Pore Pressures In Sediments Research Articles

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

AbstractThis study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (<19 nM and <70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region.

The influence of heat perturbation on natural gas hydrate sediment in deepwater drilling and cementing process

ABSTRACT The deepwater drilling and cementing techniques achieved significant development in recent years. However, the engineering response mechanism of natural gas hydrate sediment (NGHS) during drilling and cementing process is far from clear, which challenges the safe development of offshore petroleum resources. To reveal the influence of heat perturbation on NGHS of hydration exotherm of cement slurry and drilling fluid circular temperature increment in the process of subsequent drilling operation in shallow seabed, a mathematical model of hydrate dissociation is developed to simulate the influence by using the numerical simulation approach in this paper, and taking a natural gas hydrate reservoir at Liwan slope in the northern South China Sea as an example. The mathematical model comprehensively takes into account heat conduction, heat consumption of hydrate dissociation, hydrate dissociation dynamics, fluid seepage around the wellbore, and their coupling reaction. The analysis shows that the increase of heat transmitted from drilling and cementing process to NGHS facilitates the hydrate dissociation, leading to the growth of the sediment pore pressure in the near wellbore, the fluctuation range of pore pressure is rapidly diffused and significantly larger than the wellbore radius. The gas from the hydrate dissociation will permeate to the wellbore-distant area and reform hydrate with water. The gas will not to totally reformed hydrate in the short term, partially gathering around the wellbore, and the maximum gas saturation in the sediment is between 10% and 12%. The hydrate reformation area will be several times than the wellbore radius. But the maximum hydrate saturation will not exceed a certain value. The transition area of hydrate dissociation area to hydrate reformation area is quite narrow, and the range of high pore pressure is identical to that of the hydrate reformation. The hydrate dissociation will lead to NGHS instability, so the influence of heat perturbation on NGHS should be paid attention to in deepwater drilling and cementing process.

Influence of a reservoir bed on diapirism and drilling hazards near a salt diapir: a geomechanical approach

We use a 2D forward finite-element model to explore how a laterally continuous permeable bed impacts the geological evolution and the geomechanical properties of a salt basin. We show that a permeable bed tilted by rise of a salt diapir substantially increases pore pressure in sediments near the diapir through hydraulically connecting these sediments to deep, high-overpressure sediments far from the diapir. The pore-pressure increase near the diapir has the following significant consequences: it causes a faster rise of the diapir; brings sediments near the diapir close to shear failure in situ ; causes unloading of sediments around the crest of the permeable bed; and reduces the margin of appropriate mud weights for drilling near the diapir. The rise of the salt diapir induces concentrated lateral deformation and thereby overpressure in mudrocks encasing the permeable bed in an area near the bottom of the basin. This anomalously high overpressure is in marked contrast with the overpressure in the permeable bed, resulting in a large pore-pressure gradient between the permeable bed and encasing mudrocks. Our study provides insight into the importance of permeable beds to the structural evolution of a salt basin and to the exploration and production of hydrocarbon near salt diapirs.

Coupled Numerical Modeling of Gas Hydrate‐Bearing Sediments: From Laboratory to Field‐Scale Analyses

AbstractMethane hydrates are ice‐like compounds made of gas methane and water. Hydrates are stable under low‐temperature and high‐pressure conditions constraining their occurrence in sediments to marine and permafrost settings. A shift from the stability condition triggers an endothermic hydrate dissociation with the associated release of gas and water, impacting (among others) on sediment pore pressure, temperature, and deformations. Therefore, the behavior of hydrate‐bearing sediments (HBS) is controlled by strongly coupled thermo‐hydro‐chemo‐mechanical actions. The analysis of available data from past field and laboratory experiments and the optimization of future field production studies require a formal and robust numerical framework able to capture the complex behavior of this type of soil. In this paper we used a fully coupled thermo‐hydro‐mechanical framework to study different problems involving HBS, from laboratory experiments involving natural hydrate samples to gas production tests. We also develop an analytical solution for the case of gas production via radial depressurization from a confined HBS reservoir. The analyses show the complexity of the thermo‐hydro‐mechanical phenomena associated with this type of system and contribute to better understand the behavior of HBS.

Stress and Pore Pressure in Mudrocks Bounding Salt Systems

We simulate the evolution of stress and pore pressure in sediments bounding salt systems. Our evolutionary geomechanical models couple deformation with sedimentation and porous fluid flow. We find that high differential stresses develop near rising diapirs and below salt. Salt emplacement induces significant excess pressures that are comparable to the weight of the salt sheet. In addition, we show that the shear-induced component of the excess pressures is significant. We also find that low effective stresses result in low strength, which enables salt growth. We model salt as a solid viscoplastic and sediments as poro-elastoplastic materials, and calibrate the consolidation properties based on experimental testing on smectite-rich mudrocks typical of those in the Gulf of Mexico. Our approach can be applied to design stable well bores and provide insight into macroscale geological processes. Overall, we show that transient evolutionary models can provide estimates of stress and pore pressure in many geologic systems where large strains, pore fluids, and sedimentation interact. We close with a discussion of the need to better understand material behavior at geologic stress and timescales.

Fine-grained and coarse-grained Paleogene sublacustrine fan systems in Fushan Depression, Beibuwan Basin, South China Sea: implications for sedimentary characteristics and depositional processes

The Fushan Depression is a hydrocarbon-bearing half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The sublacustrine fan systems in the Paleogene Liushagang Formation have recently become important targets in this depression, while the depositional process and detailed characterization of these systems are little known. Analysis of drilled cores, wire-line logs, and 3-D seismic survey suggest that the Fushan Depression develops two different types of sublacustrine fan systems. The fine-grained sublacustrine fan system located in the western area is mainly composed of fine- to medium-grained light gray sandstones interbedded with layers of silt-rich shales. This kind of sediment slumped from the distal bar of the delta front due to high sediment pore pressures at the slope edge as well as the activity of the Meitai Fault. By contrast, the coarse-grained sublacustrine fan system formed in the eastern area is characterized by medium to coarse gray sandstones and conglomerates with the development of various deformed lenticular beddings, deformed pillow structures, and micro-faults. Further study suggests that its sedimentation process is closely related to the geomorphology of flexure slope-break belt. The slope changed abruptly from 3°~5° to 7°~9° across the flexure slope point, increasing the accommodation space under the flexure belt. The coarse-grained sublacustrine fan formed when coarse-grained sediment spillover the slope-break toward the increased accommodation space. As the reservoir quality of these sublacustrine fan systems is well controlled by the sedimentary characteristics and sedimentation processes, our results should be of great significance for the development of future exploration.

Channeling during settling and self-weight consolidation of cohesive sediments

As a part of the settling and self-weight consolidation of fine-grained materials, a common but not widely recognized phenomenon randomly occurs, namely channeling or the formation of narrow vertical paths or “channels.” Channel formation can have important effects on the microstructure, consolidation, and shear strength characteristics of newly formed sediments. However, only a few studies have been performed on channeling. The causes of channel formation and the factors affecting it are still not fully understood. This paper presents the results of an experimental study of channel formation during settling and self-weight consolidation in fine-grained materials. Four types of fine-grained materials and different slurry concentrations were tested using large and small settling columns, and channel formation was observed using a high-resolution digital camera. The results indicate that the primary mechanism for channel formation is the coalescence of small-scale discontinuities between clusters of soil particles formed by flocculation. The stronger the degree of flocculation, the more prevalent is the formation of channels. Pore pressure measurements show the effects of channels in dissipating excess pore pressures in sediments undergoing self-weight consolidation.

Periodicities in sediment temperature time-series at a marine shallow water hydrothermal vent in Milos Island (Aegean Volcanic arc, Eastern Mediterranean)

Time-series data sets of total bottom pressure (tidal plus atmospheric), seawater temperature and sediment temperature from a marine shallow hydrothermal vent (Milos, Hellenic Volcanic Arc, Aegean Sea) were studied to determine factors influencing periodicity at the vents. Bottom pressure and vent temperature were mainly opposite in phase, with the main fluctuations of vent temperature occurring at tidal frequencies. Although the fluctuations in atmospheric pressure were of the same order as those due to tidal pressure, the contribution of atmospheric pressure was considerably weaker at diurnal frequencies. Some sudden discontinuities in sediment temperature were recorded, at least one of these may have been caused by seismic events. Seawater temperature changes were not reflected in the sediment temperature record. Transient loadings, such as tidal loadings, barometric pressure and earth tides, may affect the pore pressure in sediments, influencing fluid expulsion and sediment temperature as a consequence. Most of the contribution to the fluctuations in sediment temperature depends on tidal loadings. Gravitational forces, in the form of earth tides, can also be involved and barometric pressure is probably responsible for long period temperature oscillations.

Measurement of hydraulic properties in deep lake sediments using a tethered pore pressure probe: Applications in the Hamilton Harbour, western Lake Ontario

Estimates of groundwater seepage flux in lake bottom sediments require knowledge of the hydraulic gradient at the sediment‐surface water interface and the hydraulic conductivity of the lake‐bottom materials. In deep waters, in situ measurement of these parameters can be accomplished through the use of piezometer probes lowered and monitored remotely from a surface vessel. In this research work a new tethered piezometer probe was developed and tested for use in collecting hydraulic property data in deep‐lake bottom sediments. The probe uses a variable‐reluctance transducer to measure the differential sediment pore pressure between two ports spaced 100 cm apart. The dissipation of pore pressure transients that develop during rapid emplacement of the probe were extrapolated in time to estimate equilibrium hydraulic gradients. In addition, various data analysis techniques were evaluated for determining sediment hydraulic conductivity and specific storage through interpretation of the pore‐pressure dissipation data. The probe was used to estimate groundwater seepage in the bottom sediments of the Hamilton Harbour, at the western end of Lake Ontario. Upward gradients were measured at nine locations within the harbor ranging from 0.010 to 0.425 and a downward gradient of −0.015 was recorded at one site along the harbor's eastern boundary. Hydraulic conductivities determined from pore‐pressure dissipation over time ranged from 6.9 × 10−9to 4.8 × 10−7 m/s. Specific storage values ranged from 0.08 to 0.19 m−1. Calculated average linear seepage velocities ranged from 4.3 × 10−8 to −8.5 × 10−9 m/s. The groundwater contribution to the harbor through the deeper, fine‐grained sediments was estimated to be 9.1 × 10−2 m3/s, or 2.9 × 106 m3/yr. This represents approximately 1.0% of the harbor basin's total volume, 15% of precipitation's contribution, 1.2% of the contribution of surface inflows (excluding the Burlington ship canal) and 0.22% of the total surface outflow passing through the Burlington shipping canal, which connects the harbor to Lake Ontario.

A Multi-Sensor Piezometer for Shallow Marine Sediments in Coastal Environments

Abstract A multi-sensor piezometer has been developed to measure dynamic excess and ambient pore pressures in shallow marine sediments under the effect of surface waves, tides, and currents. The probe consists of a shaft (1.5 m long with five porous stones at subseafloor depths ranging from 0.10 to 0.93 m) and two upper chambers (one that is hermetically sealed for the transducer electronics and another that is hydraulically connected to the free water column). Pore pressures in the sediments under the effect of surface waves are measured with differential pressure transducers referenced to the mean water column pressure (cyclic component removed by a specially designed cyclic damping system). Field deployments of the probe follow a calibration and verification sequence to assure the proper function of all necessary components. This sequence involves laboratory and dockside calibrations using a static fluid reservoir technique and the measurement of differential pressures in the free water column prior to insertion. Calibration and pore pressure data from one deployment in 9.4 m of water off the Gulf of Mexico Coast near Biloxi, Mississippi show excellent precision and cyclic response.

Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments. From our modeling, we conclude that significant overpressures have been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated. We further predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the oil window remained at approximately 2.5±0.5 km below sea floor (bsf). Similarly, the depth to the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively constant position of the top of the oil window defines a maturation front that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsidence.

Lancelot—a seabed piezometric probe for geotechnical studies

A seabed instrument called Lancelot has been developed to measure pore pressure characteristics within potentially unstable marine sediment deposits, in any water depth. An estimate of the coefficient of consolidation can be obtained from the sediment pore pressure dissipation response that occurs following penetration into the seabed. This data can be used to calculate the coefficient of hydraulic conductivity, which is useful in analyzing the seepage velocity of pore fluids moving within the sediment mass. Pore-water pressures in excess of the normal hydrostatic profile are detected and used in static and dynamic liquefaction analyses, with a view toward understanding the origin of unusual seabed features such as pockmarks, as well as short-term instability caused by wave loading. An example of static liquefaction due to seepage stress is given for sites in and adjacent to the well-known 1929 Grand Banks debris flow, along with an example of incomplete liquefaction caused by dynamic wave loading within Miramichi Inner Harbor, New Brunswick. The adoption of in situ measurement techniques is shown to produce data of a quality not normally obtainable from recovered core samples, due to pressure relief and sampling disturbance effects.

Pore pressures in marine sediments: An overview of measurement techniques and some geological and engineering applications

Pore pressures in the seabed are extremely sensitive to any imposed stress because of the low permeabilities commonly exhibited by marine sediments. Consequently, the measurement of sediment pore pressures can be used to infer either the nature of the imposed stress (if the sediment properties are known) or the physical properties of the sediment (if the imposed stresses are known). Stresses of many different types may be exerted on the seabed either through hydrostatic forces (e.g. tidal and wave effects), or directly by lithospheric forces (e.g. tectonic and thermal forces). Several techniques for measuring in situ pore pressures in the upper few metres of sediments have been developed, and one instrument, the PUPPI, will operate autonomously in water depths up to 6000 m. Basic sediment properties and processes can already be inferred from pore pressure responses using this technique. However, further application and development could greatly enhance its capability, especially for long-term monitoring of sediment conditions. In this Chapter, pore pressure measurement techniques are briefly reviewed and problems are highlighted. An outline is given of some of the many ways in which pore pressure measurements could be used to gain further insight into geological processes and to determine some of the pertinent sediment properties more accurately for engineering applications.

High pore pressure generation in sediments in front of the Barbados Ridge Complex

DSDP data for the Barbados Ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches. We model the evolution of pore pressure by using a finite element procedure. Three consecutive stages are recognized: (1) In the oceanic basin, sedimentation rate is low; there is little excess pore pressure. (2) Approaching the trench but before being incorporated into the accretionary prism, sediments are subjected to increasing tectonic compression as well as greater sediment overburden. Thus, high pore pressure can be created. An important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone. (3) Arcward of the trench, sediments above the decollement are offscraped while those beneath it are subducted. Fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth. This facilitates the formation of mud volcanoes, even oceanward of the deformation front.

Shelf sediments probed for instability clues

Determining the structural stability of sediments requires probing the microscopic world of soil particles to measure such unconventional quantities as the amount of water between sediment particles, or ‘pore pressure,’ and the mierostructure, or fabric, of clay sediments. The National Oceanic and Atmospheric Administration recently completed a 2‐year series of measurements of pore pressure in sediments where they lie: in their pressurized underwater environment.Richard Bennett and other NOAA researchers took measurements in the shallow marine waters in heavily drilled shelf areas south of the Mississippi Delta, using a 19‐m weighted cylindrical probe. Deployed from a ship, it digs into the soft sediment floor, sensing pore pressure at three locations along its length. This information is recorded by equipment on a nearby oil platform.

Excess pore pressure in Mississippi delta front sediments: Initial report

Abstract In September 1975, a differential piezometer probe was successfully implanted in the soft seafloor sediments of Block 28, South Pass, Mississippi Delta. The probe sensor is located approximately 6.4 m below the mudline in a water depth of 19 m, and has essentially continuously monitored excess pore pressure (the difference between sediment pore pressure and hydrostatic pressure at that depth) since installation. Excess pore pressure will be monitored until March 1976, when the probe will be recovered. Immediately after deployment, an excess pore pressure of 54 kPa was recorded. An ambient excess pore pressure of approximately 32 kPa remained after dissipation of that developed during probe installation. Because of the possible presence of gas in the sediments in this area, it is not known with certainty whether the measured excess pressure is pore water pressure, pore gas pressure, or some combination of the two. An excess pore pressure of about 32 ±4 kPa was monitored during Hurricane Eloise and...