Overpressure Dissipation Research Articles

How to consider groundwater flow systems in the Earth's Critical Zone? – Demonstration in the Central Pannonian Basin, Hungary

Study regionCentral Pannonian Basin, Hungary. Study focusCritical Zone (CZ) Science generally focuses on the soil and weathered bedrock in a few or tens of meters depth, thus influence of deeper groundwater on the CZ is understudied. Here we aim to introduce a hydrogeological methodology that can separate normal and abnormal pressure regimes and determine the groundwater flow pattern to characterize the connection of different groundwater flow systems to the CZ. Basin-scale evaluation of about 5500 measured hydraulic data were carried out by p(z) and h(z) profiles, tomographic maps and hydraulic cross sections. New hydrological insights for the RegionThree flow domains were separated and characterized. Namely, i) the uppermost topography-driven flow systems, which penetrate only a few hundred meters, ii) a deep overpressured regime below 1600–2100 m depth, which drives fluids upward; and iii) a newly identified transition zone between the former two, which gains its energy from overpressure dissipation and contains non-renewable water resources. Topography-driven flow systems and discharge areas of the transition zone, where its upwelling saline water contributes to surface salinization, are parts of the CZ. Discharge areas of the transition zone cover about 50% of the Great Hungarian Plain. The overpressured system can only influence the CZ through the transition zone. The approach and methodology can be used in any terrestrial sedimentary basin where a deep overpressured regime exists.

Impact of CO2 sequestration on pressure systems in saline aquifer: Simulation of differential pressure evolution in the caprock-reservoir

CO2 sequestration in the saline aquifer is one of the most promising means of geological sequestration, and the realization of safe CO2 sequestration without leakage is the common goal of sequestration projects. Mature continental sedimentary basins typically have a long drilling and production history, and several wellbores inevitably penetrate the saline aquifer. These wellbores have been identified as risky locations for CO2 leakage. Scale-up CO2 injection can cause large-scale perturbations to the saline aquifer pressure system, affecting the sealing integrity of wells within the sequestration site. Considering that differential pressure is a key driver of wellbore leakage, this paper establishes and validates a three-dimensional cap-reservoir model to simulate the evolution of the differential pressure between the cap-reservoir due to CO2 sequestration. The simulation results show significant differences in the magnitude and rate of pressure buildup in the cap and reservoir during injection and in the rate of overpressure dissipation after stopping injection. These differences caused the dynamic evolution of the interlayer pressure difference. They formed peaks in the injection and stopping stages, and the peak in the injection stage was higher than in the stopping stage. The peak decreases as an exponential function with increasing distance from the injection well. In the constant rate injection mode, the peak rises linearly with increasing injection rate, and the effect of injection temperature on the peak can be neglected. A sensitivity analysis of reservoir hydrogeological parameters was done. From the perspective of controlling differential pressure, an ideal saline aquifer should be characterized by deep burial, low salinity, high permeability, and large effective pore space. This study is significant for CO2 sequestration site optimization and CO2 leakage risk control.

Coupled solid-fluid response of deep tunnels excavated in saturated rock masses with a time-dependent plastic behaviour

This article provides a general numerical approach for modelling the response of deep tunnels excavated in saturated time-dependent plastic rock masses, considering a coupled solid-fluid interaction and time-dependent plastic behaviour. In order to do that, a Burgers-viscoplastic strain-softening model has been developed and implemented into the finite element method software CODE_BRIGHT, and a coupled solid-fluid model is used to simulate the interaction between solid deformations and fluid flow. Parametric analyses are then performed to analyse the influence on the tunnelling response of different time-dependent models, different standstill times and different excavation rates. It has been observed that the time-dependent model selection is crucial to simulate the response of underground excavations. Additionally, the coupled solid-fluid results are significantly different from the purely mechanical ones. The liquid pressure build-up in the vicinity of the tunnel face and the overpressure dissipation with time due consolidation can be accounted for. Moreover, the higher the excavation rate, the larger build-up of liquid pressures occurs.

Coupled poroelastic solutions for the reservoir and caprock layers with the overburden confinement effects

Coupled theory of poroelasticity is used to develop a generic analytical solution for the time-dependent solid stress and pore fluid pressure of the subsurface rocks. A uniaxial model comprising three layers of rock formations, namely, the reservoir, caprock and burden rock is considered. The model entails different mechanical and flow properties for the considered rock layers. A linear stress–displacement formulation at the interface of the caprock and overburden defined and is calibrated to account for the confinement effect of the burden rock. The closed-form solution to the considered coupled, poroelastic problem is derived in Laplace transform space. The time-domain solution is retrieved by numerical inversion of the solution in Laplace transform space.The obtained general solution is used to assess the coupled pore fluid pressure and stress evolution of rock layers in the following applied problems. (1) Caprock integrity upon fluid injection into the reservoir. (2) Seal rock efficiency of containing fluid transport from an abnormally pressured reservoir compartment. Findings indicate that the time evolution of pore fluid transport within and in between the rock layers is influenced by the relative magnitude of both flow and mechanical properties of the reservoir and caprock. In particular, the contrast between the elastic moduli of the reservoir and caprock are found to have substantial effect on the rate and magnitude of overpressure dissipation from the reservoir. Coulomb’s shear failure criterion, together with Terzaghi’s effective stress criterion for tensile failure, is used to assess the time-dependent failure tendency of the caprock upon exposure to excess pressure of the reservoir in the case of fluid injection problem. Findings suggest that higher injection rate, stiffer overburden, thinner reservoir and stiffer reservoir rock would enhance the failure tendency of the caprock.

Characterizing marshland compressibility by an in-situ loading test: design and set-up of an experiment in the Venice Lagoon

Abstract. The fate of coastal marshlands in the near future will strongly depend on their capability to maintain their elevation above a rising mean sea level. Together with the deposition of inorganic sediments during high tides, organic soil production by halophytic vegetation, and organic matter decomposition, land subsidence due to natural soil compression is a major factor controlling the actual elevation of salt-marsh platforms. Due to their high porosity and compressibility, the marsh sedimentary body undergoes large compression because of the load of overlying more recent deposits. The characterization of the geotechnical properties of these deposits is therefore of paramount importance to quantify consolidation versus accretion and relative sea level rise. However, undisturbed sampling of this loose material is extremely challenging and lab tests on in-situ collected samples are not properly representative of in-situ conditions due to the scale effects in highly heterogeneous silty soils such as those of the Venice lagoon. To overcome this limitation, an in-situ loading test was carried out in the Lazzaretto Nuovo salt-marsh in the Venice Lagoon, Italy. The load is obtained by a number of plastic tanks that are filled with seawater, reaching a cumulative load of 40 kN applied on a 2.5×1.8 m2 surface. Specific instrumentations were deployed before positioning the tanks to measure soil vertical displacement at various depths below the load (0, 10, and 50 cm) and distances (0, 40, and 80 cm) from the load centre. Moreover, six pressure transducers were used to record overpressure dissipation over time. The collected datasets will be interpreted through a 3-D flow-deformation model that, once calibrated, provides reliable estimates of the compressibility values for each monitored depth interval.

Phase difference between groundwater storage changes and groundwater level fluctuations due to compaction of an aquifer-aquitard system

Drainage and deformation of intercalated confining layers due to internal stress change (discharge/recharge cycle) in an aquifer-aquitard system not only can have great effects on the groundwater storage (GWS), but also can cause unsynchronized water level (WL) fluctuations. The phase relationship between GWS and WL is crucial for the accuracy and attribution of GWS changes. We identify the dominating episodic components of GWS and WL through the Singular Spectrum Analysis (SSA) analysis and Harmonic analysis. First, we analyzed a generic aquifer-aquitard system using numerical simulations which showed that the dissipation of overpressure from the aquitard and flow from it is the inherent cause of a phase shift between the GWS and WL observations. Water released from confining layers with compaction time constant of zero (no-delay) to century time scale results in detected phase shift between total GWS and WL in the aquifer. Then, we analyzed the complex and varied phase relationship between GWS derived using the Gravity Recovery and Climate Experiment (GRACE) data and measured WL time series in the subsiding North China Plain (NCP) aquifer. The spatially varied phase relationship between GWS and measured WL is reasonably related to varied land subsidence development features and may be related with the compaction time constant and thickness of the confining layers. Results of the generic numerical model and the NCP observations suggest that elastic/inelastic response need to be considered in the interpretation and correction of GWS changes for a compacted aquifer-aquitard system.

Permeability Evolution in Variably Glassy Basaltic Andesites Measured Under Magmatic Conditions

AbstractHeat from inflowing magma may act to seal permeable networks that assist passive outgassing at volcanic conduit margins and in overlying domes, reducing the efficiency of overpressure dissipation. Here we present a study of the evolution of permeability—measured under magmatic conditions—with increasing temperature in glassy and glass‐poor basaltic andesites from Merapi volcano (Indonesia). Whereas the permeability of glass‐poor rocks decreases little up to a temperature of 1,010°C, glassy specimens experience a pronounced decrease in permeability above the glass transition once the viscosity of the crystal suspension is low enough to relax under external stresses. Changes in temperature alone are thus not enough to significantly modify the permeability of the glass‐poor rocks that commonly form Merapi's dome. However, the presence of glass‐rich domains in a dome may lead to local sealing of the volcanic plumbing between eruptions, exacerbating localized overpressure development that could contribute to explosivity.

Numerical simulations of Holocene salt-marsh dynamics under the hypothesis of large soil deformations

Abstract Salt marshes are vulnerable environments hosting complex interactions between physical and biological processes. The prediction of the elevation dynamics of a salt-marsh platform is crucial to forecast its future behavior under potential changing scenarios. An original finite-element (FE) numerical model accounting for the long-term marsh accretion and compaction linked to relative sea level rise is proposed. The accretion term considers the material sedimentation over the marsh surface, whereas the compaction reflects the progressive consolidation of the porous medium under the increasing load of the overlying younger deposits. The modeling approach is based on a 2D groundwater flow simulator coupled to a 1D vertical geomechanical module, where the soil properties may vary with the effective intergranular stress. The model takes also into account the geometric non-linearity arising from the consideration of large solid grain movements by using a Lagrangian approach with an adaptive FE mesh. The numerical experiments show the potentiality of the proposed 2D model, which consistently integrates in modeling framework the behavior of spatially distributed model parameters. High sedimentation rates and low permeabilities largely impact on the mechanism of soil compaction following the overpressure dissipation.

Overpressure dissipation mechanisms in sedimentary sections consisting of alternating mud-sand layers

In stable subaqueous environments, sedimentary deposition often results in thick mudstone-rich successions, with mainly argillaceous sediments, containing some intercalated sandstone beds and siltstones laminae. Such depositional sequences are commonly hosting abnormally pressured fluids in some part of their structure. Abnormally pressured compartments – generally overpressured ones – are indicative of effective sealing conditions along their boundaries in order to prevent the dissipation of hydraulic pressure. This work focuses on the various mechanisms of overpressure dissipation in such overpressured mudstone-dominated systems. The study is based on the mathematical and numerical simulation of hydraulic phenomena occurring in various geometries of sandstone-mudstone series. Using 1D models, it is shown that, once overpressure is developed in such a compartment, it can be kept inside for a very long time even if no other pressure process is acting. A basic mechanism is that the mudstone-sandstone succession reacts actively to pressure dissipation: when overpressure is released outward of the compartment, subsequent compaction tends to occur in the neighbouring sediments; this in turn results in a continued pressure generation and tends to maintain the overpressure in the compartment at geological time scale. Natural hydraulic fracturing tends to dissipate overpressure but it does not release the entire overpressure because fracturing is a threshold mechanism which keeps the stress level in the seals at their hydraulic fracturing limit. The opening of faults may modify the pressure pattern through the rapid generation of efficient fluid flow paths; however, its effect is a quasi-instantaneous release of overpressures in sandstones but not in mudstones. 2D numerical simulation suggests that, once the fault is closed, the overpressures remaining in mudstones are rapidly transmitted into adjacent sandstones which then tend to recover their previous high pressure. The most efficient mechanisms of overpressure dissipation in mudstone overpressured compartments are large-scale and long-term tectonic uplift and resulting erosion of overburdens. Studies on the conditions and mechanisms of overpressure dissipation in mudstone-rich successions provide insight into processes of hydrocarbon migration and accumulation processes and the efficiency of cap rock in basins that have experienced multiple episodes of tectonic disturbance.

Exhumation charge: The last gasp of a petroleum source rock and implications for unconventional shale resources

Petroleum (oil and gas) forms from the bacterial or thermal breakdown of kerogen during progressive burial in sedimentary basins. During times of petroleum generation, kerogens in organic-rich source rocks expel petroleum to form a fluid phase in the pore system, capable of migrating under hydrodynamic and buoyancy forces to ultimately escape to the surface or accumulate within petroleum traps in the subsurface. The relative timing of petroleum charge and trap formation is a vital component in the accumulation of petroleum deposits. Exhumed basins have been historically viewed as higher-risk targets for conventional petroleum exploration because of, inter alia, the switch-off of petroleum generation in the source rock at the commencement of cooling during exhumation. However, even at the switch-off point, the source rock may retain a significant volume of petroleum sorbed in kerogen and within its pore system. Herein we demonstrate that if the source rock is exhumed to shallower depths after peak burial, pore pressure reduction and the associated volumetric expansion of the petroleum—particularly of the gaseous—phase in the pore system will result in the discharge of additional petroleum into the adjacent carrier bed or reservoir formations. Because most onshore sedimentary basins are characterized by major exhumation events at some point in their history, this represents an additional and underappreciated mechanism for a late-stage petroleum charge in exhumed sedimentary basins. The modeling also indicates that both the initial, pre-exhumation, total gas storage capacity and the exhumation gas charge are likely to be volumetrically more significant for gas-bearing source rocks that have been exposed to higher initial pressures and lower thermal gradients. The concepts presented here also have implications for petroleum resources retained within unconventional shale reservoirs because high-graded shale plays may be associated with systems where the magnitude or rate of relative overpressure dissipation has limited exhumation charge from the unconventional to conventional reservoirs within the basin.

Basin and petroleum system modelling of the East Coast Basin, New Zealand: a test of overpressure scenarios in a convergent margin

AbstractIn the East Coast Basin (ECB), an active convergent margin of the North Island, New Zealand, the smectite‐rich Eocene Wanstead Formation forms an effective regional seal, creating high overpressure in the underlying Cretaceous through Palaeocene units due to disequilibrium compaction. This study examines the evolution of pore pressure and porosity in Hawke Bay of the ECB based on stepwise structural reconstruction of a stratigraphic and structural framework derived from interpretation of a regional two‐dimensional seismic line. This framework is incorporated into a basin and petroleum system model to predict the generation, distribution, and dissipation of overpressure, and examine the influence of faults, erosion, structural thickening, and seal effectiveness of the Wanstead Formation on pore pressure evolution. We find that natural hydraulic fracturing is likely occurring in sub‐Wanstead source rocks, which makes it a favourable setting for potential shale gas plays. We use poroelastic modelling to investigate the impact of horizontal bulk shortening due to tectonic compression on pore pressure and the relative order of principal stresses. We find that shortening modestly increases pore pressure. When 5% or greater shortening occurs, the horizontal stress may approach and exceed vertical stress in the last 4 Myr of the basin's history. Shortening impacts both the magnitude and relative order of principal stresses through geological time. Due to the overpressured nature of the basin, we suggest that subtle changes in stress regime are responsible for the significant changes in structural deformational styles observed, enabling compressional, extensional, and strike‐slip fault regimes to all occur during the tectonic history and, at times, simultaneously.

Regional hydraulic behavior of structural zones and sedimentological heterogeneities in an overpressured sedimentary basin

The paper presents the results of a regional scale hydrogeological study conducted in two areas of the Pannonian Basin in Eastern and Southeastern Hungary. The purpose of the study was to investigate the role of fault zones and sedimentological heterogeneities in areas of overpressure dissipation and hydrocarbon entrapment. The Study Areas (∼10,400 km2 in total) were chosen so as to see the effects of their differing geological characteristics, specifically that of a regional scale Pre-Neogene basement high (Battonya High), and two regional scale depressions (Derecske Trough, Békés Basin). Groundwater flow patterns were inferred for both areas from distributions of fluid dynamic parameters presented on potentiometric surface maps, vertical hydraulic cross sections and pressure-vs.-elevation [p(z)] profiles. The data suggest that both fluid-flow patterns and the potential for hydrocarbon entrapment are predominantly controlled by the dissipation paths of overpressure. In turn, the routes of overpressure dissipation are controlled by the structural and sedimentological heterogeneities of the aquitards. Diagnostic relationships were found between the heterogeneities and fluid-potential anomalies, as well as hydrocarbon occurrences. The observed pattern of overpressure dissipation has allowed the identification of seven regions by seven basic types of pressure–elevation profiles, p(z). As well, a new concept, “the upper boundary of hydrocarbon migration” was proposed. Finally, based on the established diagnostic relationships and the deduced type-profiles of pressure–elevation, a hydrogeological methodology was developed which could be used in other areas and for other purposes, e.g., exploration for groundwater and geothermal resources, petroleum and in preparation for numerical modeling.

Dissipation of Overpressure into Ambient Mudrocks During Geological Carbon Dioxide Storage

Geological carbon dioxide (CO2) storage in deep geological formations can only lead to significant reductions in anthropogenic CO2 emissions if large amounts of CO2 can be stored. Estimates of the storage capacity are therefore essential to the evaluation of individual storage sites as well as the feasibility of the technology as a whole. One important limitation on the storage capacity is the lateral extent of the pressure perturbation, the radius of review of the storage project. We show that pressure dissipation into ambient mudrocks retards lateral pressure propagation significantly and therefore increases the storage capacity. For a three-layer model of an aquifer surrounded by thick mudrocks the far-field pressure is well approximated by a single-phase model. Through dimensional analysis and numerical simulations we show that the lateral pressure propagation follows a power-law that depends on a single parameter M ∼ log10(RkRSR12), where Rk and RS are the ratios of mudrock to reservoir permeability and storativity and Rl is the aspect ratio of the confined pressure plume. Both the coefficient and the exponent of the power-law are sigmoid decreasing functions of M. The M-values of typical geological storage sites are in the region where the power-law is changing rapidly. The combinations of large uncertainty in mudrock properties and the sigmoid shape lead to wide and strongly skewed probability distributions for the predicted radius of review. Therefore, the determination of the mudrock properties is an important component of the site characterization, if pressure dissipation has the potential significantly increase storage capacity. After injection the pressure will continue to diffuse and the radius of review may continue to increase, which will emphasize the significance of monitoring post-injection overpressure to secure the stability of the storage formation.

Effects of hydrocarbon generation on fluid flow in the Ordos Basin and its relationship to uranium mineralization

The Ordos Basin of North China is not only an important uranium mineralization province, but also a major producer of oil, gas and coal in China. The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies, but it has not been well understood in terms of the hydrodynamics of basin fluid flow. We have demonstrated in a previous study that the preferential localization of Cretaceous uranium mineralization in the upper part of the Ordos Jurassic section may have been related to the interface between an upward flowing, reducing fluid and a downward flowing, oxidizing fluid. This interface may have been controlled by the interplay between fluid overpressure related to disequilibrium sediment compaction and which drove the upward flow, and topographic relief, which drove the downward flow. In this study, we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure, in addition to sediment compaction and heating. Our results indicate that when hydrocarbon generation is taken into account, fluid overpressure during the Cretaceous was more than doubled in comparison with the simulation when hydrocarbon generation was not considered. Furthermore, fluid overpressure dissipation at the end of sedimentation slowed down relative to the no-hydrocarbon generation case. These results suggest that hydrocarbon generation may have played an important role in uranium mineralization, not only in providing reducing agents required for the mineralization, but also in contributing to the driving force to maintain the upward flow.

Oil generation as the dominant overpressure mechanism in the Cenozoic Dongying depression, Bohai Bay Basin, China

The Dongying depression in the Bohai Bay Basin is a young, prolific oil-producing basin in China. The gray to black mudstones, calcareous mudstones, and oil shales of the Eocene Shahejie Formation (Es3 and Es4) are the major source rocks that are primarily dominated by type I kerogens with a high total organic carbon of up to 18.6%. The Es3 interval is characterized by a sedimentation rate of up to 500 m/m.y. Widespread overpressures are present in the Eocene Es3 and Es4 intervals in the depression, with pressure coefficients up to 1.99 from drillstem tests. Among the sonic, resistivity, and density logs, only the sonic-log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified. Acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the Dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress. Pore pressures are accurately predicted in the Dongying depression using an Eaton (1972) exponent of 2.0 by comparing the predicted pressure coefficients with measured pressure coefficients. Disequilibrium compaction has been previously proposed as the sole cause of the high-magnitude overpressures in the Eocene strata of the Dongying depression because of rapid deposition of fine-grained sediments. However, our data indicate that the overpressures are caused by oil generation from the source rocks within the Es3 and Es4 intervals. The overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density, the apparent lack of correlation between mudstone densities and effective vertical stress, and the overpressured reservoir sandstones showing no anomalous high-matrix porosities or anomalous geothermal gradient. The depths to the top of the overpressure intervals range from 2000 to 3000 m (6562–9843 ft) following closely source rock depths. All the overpressured reservoirs and source rocks have a minimum temperature of approximately 87C, and overpressured source rocks generally have vitrinite reflectance (Ro) values of 0.6% or higher. Overpressures are not found in the strata within which the Ro values are less than 0.5%. The overpressured reservoirs in the Es3 and Es4 intervals are predominantly oil saturated or oil bearing. Organic-rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure. The calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal. The presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation. Overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing.

Modelling generation and dissipation of overpressure in sedimentary basins: an example from the Halten Terrace, offshore Norway

A pressure simulator modelling the dynamic overpressure history in sedimentary basins is developed using a pressure compartment methodology. Simple models for porosity reduction are used for controlling pressure generation, and different empirical models are used to estimate the lateral and vertical sealing/dissipation mechanisms. The equations describing the flow of formation water between the pressure compartments are used to calculate the resulting pressure dissipation on a geological time scale. The methodology is tested and applied to the Jurassic Fangst Group in the Halten Terrace area, offshore Norway. Dynamic allocation and an explicit numerical solution technique ensure a fast and accurate solution of the flow equations. The results suggest: (a) the probable mechanisms and processes that have generated the overpressure, (b) the timing and location of hydraulic fracturing and (c) the nature and behaviour of the overpressure in addition to the overpressure history.