Morton, Sarah L. C. (Kansas Geological Survey, formerly U.S. Geological Survey) | Lane, John W. (U.S. Geological Survey) | Thomas, Margaret A. (Connecticut Geological Survey, Department of Energy and Environmental Protection) | Liu, Lanbo (University of Connecticut, Department of Civil and Environmental Engineering)
Five new seismic hazard classifications for Hartford County, Connecticut (CT), were proposed by New England State Geologists (NESG) in an effort to improve the current USGS Seismic Hazard Map. These classes were derived from mapped surficial materials, but in situ information is required to verify this approach. Therefore, active and passive surface wave techniques were performed at thirty field sites to determine VS30 and compare the results to the NESG map. Passive data were processed using the Horizontal-to-Vertical Spectral Ratio (HVSR); active data were processed with the multi-channel analysis of surface waves (MASW) technique. The field investigation demonstrated that the surficial material-based system was not sufficient for 66% of the field sites and in-situ velocity information from at least two methods should be considered for improved classification. The geophysical work discussed here represents the first field de-rived VS30 values for Hartford County, CT.
Presentation Date: Tuesday, October 16, 2018
Start Time: 8:30:00 AM
Location: 204A (Anaheim Convention Center)
Presentation Type: Oral
Geophysical tomographic methods provide high-resolution information about subsurface geologic structure, hydraulic and geotechnical properties, pore-fluid distribution, and time-varying hydrologic conditions. Electrical, electromagnetic, seismic and ground-penetrating radar measurements collected from surface and (or) cross-hole configurations are inverted to produce two-dimensional, three-dimensional, or four-dimensional (three spatial dimensions plus time) images of the subsurface. Over the last two decades, advances in instrumentation have led to more rapid data acquisition, while advances in modeling, inversion, and computational resources have facilitated time-lapse monitoring, consideration of more rigorous measurement physics, coupled or joint inversion of multiple data types, improved uncertainty quantification, model order reduction, and more routine application of image appraisal techniques. Here, we provide an overview of the current state of the research, remaining challenges, and the path forward for geophysical tomography.
We acquired microtremor array data at 11 sites in the Seattle basin, Washington State, and applied the wavenumber-normalized SPAC method (krSPAC) to obtain
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 213B (Anaheim Convention Center)
Presentation Type: Oral
There is a critical and growing need for information about subsurface geological properties and processes over sufficiently large areas that can inform key scientific and societal studies. Airborne geophysical methods fill a unique role in Earth observation because of their ability to detect deep subsurface properties at regional scales and with high spatial resolution that cannot be achieved with groundbased measurements. Airborne electromagnetics, or AEM, is one technique that is rapidly emerging as a foundational tool for geological mapping, with widespread application to studies of water and mineral resources, geologic hazards, infrastructure, the cryosphere, and the environment. Applications of AEM are growing worldwide, with rapid developments in instrumentation and data analysis software. In this study, we summarize several recent hydrogeophysical applications of AEM, including examples drawn from a recent survey in the Mississippi Alluvial Plain (MAP). In addition, we discuss developments in computational methods for geophysical and geological model structural uncertainty quantification using AEM data, and how these results are used in a sequential hydrogeophysical approach to characterize hydrologic parameters and prediction uncertainty.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 213B (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: Enhanced reservoir connectivity generally requires maximizing the intersection between hydraulic fracture (HF) and preexisting underground natural fractures (NF), while having the hydraulic fracture continue to propagate across the natural fractures. Observations of downhole core samples suggest that these natural fractures are in fact veins filled with minerals such as calcite (Mighani et al., 2016). We study this interaction during the approach of a hydraulic fracture to a smooth saw-cut fracture under triaxial stress conditions. The specimen is Solnhofen limestone, a fine-grained (<5 μm grain), low permeability (<10 nD) carbonate. The differential stress (1-20 MPa) and inclination of the fault which determines the approach angle, θ (30, 60°) are the experimental variables. We conduct the experiments on both bare surface and gouge-filled fault surfaces. The gouge is a 1 mm thick crushed powder of Solnhofen limestone with <106 μm grain size. During the hydraulic fracture, acoustic emissions (AE), inferred slip velocity, axial stress and pore pressure are recorded at a 5 MHz sampling rate.
The hydraulic fracture was able to cross the bare surface fault with small induced fault slip. The fault gouge increased the coefficient of friction significantly from 0.12 (bare, polished surface) to > 0.44 (gouged layer). However, the gouge-filled fault arrested the hydraulic fracture and generated a slip event with different characteristics: 1- The stress drop was larger while the generated AE signals had lower magnitude. 2- Slip velocity recorded by the vibrometer was of the same order of magnitude for the bare and gouge- filled faults, but the slip duration increased from 29 μsec for bare surface to ~2.5 msec (~90 times longer rise time) for the gouge- filled fault. The experiments suggest that the gouge-filled fault can accommodate much larger displacement while promoting slow slip on the fault which is harder to detect as AE signals. The observed long duration slip events are similar to the field observations of the long period and long duration (LPLD) events during the stimulation of clay-rich shale formations (Zoback et al., 2012). While the intrinsic low strength, high ductility, and unfavorably oriented natural fractures in shale formations are expected to reduce the occurrence of induced seismicity, our experiments suggest an additional mechanism for the observed LPLD events, i.e. the role of fault gouge. They also suggest that the microseismic detection techniques may under-predict the stimulated volume as the activation of natural gouge-filled fractures may proceed aseismically.
Hydraulic fracture (HF) operations have been extensively used over the years to increase the productivity of low- permeability hydrocarbon reservoirs. The intersection of hydraulic fractures with present underground natural fractures is proven as a major factor in increasing the productivity of the shale gas reservoirs (Mayerhofer et al., 2010). Microseismic observations (Mayerhofer et al., 2010) and mined-back downhole samples from field operations (Warpinski and Teufel, 1987) support the importance of natural fractures and their activation during the operation. Understanding the necessary conditions for the activation of natural fractures, the expected slip magnitude and enhanced fluid transmissivity, and the impact of this slip on the hydraulic fracture path is a key to understanding the role of hydraulic fracture in enhanced recovery operations.
ABSTRACT: Geologically diverse landforms around the world show indications of energetic macroscale fracture. These fractures are sometimes displayed dramatically as so-called “A-tents”, whereby relatively thin rock sheets push upwards and fracture, forming tent-like voids beneath the ruptured sheets. The origin and formation of such features has been a topic of considerable interest and analysis for over a century. Here we show that thermally-induced stresses, coincident with particularly hot days during particularly hot years, were responsible for recent (2014-2016) energetic ruptures of rock sheets forming a granitic exfoliation dome in California, USA. Through a three-year field effort, we found that subcritical fracture occurred due to diurnal and seasonal cumulative thermal stresses. However, our analyses also indicate that subsequent critical fracture could only have been reached if thermal stresses acted in concert with existing tectonic stresses. Thus, we offer a superposition triggering mechanism (background tectonic stresses with cyclic thermal stresses) to explain these rock fracture features.
The process of natural rock fracture on the Earth's surface is rarely observed despite plentiful evidence in rock outcrops around the world. In cliff exposures (e.g., Stock et al., 2012; Collins and Stock, 2016; Ziegler et al., 2014) and in suficially deposited boulders (e.g., McFadden et al., 2005; Eppes et al., 2010, 2016), rocks show ample evidence of recent fracture. Given the tenents of geological time, it is logical to assign most such fractures a subcritical providence - that is, fracture occurring steadily over long time scales (Eppes and Keanini, 2017). Some evidence exists, however, of more rapid rock fracture - for example, those indicated by exfoliation fractures and related A-tents of rock, wherein slabs of rock are perched upon one another, seemingly uplifted away from the Earth's surface (e.g., Ericson and Olvmo, 2004; Twidale and Bourne, 2009). These and similar features (commonly termed exfoliation sheets) have long drawn the attention of geomorphologists (e.g., Gilbert, 1904; Jahns, 1943; Twidale, 1973; Holzhausen, 1989) but considerably less attention from the rock mechanics field (e.g., Martel, 2006, 2017).
Recently, the miscible CO2-EOR tertiary process used in the main pay zone (MP) of suitable reservoirs has broadened to include exploitation of the underlying residual oil zone (ROZ) where a significant amount of oil may remain. The objective of this study is to identify the ROZ and to assess the remaining oil in a brownfield ROZ by using core data and conventional well logs with probabilistic and predictive methods.
Core and log data from three wells located in the East Seminole Field in Gaines County, Texas, were used to identify the MP and ROZ in the San Andres Limestone, and to predict oil saturations. The core measurements were used to calculate probabilistic in-situ oil saturations within the MP and the ROZ as a function of depth. Well logs, in combination with core data and calculated saturations, on the other hand, were used to develop two expert systems using artificial neural networks (ANN); one to identify the ROZ and MP, and the other to predict oil saturation. These systems were also supported by a classification and regression tree (CART) analysis to delineate the rules that lead to classifications of zones.
Results showed that expert systems developed and calibrated by combining core and well log data can identify MP and ROZ with a success score of more than 90%. Saturations within these zones can be predicted with a correlation coefficient of around 0.6 for testing and 0.8 for training data. The analyses showed that neutron porosity and density well log readings are the most influential ones to identify zones in this field and to predict oil saturations in the MP and ROZ. To explain the relationships of input data with the results, a rule-based system was also applied, which revealed the underlying petrophysical differences between MP and ROZ.
This new predictive approach using machine learning techniques, could potentially address the challenges that previous studies have come up against in defining the ROZ within the formation and quantifying remaining oil saturations. The method can potentially be applied to additional fields and help reliably identify the ROZ and estimate saturations for future resource evaluations.
Low-temperature hydrous pyrolysis (LTHP) at 300°C (572°F) for 24 h released retained oils from 12- to 20-mesh-size samples of mature Niobrara marly chalk and marlstone cores. The released oil accumulated on the water surface of the reactor, and is compositionally similar to oil produced from the same well. The quantities of oil released from the marly chalk and marlstone by LTHP are respectively 3.4 and 1.6 times greater than those determined by tight rock analyses (TRA) on aliquots of the same samples. Gas chromatograms indicated this difference is a result of TRA oils losing more volatiles and volatilizing less heavy hydrocarbons during collection than LTHP oils. Characterization of the rocks before and after LTPH by programmable open-system pyrolysis (HAWK) indicate that under LTHP conditions no significant oil is generated and only preexisting retained oil is released. Although LTHP appears to provide better predictions of quantity and quality of retained oil in a mature source rock, it is not expected to replace the more time and sample-size efficacy of TRA. However, LTHP can be applied to composited samples from key intervals or lithologies originally recognized by TRA. Additional studies on duration, temperature, and sample size used in LTHP may further optimize its utility.
The quantity and quality of oil retained in mature source rocks are important attributes in determining the potential of tight-oil accumulations. Retort methods using crushed rock such as “Tight Rock Analysis” (TRA) have been used to determine oil quantities (Handwerger et al. 2011 and 2012). Released TRA oil quantities are determined by volatilization of the retained oil in open-system pyrolysis at 316°C (600°F). Although this retort approach provides a rapid method for evaluating retained oil in numerous core samples in a timely manner, volatilization is not operative in the subsurface extraction of oil from tight-oil accumulations. As a result, TRA may not provide an accurate account of the quantity or quality of retained oil. Low-temperature hydrous pyrolysis (LTHP) provides an alternative to acquiring quantities and quality of retained oil in mature source rocks. LTHP, like TRA uses mature source rocks that are crushed between 12 and 20 mesh size. However, LTHP heats the rock in the presence of liquid water in a closed system at 300°C (572°F) for 24 hours. This condition is below the thermal-stress level typically required to generate oil from the thermal decomposition of bitumen and kerogen, but sufficient to release retained oil in a mature source rock. Under this condition, thermal expansion of pore fluids and reduced capillary forces releases retained oil, which accumulates on the water surface in the reactor during heating. The chalk and marlstone sequence of the Cretaceous Niobrara Formation in the Denver Basin provides an excellent test of this approach with both lithologies being a source and reservoir of retained oil to different degrees.
The well-documented 2014-2016 exfoliation of Twain Harte Dome, located in the western foothills of California’s Sierra Nevada Mesozoic granitic batholith, provides an unprecedented opportunity to study progressive fracture of exfoliation domes. The dome seemingly spontaneously fractured on five occasions during the late summer of 2014, with subsequent fracturing in the summers of 2015 and 2016. The warm-weather timing of all events strongly suggests that thermally-induced environmental conditions are responsible for triggering exfoliation. Some of the exfoliation events consisted of rock slabs being thrust ~40 cm into the air in a matter of seconds, with both audible and observable explosive-like energy release. We initiated a research project to investigate the triggering conditions and fracture mechanics of dome exfoliation under this active setting to serve as a proxy for understanding precursor signals that might occur prior to rock falls on near-vertical cliffs. Here we present the portion of our data that captures the collapse of the slab that was deformed during the 2014 events. The data suggest that the collapse and other ongoing deformation is occurring as a result of progressive fracture driven by diurnal heating and cooling.
2 Exfoliation sheet monitoring
Following the August 2014 fracture events, we installed three vibrating-wire-type strain gauge “crackmeters” (Geokon modified 4420; see Collins & Stock 2016) between a newly detached portion of the uppermost slab of the dome and the intact rock beneath (Fig. 1). Two crackmeters monitored open fractures and a third crackmeter acted as a control to differentiate between thermal strains in the instrumentation verses thermal strains in the granitic slab. We also deployed pendent-type temperature and light sensors (Onset Hobo UA-002-08) to act as a proxy for surface rock temperatures. Our instrumentation captured data at 5-minute intervals beginning on 22 August 2014 for 41 days until the upper slab detached and crushed the crackmeters.
Attanasi, Emil D. (U.S. Geological Survey)
The precipitous decline in oil prices during 2015 has forced operators to search for ways to develop low-cost and low-risk oil reserves. This study examines strategies to low cost development of legacy reservoirs, particularly those which have already implemented a carbon dioxide enhanced oil recovery (CO2 EOR) program. Initially the study examines the occurrence and nature of the distribution of the oil resources that are targets for miscible and near-miscible CO2 EOR programs. The analysis then examines determinants of technical recovery through the analysis of representative clastic and carbonate reservoirs. The economic analysis focusses on delineating the dominant components of investment and operational costs. The concluding sections describe options to maximize the value of assets that the operator of such a legacy reservoir may have that include incremental expansion within the same producing zone and to producing zones that are laterally or stratigraphically near main producing zones. The analysis identified the CO2 recycle plant as the dominant investment cost item and purchased CO2 and liquids management as a dominant operational cost items. Strategies to utilize recycle plants for processing CO2 from multiple producing zones and multiple reservoir units can significantly reduce costs. Industrial sources for CO2 should be investigated as a possibly less costly way of meeting EOR requirements. Implementation of tapered water alternating gas injection schemes can partially mitigate increases in fluid lifting costs.