Impermeable Rock Research Articles

Modeling injection-induced seismicity through calculation of radiated seismic energy

Seismicity induced by the injection of fluid into the fractured ground is one of the most challenging issues facing geothermal and deep wastewater disposal industries. This paper introduces an energy-based numerical methodology to study roles of fluid injection in triggering rupture (seismic slip) along preexisting faults. The methodology is developed in the Universal Distinct Element Code (UDEC) using its quasi-static and dynamic schemes and calculates the total seismic energy radiated by a rupture when more energy is made available in the system than can be stored or consumed. As an example of the application of the developed methodology, we study effects of fluid injection on rupture dynamics by pressurizing a single fault surrounded by impermeable rock, representing a simplified analogy for the injection process in deep wastewater disposal and geothermal activities. We discuss effects of raising the fluid pressure on initiating rupture over well-oriented (or critically loaded) and misoriented faults. Results show that fluid injection can trigger a rupture along both well-oriented and misoriented faults although the notion of seismicity may be observed along the well-oriented fault as early as the beginning of the injection process. The well-oriented fault generates higher seismic energy magnitude as more energy is available for rupture due to the higher peak shear stress and stress drop on the fault. Making simplifying assumptions, this study also found that fluid can be injected under a high-pressure increment before and after the fault initial activation while the radiated seismic energy remains relatively insignificant. However, gradually increasing the fluid pressure at the onset of rupture reduces the radiated seismic energy by 30%. Comparing the seismic moment and radiated seismic energy for each event reveals that while radiated seismic energy varies between different values of pressure increment, the calculated seismic moment stays constant, showing the possible ineffectiveness of the seismic moment in representing the intensity of injection-induced ruptures.

Vaca Muerta Rising: Building Unconventional Production From the Ground Up in Argentina

Vaca Muerta Rising: Building Unconventional Production From the Ground Up in Argentina What is unconventional exploration and production in Spanish? YPF goes with “no convencional.” The translation, not conventional, is a literal description for the sort of ultra-tight reservoir rock in the Vaca Muerta, the huge formation that the Argentine oil company says can return the country to the ranks of energy-exporting nations. Not conventional is also the essential mindset required to do something nobody else in the world has accomplished: develop a massive shale play outside the US and Canada. The potential is there. The US Energy Information Administration ranks Argentina second in the world among unconventional formations for technically recoverable gas and fourth for oil. When YPF announced the discovery of a world-class shale play in 2011, it marked the start of years of work to solve a multidimensional puzzle presenting daunting geological, business, and political problems that has stymied the global spread of unconventional development. In the 6 years since then, YPF has shown it is possible to make money drilling and completing wells when oil prices are around $50/bbl while developing a block with Chevron. Vaca Muerta Rising: Faster and Cheaper Without Losing Better After Gustavo Astie, executive manager for unconventionals at YPF, presented what he thought was an aggressive growth plan for the coming year to YPF management, he was asked: Would it be possible to go faster? The answer was, there are limits. In the past 6 years, YPF has built a foundation for profitably developing the Vaca Muerta, but it is just now beginning to see if what worked in the initial pilots can be transferred elsewhere in the huge formation. When it comes to the pace of development, YPF needs to work with corporate partners providing much-needed money as well as expertise. Those partners include some of the biggest, most technically savvy companies in the industry, which took their time getting into unconventional development. Vaca Muerta Rising: A Trip Along the Learning Curve A drive down a gravel road in the Loma Campana follows the steep early learning curve in the Vaca Muerta. There are large pump jacks on old single wells left from the early days when vertical wells were drilled on scattered sites in a place that looks a lot like West Texas. There are pads with four vertical wells forming a rectangle. That pattern reduced the time needed to drill the wells by shortening the trip from well to well for older rigs not built to walk or slide on skids. Still it used two rigs, and the wells were far less productive than horizontal ones. A flexible pipeline snakes along the edge of the road leading to a fracturing site where it supplies the fresh water needed for the job. Flexible water lines have become a common sight, replacing fleets of trucks pounding down the bumpy, unpaved roads. Vaca Muerta Rising: Forces Favoring Shale Development are Aligned in Argentina, for Now It is easy to fixate on what it will take to extract huge volumes of oil and gas from the nearly impermeable rock within the Vaca Muerta. But in terms of the future of the huge Argentine unconventional formation, “the aboveground risk is far more important in the pace of development,” said Robert Lewis, a senior research analyst covering Latin America upstream for IHS Markit. Scaling up this unconventional play will require spending billions of dollars a year and support from government policies that promote a stable investment climate, the ability to move money and goods in and out of the country, affordable deals with labor unions, and improved infrastructure. Vaca Muerta Rising: Creating a Family-Friendly Oilfield Boomtown Añelo is a small town in an arid, sparsely populated area with a new supermarket, police station, bank, skate park, hotel, and hospital. In the past decade the population has roughly tripled to 7,000, and in 5 years, it is expected to nearly triple again to 20,000. The catalyst for this boom is the development of the Vaca Muerta, an enormous unconventional oil formation that extends under the town. As the town nearest to YPF’s office for this huge play, Añelo has recently attracted a cluster of service company offices, warehouses, and equipment yards on the edge of town. And more growth is expected as the national oil company scales up development. For YPF, the goal is to turn the fast-growing town into a good place to live for workers and their families. But that is hardly a sure thing.

Three‐dimensional texture of natural pseudotachylyte: Pseudotachylyte formation mechanism in hydrous accretionary complex

AbstractMelt‐origin pseudotachylyte is the most reliable seismogenic fault rock. It is commonly believed that pseudotachylyte generation is rare in the plate subduction zone where interstitial fluids are abundant and can trigger dynamic fault‐weakening mechanisms such as thermal pressurization. Some recent studies, however, have discovered pseudotachylyte‐bearing faults in exhumed ancient accretionary complexes, indicating that frictional melting also occurrs during earthquakes in subduction zones. To clarify the pseudotachylyte generation mechanism and the variation of slip behavior in the plate subduction zone, a pseudotachylyte found in the exhumed fossil accretionary complex (the Shimanto Belt, Nobeoka, Japan) was re‐focused and microscopic and three‐dimensional observations of the pseudotachylyte‐bearing fault were performed based on optical, electron, and X‐ray microscope images. Based on the patterns contained in the fragment, the pseudotachylyte is divided into four domains, although no clear domain boundaries or layering structures are not found. Three‐dimensional observation also suggests that the pseudotachylyte were fragmented or isolated by cataclasite or carbonate breccia. The pseudotachylyte was rather injected into the surrounding carbonate breccia, which is composed of angular fragments of the host rock and a matrix of tiny crystalline carbonate. The pseudotachylyte volume was extracted from the X‐ray microscope image and the heat abundance consumed by the pseudotachylyte generation was estimated at 2.18 MJ/m2, which can be supplied during a slip of approximately 0.5 m. These observations and calculations, together with the results of the previous investigations, suggest hydrofracturing and rapid carbonate precipitation that preceded or accompanied the frictional melting. Dynamic hydrofracturing during a slip can be caused by rapid fluid pressurization, and can induce abrupt decrease in fluid pressure while drastically enhancing the shear strength of the shear zone. Consequently, frictional heating would be reactivated and generate the pseudotachylyte. These deformation processes can explain pseudotachylyte generation in hydrous faults with the impermeable wall rock.

Flood risk management in the Souss watershed

Flooding is the most devasting natural hazards that causes more damage throughout the world. In 2016, for the fourth year in a row, it was the most costly natural disaster, in terms of global economic losses: $ 62 billion, according to a Benfield’s 2016 annual report on climate and natural disasters [1]. The semi-arid to arid Souss watershed is vulnerable to floods, whose the intensity is becoming increasingly alarming and this area does not escape to the effects of this extreme event.. Indeed, the susceptibility of this region to this type of hazard is accentuated by its rapid evolution in terms of demography, uncontrolled land use, anthropogenic actions (uncontrolled urbanization, encroachment of the hydraulic public domain, overgrazing, clearing and deforestation).), and physical behavior of the environment (higher slope, impermeable rocks, etc.). It is in this context, that we have developed a strategic plan of action to manage this risk in the Souss basin in order to reduce the human, economic and environmental losses, after the modeling of the flood hazard in the study area, using georeferenced information systems (GIS), satellite remote sensing space and multi-criteria analysis techniques, as well as the history of major floods. This study, which generated the high resolution 30m flood hazard spatial distribution map of with accuracy of 85%, represents a decision tool to identify and prioririze area with high probability of hazard occurrence. It can also serve as a basis for urban evacuation plans for anticipating and preventing flood risk in the region, in order to ovoid any dramatic disaster.

Experimental Investigation of Thermal Cracking and Permeability Evolution of Granite with Varying Initial Damage under High Temperature and Triaxial Compression

Thermal cracking and permeability evolution of granite under high temperature and triaxial compression are the key to designing high‐level waste disposal sites. In this paper, uniaxial compression tests of granite specimens with different axial compression are designed, and then a solid‐head‐designed coupling triaxial testing system is applied to study thermal cracking and permeability evolution of granite specimen with different damage at different inlet gas pressures (1, 2, 4, and 6 MPa) and temperatures (ranging from 100 to 650°C). The test results show that granite, nearly impermeable rocks, can show a striking increase of permeability by heating beyond the critical temperature. When the initial axial pressure is 60% or 70% of the uniaxial compressive strength, the growth of granite permeability exhibits three stages during 100∼650°C heating process. Permeability increases by two orders of magnitude, but it does not reach the maximum value (i.e., a network of interconnected cracks is not fully formed in the specimen). With increasing initial damage, permeability shows a sharp increase. Permeability increases by three orders of magnitude, it is in equilibrium state, and a network of interconnected cracks is fully formed in the specimen. Permeability of granite has a critical temperature at which permeability increases sharply. When the temperature is lower than the critical temperature, the magnitude of permeability is 10−18 m2 with a slight increase. When temperature is higher than the critical temperature, the magnitude of permeability is 10−15 m2 with a sharp increase. The critical temperature is related to the initial damage of specimen, and the critical temperature is smaller with the initial damage going larger. Therefore, studying thermal cracking and permeability evolution of granite with different initial damage under high temperature and triaxial compression is expected to provide necessary and valuable insight into the design and construction of high‐level waste disposal structures.

Mechanizmy i uwarunkowania rozwoju progów morfologicznych Gór Stołowych = Mechanisms and controls of escarpment evolution in Poland’s Stołowe Mountains

The Stolowe Mountains,situated in the Middle Sudetes, form Poland’s only tableland, which is underlain by an alternating succession of sedimentary rocks, all of Permian and Late Cretaceous age. The morphology of the Stolowe Mountains is characterised by the presence of isolated mesas and plateaus. Nearly-level summits are bounded by precipitous escarpments up to 300 m tall. These are bipartite in profile, with stronger sandstones forming vertical cliffs and less-resistant marls and mudstones being truncated by concave slope sections. There seems to be general agreement that long-term escarpment retreat is the dominant pathway by which this tableland evolved geomorphically. However, ideas on the processes contributing to the scarp recession vary. Since the early 20th century, the dominant concept has ascribed a major role to catastrophic mass movements. Łozinski (1909) for example pointed to rock falls, and linked their origin with intense mechanical weathering. For their part, both Czeppe (1952) and Dumanowski (1961, 1967) underlined the crucial importance of subsurface water flow at the point of contact between the permeable sandstones and impermeable fine-grained rocks. In their opinion, that phenomenon results in the undercutting and destabilisation of rock faces. Pulinowa (1972, 1989) was of a similar view and suggested that plastic deformations of underlying marls and mudstones caused subsidence and the toppling of marginal parts of the sandstone caprock. Although deep clefts are a prominent feature of Mt Szczeliniec Wielki, no significant movements have been recorded in recent decades (e.g. Cacon, 2008). Thus, from a present-day standpoint, the different outlooks mentioned above can be considered highly influenced by paradigms, while lacking in support where empirical data are concerned. In contrast, systematic study based on quantitative measurements has been engaged in recently by Duszynski and Migon (2015) and Duszynski et al. (2016). They reveal that non-catastrophic disintegration of cliff lines appears to be a much more common phenomenon than rock failure. An alternative scenario assumes that the marginal parts of plateaus are separating along joints, due to underground erosion and removal of rock residuum. In this way, after a long period of time, a once-solid rock face becomes a mess of joint-bounded blocks. Transport downslope is then more apparent than real as progressively lower topographical positions are occupied, while the lower escarpment slope recedes. Landslides in the middle and lower parts of the slope (Duszynski et al., 2017), block ploughing (Pulinowa, 1989; Duszynski and Parzoch, 2016) and erosional incision (Migon and Kasprzak, 2016) all do contribute to escarpment retreat, but their role is limited to single localities. Although our knowledge regarding the evolution of the Stolowe Mountains has improved greatly, we still lack chronological data, and are hence unable to pinpoint the environmental conditions proving particularly favourable to escarpment recession. There is also a need to determine whether the arenisation process often described in the foreign literature (e.g. Wray and Sauro, 2017) is actually responsible for the slow detachment of sand grains from quartz sandstone.

Subsurface CO2 storage estimation in Bakken tight oil and Eagle Ford shale gas condensate reservoirs by retention mechanism

This paper describes the CO2 geological sequestration process in unconventional reservoirs in northern and southern United States such as Bakken tight oil and Eagle Ford shale gas condensate reservoirs. The hysteresis modelling and retention mechanism was performed in this research and this is one of the efficient and proven method to store CO2 in the subsurface. This can be achieved through CO2 EOR process while injecting CO2, the fluid will be trapped in the pore spaces between the impermeable rocks and oil can be recovered simultaneously. A total of four cases was taken for the analysis, such as the Bakken and Eagle Ford reservoirs with CO2 huff-n-puff process and another two cases with CO2 Flooding. Injection pressure, injection rate, injection time, number of cycles, carbon dioxide soaking time, fracture half-length, fracture conductivity, fracture spacing, porosity, permeability, and initial reservoir pressure as is taken as inputs and cumulative oil production, and oil recovery factor was taken as outputs. The reservoirs were modelled for 30 years of oil production and the factor year was taken as Decision Making Unit (DMU) and the models was calculated at each year. The retention was successfully calculated in all four models and percentage of retention above 90% was observed in all four cases and the injection pressure has the most dominating effect on the CO2 geological sequestration. It was also revealed that the CO2 huff-n-puff performance in Bakken reservoir is not that much more effective since the retention rate decreases during soaking period and flooding was found to be a suitable method in this formation. Even in Eagle Ford formation, the average performance of CO2 flooding process is better than the huff-n-puff, but the latter process was quite effective in this shale gas condensate reservoir.

Solution for a plane strain rough‐walled hydraulic fracture driven by turbulent fluid through impermeable rock

SummaryThe impact of turbulent flow on plane strain fluid‐driven crack propagation is an important but still poorly understood consideration in hydraulic fracture modeling. The changes that hydraulic fracturing has experienced over the past decade, especially in the area of fracturing fluids, have played a major role in the transition of the typical fluid regime from laminar to turbulent flow. Motivated by the increasing preponderance of high‐rate, water‐driven hydraulic fractures with high Reynolds number, we present a semianalytical solution for the propagation of a plane strain hydraulic fracture driven by a turbulent fluid in an impermeable formation. The formulation uses a power law relationship between the Darcy‐Weisbach friction factor and the scale of the fracture roughness, where one specific manifestation of this generalized friction factor is the classical Gauckler‐Manning‐Strickler approximation for turbulent flow in a rough‐walled channel. Conservation of mass, elasticity, and crack propagation are also solved simultaneously. We obtain a semianalytical solution using an orthogonal polynomial series. An approximate closed‐form solution is enabled by a choice of orthogonal polynomials embedding the near‐tip asymptotic behavior and thus giving very rapid convergence; a precise solution is obtained with 2 terms of the series. By comparison with numerical simulations, we show that the transition region between the laminar and turbulent regimes can be relatively small so that full solutions can often be well approximated by either a fully laminar or fully turbulent solution.

Purification of flowback fluids after hydraulic fracturing of Polish gas shales by hybrid methods

ABSTRACTHydraulic fracturing is a process that is used for the stimulation of impermeable rocks in order to release the trapped natural gas. Flowback fluid contains many different chemical ingredients used in the fracturing process; it also holds compounds leached from the bedrock. The production of a large amount of toxic fluids leads to the necessity of purification for recycling to reduce environmental impact. In this paper, the results of studies on the purification of flowback fluids by the combination of various processes such as filtration, advanced oxidation, and ion exchange are presented. The technological scheme of the treatment is proposed.

Blade-Shaped Hydraulic Fracture Driven by a Turbulent Fluid in an Impermeable Rock

AbstractWater-driven hydraulic fractures with high flow rates are more common now than ever in the oil and gas industry. Although these fractures are small, the high injection rate and low viscosit...

Fluid-driven metamorphism of the continental crust governed by nanoscale fluid flow.

The transport of fluids through the Earth's crust controls the redistribution of elements to form mineral and hydrocarbon deposits, the release and sequestration of greenhouse gases, and facilitates metamorphic reactions that influence lithospheric rheology. In permeable systems with a well-connected porosity, fluid transport is largely driven by fluid pressure gradients. In less permeable rocks, deformation may induce permeability by creating interconnected heterogeneities, but without these perturbations, mass transport is limited along grain boundaries or relies on transformation processes that self-generate transient fluid pathways. The latter can facilitate large-scale fluid and mass transport in nominally impermeable rocks without large-scale fluid transport pathways. Here, we show that pervasive, fluid-driven metamorphism of crustal igneous rocks is directly coupled to the production of nanoscale porosity. Using multi-dimensional nano-imaging and molecular dynamics simulations, we demonstrate that in feldspar, the most abundant mineral family in the Earth's crust, electrokinetic transport through reaction-induced nanopores (10-100 nm) can potentially be significant. This suggests that metamorphic fluid flow and fluid-mediated mineral transformation reactions can be considerably influenced by nanofluidic transport phenomena.

Structural controls on fluid circulation at the Caviahue-Copahue Volcanic Complex (CCVC) geothermal area (Chile-Argentina), revealed by soil CO2 and temperature, self-potential, and helium isotopes

Natural geothermal systems are limited areas characterized by anomalously high heat flow caused by recent tectonic or magmatic activity. The heat source at depth is the result of the emplacement of magma bodies, controlled by the regional volcano-tectonic setting. In contrast, at a local scale a well-developed fault-fracture network favors the development of hydrothermal cells, and promotes the vertical advection of fluids and heat. The Southern Volcanic Zone (SVZ), straddling Chile and Argentina, has an important, yet unexplored and undeveloped geothermal potential. Studies on the lithological and tectonic controls of the hydrothermal circulation are therefore important for a correct assessment of the geothermal potential of the region. Here, new and dense self-potential (SP), soil CO2 and temperature (T) measurements, and helium isotope data measured in fumaroles and thermal springs from the geothermal area located in the north-eastern flank of the Copahue volcanic edifice, within the Caviahue Caldera (the Caviahue-Copahue Volcanic Complex - CCVC) are presented. Our results allowed to the constraint of the structural origin of the active thermal areas and the understanding of the evolution of the geothermal system. NE-striking faults in the area, characterized by a combination of SP, CO2, and T maxima and high 3He/4He ratios (up to 8.16±0.21Ra, whereas atmospheric Ra is 1.382×10−6), promote the formation of vertical permeability preferential pathways for fluid circulation. WNW-striking faults represent low-permeability pathways for hydrothermal fluid ascent, but promote infiltration of meteoric water at shallow depths, which dilute the hydrothermal input. The region is scattered with SP, CO2, and T minima, representing self-sealed zones characterized by impermeable altered rocks at depth, which create local barriers for fluid ascent. The NE-striking faults seem to be associated with the upflowing zones of the geothermal system, where the boiling process produces a high vapor-dominated zone close to the surface, whereas the WNW-striking faults could act as a boundary of the Copahue geothermal area to the south.

Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR

Thermal cracking behavior of granite at high temperature and high pressure (HTHP) is the key to the performance of Hot Dry Rock (HDR) geothermal energy extraction system. In this study, permeability tests accompanying acoustic emission (AE) tests in granites are first conducted under HTHP by 600 °C 20MN servo control rock triaxial testing machine. The test results show that granites, nearly impermeable rocks, can show a striking increase of permeability by heating from the critical temperature. The growth curve of granite permeability shows two phases because of the multi-period of thermal-cracking in the heating process from room temperature to 500 °C. The coupled effect of temperature and pressure shows that critical temperature of permeability change decreases with increasing confining pressure. Then, a detailed characterization of the sample microstructure is presented using Micro-CT method. It is discovered that thermal cracking mainly occurs at grain boundaries in forms of inter-granular microcracks along apparent weaknesses, and develops with increasing temperature. Meanwhile intra-granular cracks are observed when heating to 500 °C, indicating that thermal cracking in granite under HTHP is induced by both intra-granular and inter-granular thermal stress. At last, experimental stimulation and application for geothermal mining of HDR are discussed.

Hydrogeological characterisation of groundwater over Brazil using remotely sensed and model products

For Brazil, a country frequented by droughts and whose rural inhabitants largely depend on groundwater, reliance on isotope for its monitoring, though accurate, is expensive and limited in spatial coverage. We exploit total water storage (TWS) derived from Gravity Recovery and Climate Experiment (GRACE) satellites to analyse spatial-temporal groundwater changes in relation to geological characteristics. Large-scale groundwater changes are estimated using GRACE-derived TWS and altimetry observations in addition to GLDAS and WGHM model outputs. Additionally, TRMM precipitation data are used to infer impacts of climate variability on groundwater fluctuations. The results indicate that climate variability mainly controls groundwater change trends while geological properties control change rates, spatial distribution, and storage capacity. Granular rocks in the Amazon and Guarani aquifers are found to influence larger storage capability, higher permeability (>10-4 m/s) and faster response to rainfall (1 to 3months' lag) compared to fractured rocks (permeability <10-7 m/s and lags > 3months) found only in Bambui aquifer. Groundwater in the Amazon region is found to rely not only on precipitation but also on inflow from other regions. Areas beyond the northern and southern Amazon basin depict a ‘dam-like’ pattern, with high inflow and slow outflow rates (recharge slope > 0.75, discharge slope < 0.45). This is due to two impermeable rock layer-like ‘walls' (permeability <10-8 m/s) along the northern and southern Alter do Chão aquifer that help retain groundwater. The largest groundwater storage capacity in Brazil is the Amazon aquifer (with annual amplitudes of > 30cm). Amazon's groundwater declined between 2002 and 2008 due to below normal precipitation (wet seasons lasted for about 36 to 47% of the time). The Guarani aquifer and adjacent coastline areas rank second in terms of storage capacity, while the northeast and southeast coastal regions indicate the smallest storage capacity due to lack of rainfall (annual average is rainfall <10cm).

Numerical Investigation of Temperature Distribution in a Confined Heterogeneous Geothermal Reservoir Due to Injection-production

The present study deals with the modeling of transient temperature distribution in a heterogeneous geothermal reservoir in response to the injection-production process. The heterogeneous geothermal aquifer considered here is a confined aquifer with homogeneous layers of finite length and overlain and underlain by impermeable rock media. The heat transport modes considered are advection, conduction in the geothermal reservoir and heat transfer to the confining rock media. Results show that heterogeneity plays a very significant role in determining the transient temperature distribution and controlling the advancement of the thermal front in the reservoir. A one-dimensional (1D) analytical model for temperature distribution in the geothermal reservoir is also derived in this study. Results from a simpler version of the numerical model are compared with the results from the analytical solution which are in good agreement with each other. (C) 2017 The Authors. Published by Elsevier Ltd.

Quantitative analysis and implications of drainage morphometry of the Agula watershed in the semi-arid northern Ethiopia

This study aimed at quantitative analysis of morphometric parameters of Agula watershed and its sub-watersheds using remote sensing data, geographic information system, and statistical methods. Morphometric parameters were evaluated from four perspectives: drainage network, watershed geometry, drainage texture, and relief characteristics. A sixth-order river drains Agula watershed and the drainage network is mainly dendritic type. The mean bifurcation ratio (Rb) was 4.46 and at sub-watershed scale, high Rb values (Rb > 5) were observed which might be expected in regions of steeply sloping terrain. The longest flow path of Agula watershed is 48.5 km, with knickpoints along the main river which could be attributed to change of lithology and major faults which are common along the rift escarpments. The watershed has elongated shape suggesting low peak flows for longer duration and hence easier flood management. The drainage texture analysis revealed fine drainage which implies the dominance of impermeable soft rock with low resistance against erosion. High relief and steep slopes dominates, by which rough landforms (hills, breaks, and low mountains) make up 76% of the watershed. The S-shaped hypsometric curve with hypsometric integral of 0.4 suggests that Agula watershed is in equilibrium or mature stage of geomorphic evolution. At sub-watershed scale, the derived morphometric parameters were grouped into three clusters (low, moderate, and high) and considerable spatial variability was observed. The results of this study provide information on drainage morphometry that can help better understand the watershed characteristics and serve as a basis for improved planning, management, and decision making to ensure sustainable use of watershed resources.

Modeling initiation and propagation of a hydraulic fracture under subcritical conditions

A numerical model has been developed for simulating the initiation and propagation of a plane strain or axisymmetric hydraulic fracture from an openhole wellbore in an impermeable homogeneous rock formation. The main novelty is inclusion of a subcritical growth law, thereby allowing consideration of hydraulic fracture growth when the wellbore pressure is otherwise considered insufficient to initiate fracturing. To enable tracking the moving crack front in the simulations, we develop a new tip asymptotics based on a subcritical crack growth law. The results are first validated against available analytical solutions for plane strain and axisymmetric hydraulic fractures. A comparison is presented between the solutions of the subcritical growth model and a conventional hydraulic fracture model in which fracture growth is not allowed until the stress intensity factor equals the fracture toughness of the rock. This comparison, as well as a study of the influence of the relevant parameters appearing in the subcritical growth law, indicates significant influence of subcritical growth on the evolution of the crack length and the wellbore pressure. Notably, this model provides the capability to simulate delayed growth of hydraulic fractures under pressures that are insufficient to generate instantaneous growth, which is a behavior observed in experiments but not considered by conventional hydraulic fracturing models.

A 2D volume conservative numerical model of hydraulic fracturing

A volume conservative numerical approach is suggested for the modelling of propagation of a hydraulic fracture away from an injection well. A regular finite element grid is applied at the reservoir scale without any special gridding of the fracture. Models for critical fluid pressure are used to derive exact solutions for the propagation of hydraulic fractures in impermeable and homogeneous rocks. The numerical model is verified by these exact solutions for hydraulic fracturing. Numerical examples of hydraulic fracturing of permeable and inhomogeneous rocks are given, where the volume balance is given as a function of time.

HYPOGENIC FEATURES IN MARONIA CAVE, THRACE, GREECE. EVIDENCE FROM MORPHOLOGIES AND FLUID INCLUSIONS.

The evolution of caves is usually controlled by meteoric water seepage into karst systems (epigenic process). In some cases, caves are formed by ascending fluids the aggressiveness of which is gained in depth. Such cavities are defined as hypogenic. Many caves considered previously as epigenic are now reinterpreted as hypogenic. Most Greek hypogenic caves are related with confined speleogenesis in karstic rocks near to impermeable rock exposures. At the present study the hypogenic features of the Maronia cave in Thrace of Greece, are described. The cave system is developed in a relatively thin layer of eroded Nummulitic limestones with a Middle Eocene age. Medium scale morphological characteristics such as cupolas and feeders indicate rising flow of solutions. In addition, fluid inclusion studies in selected calcite spars from the cave show elevated temperatures of formation (93 to 164ºC with two peaks at 100 and 140 ºC) from circulating hydrothermal fluids.

The acid crater lake of Taal Volcano, Philippines: hydrogeochemical and hydroacoustic data related to the 2010–11 volcanic unrest

Abstract Studies of the water chemistry of Taal crater lake and echo-sounding surveys have provided new insights into its chemical and physical dynamics. During the volcano-seismic unrest of April 2010–June 2011, the waters of Taal crater lake showed changes in chemical composition and increases in CO 2 emissions associated with the seismic unrest. The chemical and isotopic data show that the lake water has contributions from both seawater and meteoric water and receives injections of deep hydrothermal water and gases during periods of intense volcano-seismic unrest. These inflationary periods may lead to faulting of the impermeable cap rock that usually seals the deeper Taal hydrothermal reservoir in response to degassing and convective movements in the underlying Taal magma chamber.