Two-Dimensional Soil Geometric Tortuosity Model Based on Porosity and Particle Arrangement

Porosity and particle arrangement are important parameters affecting soil tortuosity, so it is of great significance to determine the intrinsic relationship between them when studying soil permeability characteristics. Theoretical derivation and geometric analysis methods are used to derive a two-dimensional geometric tortuosity model. The model is a function of particle arrangement parameters (m and θ) and porosity. An analysis of the model and its parameters shows that: (1) The arrangement of particles is one of the reasons for the different functional relationship between tortuosity and porosity, which proved that the tortuosity is not only related to the porosity but also affected by the particle arrangement. (2) The greater the anisotropy parameter m is, the greater the tortuosity is, indicating m varies when fluid passes through the soil from different sides resulting in different values of permeability. (3) The tortuosity increases with the increase in the blocking parameters θ. (4) With increasing porosity, the influence of the parameters m and θ on the tortuosity gradually decreases, suggesting that the influence of particle arrangement on tortuosity gradually decreases. The results presented here increase the understanding of the physical mechanisms controlling tortuosity and, hence, the process of fluid seepage through soil.

Multiscale Multiphysics Modeling of the Infiltration Process in the Permafrost

In this work, we design a multiscale simulation method based on the Generalized Multiscale Finite Element Method (GMsFEM) for numerical modeling of fluid seepage under permafrost condition in heterogeneous soils. The complex multiphysical model consists of the coupled Richards equation and the Stefan problem. These problems often contain heterogeneities due to variations of soil properties. For this reason, we design coarse-grid spaces for the multiphysical problem and design special algorithms for solving the overall problem. A numerical method has been tested on two- and three-dimensional model problems. A a quasi-real geometry with a complex surface is considered for the three-dimensional case. We demonstrate the efficiency and accuracy of the proposed method using several representative numerical results.

Morphology, internal architectures and formation mechanisms of mega-pockmarks on the northwestern South China Sea margin

Pockmarks, as depression morphology related to fluid escape on the seafloor, are revealed by three-dimension (3D) seismic data on the northwestern South China Sea (SCS) margin. The pockmarks can be classified into two groups by their various shapes in plan-view, which are circular group and elongating group. These pockmarks in the study area could be defined as mega-pockmarks, as their maximum diameters can reach to 7.5 km. They commonly develop more than one crater, which are central crater and secondary crater. The seismic data illuminated their complicated internal architectures in the subsurface, as well as their evolution periods, such as initiation stage, mature stage and abandonment stage. According to the buried structures and their genesis mechanism, the mega-pockmarks could be classified into linear faults-associated pockmarks and volcano-associated pockmarks. The linear faults-associated pockmarks root on the top Middle Miocene, where the linear faults distribute. The linear faults on the top of fluid reservoir in Middle Miocene act as conduits for fluid seepage. The fluid seepage is driven by the break of balance between the hydrostatic and pore pressure. When the fluid seepage initiate, they will migrate along the linear faults, making the linear feature of pockmarks on the seafloor. Both thermogenic gas from deep intervals and biogenic gas in shallow intervals may be fluid source for the genesis of pockmarks. On the other hand, the volcanic activities control the genesis and evolution of volcano-associated pockmarks. The volcano-associated pockmarks root on the craters of volcanoes. The volcanoes underneath the pockmarks provide volcanic hydrothermal solutions, such as phreatomagmatic eruptions through the volcanic craters. The confined fluid seepages make the pockmarks on exhibiting more circular shape on the seafloor. Long-term, multi-episode fluid expulsions generate the complicated internal architecture that leads to multi-cratered mega-pockmarks on the northwestern margin of SCS.

Influence of Pore Throat Structure and the Multiphases Fluid Seepage on Mobility of Tight Oil Reservoir

Mobility is the main factor restricting the production of tight oil. In order to explore the influence of pore throat structure and fluid seepage on the mobility, six tight sandstone samples are selected by high-pressure mercury intrusion, nuclear magnetic resonance, water driving oil experiments, and oil-water relative permeability experiments to discuss the influence of pore structure and multiphases on the mobility of tight oil. The results indicate that with the increase in effective porosity, more oil and water are exchanged, and the mobility of the oil phase is enhanced. The large pore is positively correlated with the mobility of tight oil while the relationship between the mobility of small pore and effective porosity remains unclear. Particularly, the mobility of the tight oil is determined by the matching relationship between the pore throat radius and the sorting of the tight reservoir. Specifically, the smaller the two-phase copermeation zone, the greater the bound water saturation; the greater the slope of the oil phase permeability curve, the less the space for the two phases to flow together; the more the oil blocked by water in the reservoir, the worse the phase mobility. The mobility of tight oil can be divided into four categories by pore throat radius, pore throat sorting coefficient, and bound water saturation.

Shallow-marine serpentinization-derived fluid seepage in the Upper Cretaceous Qahlah Formation, United Arab Emirates

Serpentinization of ultramafic rocks in the sea and on land leads to the generation of alkaline fluids rich in molecular hydrogen (H2) and methane (CH4) that favour the formation of carbonate mineralization, such as veins in the sub-seafloor, seafloor carbonate chimneys and terrestrial hyperalkaline spring deposits. Examples of this type of seawater–rock interaction and the formation of serpentinization-derived carbonates in a shallow-marine environment are scarce, and almost entirely lacking in the geological record. Here we present evidence for serpentinization-induced fluid seepage in shallow-marine sedimentary rocks from the Upper Cretaceous (upper Campanian to lower Maastrichtian) Qahlah Formation at Jebel Huwayyah, United Arab Emirates. The research object is a metre-scale structure (the Jebel Huwayyah Mound) formed of calcite-cemented sand grains, which formed a positive seafloor feature. The Jebel Huwayyah Mound contains numerous vertically orientated fluid conduits containing two main phases of calcite cement. We use C and O stable isotopes and elemental composition to reconstruct the fluids from which these cements precipitated and infer that the fluids consisted of variable mixtures of seawater and fluids derived from serpentinization of the underlying Semail Ophiolite. Based on their negative δ13C values, hardgrounds in the same section as the Jebel Huwayyah Mound may also have had a similar origin. The Jebel Huwayyah Mound shows that serpentinization of the Semail Ophiolite by seawater occurred very soon after obduction and marine transgression, a process that continued through to the Miocene, and, with interaction of meteoric water, up to the present day.

Numerical Simulation on Mesoscale Mechanism of Seepage in Coal Fractures by Fluid-Sloid Coupling Method

Trying to reveal the mechanism of gas seepage in coal is of significance to both safe mining and methane exploitation. A series of FEM numerical models were built up and studied so as to explore the mesoscale mechanism of seepage in coal fractures. The proposed mesoscale FEM model is a cube with micron fractures along three orthogonal directions. The distribution of velocity and pressure under fluid-solid coupling was obtained, and furthermore, the seepage flow flux and an equivalent permeability of the whole model were calculated. The influences of fracture width, outlet velocity, and in situ stress level on seepage were investigated. The numerical results show that nonlinear Darcy seepage occurs during low velocity zone. The permeability is increased linearly with the increasing of facture width and outlet velocity. A certain change of lateral coefficient of in situ stress also affects seepage. The permeability is increased sharply once deviating the isotropic spherical stress state, but it is no longer changed obviously after the lateral coefficient has been increased or decreased more than 20%. The mesoscale seepage mechanism in coal fractures has been preliminarily revealed by considering fluid-solid coupling effect, and the key factors influencing fluid seepage in coal fractures were demonstrated. The proposed methods and results will be helpful to the further study of seepage behaviour in coal with more complex structures.

A Comparison of the Measured Properties of Annular Cement with Ultrasonic Cement Evaluation Logs

Different measurement methods have been utilized to investigate the quality of the cement sheath inside two cemented sandwich sections including high-resolution ultrasonic cement evaluation logs, analysis of samples, mechanical loading, and fluid seepage measurements. The sections were recovered from a North Sea well during a permanent plug and abandonment operation. The measurements have been analyzed with an aim to describe in detail the spatial variations in the cement properties and relate them to the logs. Ultrasonic cement evaluation logs were recorded to map the acoustic properties of the annular cement in the casing sections. Logging passes were recorded using different annular fluids and with different internal casing pressures to investigate the potential effect on the casing to cement bond response. The casing annulus was pressure tested using water and gas, and seepage rates were recorded whilst varying the annulus and the inner casing pressures. The sections tested were instrumented with an array of annulus pressure sensors. On one section, strain gauges were installed on the casing outer surface to record the transfer of strain through the cement sheath to the outer casing. The eccentricity of the inner casing was up to 70% compared with the outer casing which results in a substantial variation of the cement sheath thickness. Accordingly, the pressure sensors and strain gauge arrays were positioned to capture both axial and azimuthal variations of the cement sealing properties. Cement mechanical, chemical, and acoustic bulk properties were also measured on core plugs taken from the cement sheath. The log recordings and sensor measurements showed that the cement sheath properties vary considerably, both along the section length and from the narrow to the wide side of the annulus in the casing sandwich sections. The sealing quality of the cement sheath measured by pressure testing could be correlated with the log response. We observed a nearly linear reduction in seepage rates when increasing the inner casing pressure due to the reduction in size of the annular leakage path. Analysis of bulk properties confirm the presence of cement defects such as mud contamination and microannuli. The logs identified features related to the test cell construction that demonstrated the log spatial resolution and enabled an accurate spatial comparison to be made between the logs and cement sheath sealing properties. A comprehensive data set has been recorded on casing in casing-cemented sandwich sections with axial and azimuthal variations in the cement sheath quality. The data analysis has improved the understanding of the cement sheath mechanical properties, the seal quality, and the response of the ultrasonic cement evaluation logs.

Borehole enlargement rate as a measure of borehole instability in hydrate reservoir and its relationship with drilling mud density

Borehole collapse will pose a threat to the safety of equipment and personnel during drilling operation. In this paper, a finite element multi-field coupling model for investigating borehole collapse in hydrate reservoir was developed. In this model, fluid seepage, heat transfer, hydrate dissociation and borehole deformation are all considered. Based on which, effects of drilling fluid density on both of hydrate dissociation and borehole collapse are investigated. The investigation results show that disturbance of drilling fluid invasion to hydrate reservoir will lead to hydrate dissociation around wellbore, and dissociation range narrows obviously with the increase in drilling fluid density. When the relative fluid density is 0.98, natural gas hydrates in reservoir with a width of about 16.65 cm around wellbore dissociate completely. However, dissociation range of natural gas hydrate has decreased to 12.08 cm when the relative fluid density is 1.10. Moreover, hydrate dissociation around wellbore caused by drilling fluid invasion may lead to borehole collapse, and borehole collapse can be significantly restrained with the increase in relative fluid density. Borehole enlargement rate is 33.67% when the relative fluid density is 0.98, but nearly no collapse area displays around wellbore when the relative fluid density increases to 1.12. In addition, investigation herein can provide an idea for designing drilling fluid density in hydrate reservoir when different allowable borehole enlargement rate is considered. The minimum fluid density designed for avoiding disastrous borehole collapse increases nonlinearly when higher requirements for borehole stability are proposed.