number of reasons why an offset well in a shale formation might produce differentially including anisotropic stress conditions and heterogeneous reservoir conditions over short distances. McDaniel et al. (1997) conducted simple laboratory studies where coal samples were immersed in cryogenic nitrogen. The coal samples experienced significant shrinkage and fractured into smaller cubicle units, with the creation of microfractures orthogonal to the surface exposed to the cold fluid. The researchers found that repeated exposure cycles to the cryogen caused the coal to break into smaller and smaller pieces, becoming rubblelized. If the creation of fractures due to thermal stresses can occur in coal bed formations, it may have the potential to occur in other types of rock as well.
ABSTRACT: Felt induced seismicity due to hydraulic fracturing has been observed at many fields in the past few years. Though spatial and temporal correlations establish the link between hydraulic fracturing activities and felt induced seismicity, the underlying stress/pressure change mechanisms that cause fault reactivation are unclear. This study reports a systematic analysis of several felt induced seismicity field observations using geomechanical models to understand the mechanisms that cause the fault reactivation. The field observations indicate that felt induced seismicity occurs at 100s of meters to few kms away from the wellbore and there is a time delay of hours to days. The numerical modeling results of hydraulic fracturing in a low permeability homogeneous medium show that stress and pressure changes attenuate rapidly with distance, unable to explain reasons for the felt induced seismicity at few kms away from the wellbore. Also wells in close proximity, completed similarly produce vastly different seismicity, indicating that local heterogeneity is important. An alternative mechanism, such as a direct hydraulic connection between a hydraulic fracture and fault, found to be consistent with the field observations. The insights gained from the numerical modeling of fracture fault interaction models can be used to develop felt induced seismicity mitigation plans.
It is well known that microseismic events (magnitudes of −2 and −3), which are not felt at the earth’s surface, are created by hydraulic fracturing and that knowledge is used to identify the extent of stimulation. However, felt induced seismic events (magnitude > 2.5) due to hydraulic fracturing have historically been rare, with only 70 felt events for 3 million hydraulic fractures up to early 2010s (US National Academy, 2012). Also, felt induced seismicity due to hydraulic fracturing has been observed in many unconventional plays in recent years. The magnitude 3.8 event in year 2011 in the United Kingdom (de Pater and Baisch, 2011), magnitude 3.0 event in year 2013 in Ohio (Skoumal et al., 2015) and magnitude >4 events in Alberta and British Columbia in years 2013-2016 (Schultz et al., 2017, BC O&G Commission, 2012 and 2014) are some of the recent examples of felt induced seismicity due to hydraulic fracturing.
ABSTRACT: In burst-prone mines, seismic monitoring is a central component of risk analysis and mitigation efforts. In order to maximize the effectiveness of such analyses, event magnitude and location errors should be minimized. However, minimizing location errors can be difficult due to unknowns in the geologic media through which the seismic energy propagates. The National Institute for Occupational Safety and Health (NIOSH) is conducting research to improve the quality of the event locations contained in a seismic catalog produced by a surface network monitoring a deep underground silver mine in northern Idaho. Travel times of several events with relatively well-known locations, as determined by an in-mine network, were modeled using the fast marching method. The Nelder-Mead simplex method was then employed, in conjunction with global Monte Carlo sampling, to determine a suitable 1-D layered P-velocity model. Using the new model with station delays on the surface network, epicentral location errors were reduced by approximately a factor of three and depth errors were reduced by more than a factor of ten. Significantly lower location errors can support higher-quality analyses to help improve safety in deep underground mines. Planned development of a 3-D model and additional instrument deployments will further increase location quality.
Seismic monitoring has been conducted in hardrock mines since the early 1900s, when mining-induced tremors in South Africa were recognized as hazards to mine workers and nearby communities (Durrheim, 2010). Since that time, monitoring and analysis of mining- induced seismicity (MIS, see Gibowicz and Kijko, 1994, for review) has become an important aspect of ground control strategy for many seismically active mines. Seismic event monitoring can be used to identify potentially hazardous geological features (e.g,, Spottiswoode and Milev, 1998), provide information for quantifying and mitigating seismic hazards (Potvin, 2009; Hudyma and Potvin, 2010), relate damage to operational parameters (Hodgson and Joughin, 1966), and determine failure mechanisms (McGarr, 1992; Trifu et al., 2000), among other important research areas that can have significant impacts on mine safety.
Yang, Liu (Institute of Mechanics) | Shi, Xian (China University of Petroleum (East China)) | Zhang, Kunheng (China University of Petroleum (East China)) | Ge, Hongkui (China University of Petroleum (East China)) | Gao, Jian (Research Institute of Petroleum Exploration and Development) | Tan, Xiqun (Research Institute of Petroleum Exploration and Development) | Xu, Peng (Research Institute of Petroleum Exploration and Development) | Li, Lingdong (Research Institute of Petroleum Exploration and Development)
ABSTRACT: The fact that salt ions in shale pores diffuse into fracturing fluids is key factor to lead to recovered water with high salinity. In this paper, the authors conduct the test of mineral composition and SEM to understand the reservoir characteristics. The diffusion experiments are conducted on crushed samples, and a new method is proposed to differentiate between matrix and microfractures by using diffusion data. A large amount of salt ions exit in shale pores and can diffuse into fracturing fluids after fracturing operations. To a great extent, ion diffusion rate is determined by the development of microfractures. The crushed samples with smaller grain diameter contain have lower diffusion rate due to the low probability of microfractures development. When the grain diameter is lower than critical value, the crushed samples cannot contain microfractures. As for Longmaxi formation sample, the fracture-matrix boundary is about 80mesh.The research contributes to understanding the reservoir characteristics and salinity profiles of gas shale.
The field observations show that the salinity of recovered water is generally high. What’s more, the salinity increases continuously over time and even exceeds 10%. It should be noted that the salinity of slick water is about 0.1%. The researchers tend to attribute this observation to the salt ions diffusion into fracturing fluids (Wang et al., 2016).
The salt ions concentration and type in recovered water can act as the indicator to evaluate the development of fracture network. Unlike primary fractures, the secondary fractures are induced fractures that are covered by connate water film. The connate water film can mixes easily with fracturing fluids to increase the salinity of fracturing fluids (Woodroof et al., 2003). The secondary fractures with smaller aperture size tend to forms high exposure area that can enhance the ion diffusion capacity. In addition, the ion type in secondary fractures is different from that in primary fractures (Gdanski et al., 2007). The study found that Ba2+ exits in secondary fractures and the development of microfractures are evaluated based on the concentration of Ba2+ (Agrawal and Sharma, 2013).
Zhang, Yingbin (Southwest Jiaotong University, Kyushu University) | Zhao, John X. (Southwest Jiaotong University) | Huang, Xiaofu (Southwest Jiaotong University) | Chen, Guangqi (Kyushu University) | Hamasaki, Tomohiro (West Nippon Expressway Company Limited)
ABSTRACT: Rockfall is one of the most frequent geo-hazards, which often occurs in mountain areas and along land transportation. It often causes serious consequences in terms of lives lost, homes destroyed and infrastructures disrupted because its high energy and significant mobility, although it usually impacts only small areas compared to other geo-hazards. Rockfall could be activated by various triggers, in which earthquake is one important trigger. Estimate the run-out of earthquake-induced rockfall is important for choosing suitable protection measures. A growing body of evidence, including site videos and eyewitness testimonies, suggests that the ground still intensively quake when the process of rockfall. The main purpose of this paper is to clarify whether the seismic loading has any influence on the mobility of rockfall through shaking table test. Thousands of tests are conducted to investigate the effects of the amplitude and the frequency of simple harmonic seismic loading on the horizontal and the lateral displacements of triangular prism, quadrangular prism, pentagonal prism and hexagonal prism. The results show the significant effects of seismic loading on the run-out distance and the lateral displacements of rockfall.
Rockfall is one of the most frequent geo-hazards, which often occurs in mountain areas and along land transportation. It often causes serious consequences in terms of lives lost, homes destroyed and infrastructures disrupted because its high energy and significant mobility, although it usually impacts only small areas compared to other geo-hazards (Guzzetti et al., 2002). In addition, rockfall event occurs accidently in most cases, that is hardly unpredictable and usually without any obvious warnings (Masuya et al., 2009). As for examples, there are many reports on rockfall induced fatalities and economic losses particularly along roads from China, Japan and other parts of the world (Hungr et al., 2004). Therefore, in order to mitigate rockfall risk, it is important to clear the disaster-caused mechanism and take preventive measures correspondingly.
Different rockfall analysis methods have been proposed at different scales through empirical models, process-based models and Geophysical Information System (GIS)-based models. However, most of these approaches assess the rockfall using heuristic approaches, expert judgement or analysis of relatively small historical datasets. This makes them inapplicable in seismic areas, where the rockfall is mostly caused by earthquake events (Valagussa et al., 2014).