Explicit Pore-Pressure Evaluation: Concept and Application

The article is devoted to the issue of definition in situ vertical effective stress in weakly filtering soils (WFS). Shown. that the calculation of the vertical effective stress in soils, taking into account the weighing action of water, is valid only for the special case of the hydrostatic distribution of pore pressure over depth. In this regard, it is proposed to determine the vertical effective stress by the difference between total and pore pressure, in accordance with the principle of effective stresses of K. Terzagi, as more universal. The existing variants of pore pressure calculations in weakly filtering soils are considered. The example shows that zeroing the pore pressure leads to an overestimation of the in situ effective stress in the WFS, and the hydrostatic distribution, as a rule, leads to its underestimation. A computational method for determining the vertical effective stress in the WFS by linear interpolation is presented. Formulas for determining pore pressure and vertical effective stress in the WFS layer are proposed. The features of the application of the interpolation method for different values of groundwater levels in aquifers in depth are considered. A necessary condition for calculating the pore pressure by interpolation is information about the level of groundwater in the underlying aquifer, which must be taken into account when performing engineering-geological surveys. Examples of comparison of the results obtained by various calculation methods with the data of direct measurements of pore pressure by the dissipation method and the method of filled wells are given. The analysis showed that the values of pore pressure in the WFS layers calculated by linear interpolation correspond to direct measurements of pore pressure to the greatest extent. Thus, the linear interpolation method is the most reliable calculation method and can be recommended for use when performing geotechnical calculations.

A detailed understanding of the present-day stress state is key to understanding naturally occurring seismicity and safely and successfully conducting subsurface operations. For example, in the case of geothermal energy applications, the role of faults and fractures in both productivity and induced seismicity critically depends on the present-day stress state. In addition, knowledge of the subsurface stress state is also of significant importance to mitigate drilling risks. Here, in particular, the least principal stress controls the maximum allowable wellbore pressure before the drilled formation is unintentionally fractured. In its simplest form, the state of stress can be described by the magnitude of vertical stress and two horizontal stresses and their azimuthal orientations. Ideally, the state of stress includes the counteracting effect of pore fluid pressure (short: pore pressure) and is described as the effective stress tensor (effective stress is the difference between stress and pore pressure).In this study, we investigate the magnitude of the least principal stress (minimum horizontal stress) in the North Alpine Foreland Basin in SE Germany using stress measurements such as Formation Integrity (FIT) and Leak-Off Tests (LOTs).Whilst pore pressure magnitudes have been extensively studied and published in numerous publications in the North Alpine Foreland Basin in SE Germany, knowledge of the prevailing least principal stress is still quite limited, particularly in overpressured formations. So far, only subsets of the available FIT/LOT data, mainly concentrated around Munich, have been investigated. Recently, additional FIT/LOT data became available covering greater depths (up to 4,2 km) and overpressured formations. We investigate this new dataset in combination with data from previous studies to establish a minimum horizontal stress gradient model, which considers both pore pressure and rock type. To do so, we consider the ratio between the measured minimum horizontal and vertical effective stress using a previously established pore pressure magnitude model. The resulting effective stress ratio model is tested against the least principal stress measurements of deep geothermal wells in the study area's hydrostatically and overpressured regions, showing that considering both lithological and pore pressure variations is necessary to predict the least principal stress magnitudes. The established model can be used to improve the efficiency and safety of future drilling campaigns in the study area and can also serve as an input for mechanical subsurface modelling, e.g. for a better understanding of deformation or natural and induced seismicity. 

Stress and pore pressure histories in complex tectonic settings predicted with coupled geomechanical-fluid flow models

Fabrication and verification of novel 3D FBG-based transducer for directly measuring vertical and lateral effective stresses

Drilling into a geopressured zone will generally cause a change in a number of basic formation/ drilling relationships. This change is usually seen as a reversal of a gradual depth related trend in a lithologically uniform formation. Of all the geophysical methods, the reflection seismic method is essentially the only technique used to predict pore pressures. The seismic method detects changes of interval velocity with depth from velocity analysis of the seismic data. These changes are in turn related to lithology, pore fluid type, rock fracturing and pressure changes within a stratigraphic column. When the factors affecting the velocity are understood for a given area, a successful pressure prediction can be made. For clastic environments such as the Tertiary section of the Gulf of Mexico or the Niger delta, the interval velocity of the rocks increases with depth because of compaction. In these areas, deviations from normal compaction trends are related to abnormally high pore pressures. The adapted methods provide a much easier way to handle normal compaction trend lines. In addition to well log methods, pressure detection can be obtained via drilling parameters by applying Eaton’s DXC methods. Seismic velocities have long been used to estimate pore pressure, indeed both these quantities are influenced by variations in rock properties such as porosity, density, effective stress and so on, and high pore pressure zones are often associated with low seismic velocities. Pressure prediction from seismic data is based on fundamentals of rock physics and seismic attribute analysis. This paper hence tries to assess the use of seismic waves as a viable means to calculate pore pressure, especially in areas where no prior drilling history can be found. Then we applied these methods on LAGIA-8 well, Sinai, Egypt as a case study. Pore pressure prediction from Seismic is a very essential tool to predict pore pressure before drilling operation. This could prevent the well problem as well blowout and to prevent formation damage, especially in areas where no prior drilling history can be found.

In this study, an analytical method to solve the wave-induced pore pressure and effective stress in a saturated porous seabed is proposed. The seabed is considered as a saturated porous medium and characterized by Biot’s theory. The displacements of the solid skeleton and pore pressure are expressed in terms of two scalar potentials and one vector potential. Then, the Biot’s dynamic equations can be solved by using the Fourier transformation and reducing to Helmholtz equations that the potentials satisfy. The general solutions for the potentials are derived through the Fourier transformation with respect to the horizontal coordinate. Numerical results show that the permeability and shear modulus of the porous seabed has obvious influence on the response of the seabed. The vertical effective stress and attenuation velocity of pore pressure along seabed depth increase as permeability k increases. The liquefaction may be occur at the surface of seabed when shear modulus decreasing.

Velocity-based pore pressure prediction in a basin with late-stage erosion: Delaware Basin, U.S.

The relationship developed in soil mechanics between void ratio and vertical effective stress is a simple but practical way of describing the one-dimensional, mechanical compaction of fine-grained, clastic mudstones. The compression coefficients (e 100 and ß) that define this relationship are strongly influenced by grain size, which can be simply described by the sediment's clay content. In this paper, data both from the soil mechanics literature and from geological samples from the North Sea and Gulf of Mexico are used to construct the relationship between clay content and compression coefficients. The two datasets yield different values for the coefficients, but the authors believe that the coefficients derived from the large geological database should be used to describe geological compaction. Regression of the geological data generates the following relationships between clay content and compression coefficients: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} e_{100}{\,}={\,}0.3024{\,}+{\,}1.6867clay{\,}+{\,}1.9505clay^{2}\end{document} \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} {\beta}{\,}={\,}0.0407{\,}+{\,}0.2479clay{\,}+{\,}0.3684clay^{2}\end{document} There is excellent agreement between true porosities and porosities calculated using the standard soil mechanics compaction equation with these coefficients. This indicates that the mechanical compaction of natural mudstones to 40 MPa can be adequately described using the soil mechanics approach. Practical application of the work is a two-stage process which first involves the evaluation of clay content and, thus, compression coefficients directly from wireline log data. These data can then be used to (a) help define effective stress–porosity inputs for basin models and (b) estimate mudstone pore pressures. Examples of pore pressure estimates are shown from west Africa, the North Sea and the Gulf of Mexico.

The instability region in anisotropic seabed to water waves has investigated to estimate from the five independent elastic parameters of anisotropic materials and coefficient of permeability. Especially, the phase delay, pore water pressures, effective normal stresses and shear stresses have been discussed. As the results of the analysis of the 2- and 3- dimensional stresses distribution, we have to not only judge the instability of the seabed by the area where the vertical effective stresses is negative. We also have to examine the conditions leading to the formation of non-vertical pulling destruction and shear destruction.

The chemical compaction of mudstones which is dominated by the transformation of clay minerals leads to significant changes in the mineral composition and microstructure of mudstone during process of deep burial. In particular, the transformation of smectite to illite in mudstones results in noticeable impact on the pore pressure formation and the overpressure logging responses. At present, the study about the pressurization mechanism of chemical compaction and the impact on overpressure logging responses is really weak, which made it hard to pore pressure identification and pressure prediction for deep buried formations. Taking the Paleogene Shahejie Formation in the Dongying depression of the Bohai Bay Basin in eastern China as typical case, this paper analyses the characteristics of clay mineral transformation of the Shahejie Formation in the Dongying depression, the logging responses of overpressures, and the influence of chemical compaction on the prediction of pore pressure. The results showed that the chemical compaction of mudstones changes the relationship between the petrophysical properties of mudstone and vertical effective stress and the logging responses of overpressure. The typical characteristic of chemical compaction manifested as density increase continuous with the depth. The normal compaction trends of the different compaction stages are the basis for overpressure mechanisms identification and pore pressure prediction. The depth of the rapid transformation of clay minerals has a good consistency with the top of overpressure zone (2000–2800 m) in Dongying depression, which indicates that the overpressure and its logging responses may be related to the chemical compaction of mudstones. The measured pressure in intervals deeper than 3000 m is closer to the predicted pressure based on the normal compaction trend of chemical compaction.

This study investigates the behavior of four unreinforced embankments on soft soils through numerical and parametric analyses. The embankments, composed of granular material, are 3.5 m in height and 17 m in width, and are modeled over a clay layer extending 30 m horizontally, with thicknesses of 5, 10, 15, and 20 m. The soft soil was simulated with the modified Cam Clay model, and the embankment fill with the Mohr–Coulomb criterion. Forty Abaqus simulations were performed, varying soil thickness, overconsolidation ratio (OCR), recompression index (κ ), and critical state line slope (M). The analyses evaluated settlement, horizontal displacement, pore pressure, effective vertical stress and mobilized soil volume, applying multiple normalization approaches. Results highlight the strong influence of soil thickness and κ on deformation and pore pressure, the limited effect of M and OCR for thicker layers and confirm the usefulness of mobilized volume criteria for stability evaluation.

Load estimation is a critical issue in infrastructure design. This task is usually realized using the Marston solution, which is developed for vertical trenches, without consideration of the water condition. In many cases, trenches need to be excavated with inclined walls. Also, rain, snow or other water sources may flow in the trenches where conduits are buried. Once submerged by water, the state of the backfill in such trenches may change significantly in terms of total and effective stresses. This aspect has been addressed by the authors and collaborators for vertical backfilled openings. Furthermore, solutions have been proposed for estimating the stresses in backfill within inclined wall trenches under fully drained conditions (zero pore water pressure). However, there is no analytical solution available for evaluating the total and effective stresses in backfill within inclined wall trenches. In this paper, an analytical solution is presented to evaluate total and effective vertical stresses in partly or totally submerged backfilled trenches with inclined walls. Then, numerical modeling is performed and compared to the analytical solution. Results show that the proposed solution predicts the vertical stresses at the bottom of trenches quite well. With certain precautions, it could be used to estimate the backfill loads on buried conduits in inclined wall trenches.

Burial of munitions and other unexploded ordnance (UXO) on the seafloor complicates the detection and remediation of associated threats in coastal areas. We observed up to 0.6-m burial of surrogate UXO placed near the 8-m isobath during a series of winter storm events from late January to early March 2015 in Duck, NC, USA. We used time series from a pressure sensor located roughly 1 m above the bed during the experiment to apply an existing analytical solution for dynamic poroelastic wave-sediment interaction. The analytical solution yielded the spatial and temporal distributions of wave-induced pore pressures and effective stresses, which were used to evaluate five different criteria for wave-induced sediment instability. Four of the criteria have been applied previously considering different deterministic or stochastic wave types, whereas the fifth is novel and may be regarded as a generalization of the Sleath condition for plug flow. Using the estimated maximum depths of sediment instability (or bed failure) as predictors for observed surrogate UXO burial depths, we concluded that criteria based on vertical effective stress and the vertical distribution of pore pressures were most accurate. We believe that our data and the accompanying analysis are unique in the literature, where field observations of wave pressures were combined with a dynamic wave-seafloor interaction model to make predictions of sediment failure depths that were validated against actual observations of surrogate UXO burial depths.

Pore and fracture pressure modeling using direct and indirect methods in Badri Field, Gulf of Suez, Egypt

Pore and fracture pressure determinations are key considerations for the successful planning and drilling of North Sea Central Graben High Pressure High Temperature (HPHT) wells. Knowledge of these downhole pressure constraints can have a significant impact on drilling safety and economics. In this paper we present recent advancements to a previously presented methodology. Pore and fracture pressure is determined using wireline or MWD petrophysical data and an effective stress approach. The results have increased our understanding of overpressure generation mechanisms and hydrocarbon migration in this basin. Most traditional pore pressure estimation methods use a shale disequilibrium compaction model for their calculations. We assess these methods and propose that excellent results can be obtained by deriving porosity from density or deep resistivity data. This porosity, together with a lithology estimation from the gamma ray, are input into an Effective Stress Loading limb (ESL) model that calculates pore and fracture pressures through all major lithologies. Along the North Sea Central Graben axis there are two distinct pressure domains. These are separated by a low porosity (<5%) horizontal pressure seal within the Cretaceous Chalk Group. This seal occurs between ~3.5-4km and is independent of stratigraphic level within the chalk. Above ~3.5km, rapidly deposited Tertiary shales and Upper Cretaceous chalks are on the compactional loading limb. Here, moderate overpressures are generated by disequilibrium compaction. Below ~4km, low porosity (5-10%) Mesozoic sediments are unloaded by fluid expansion mechanisms to produce extreme overpressures. The upper limit for the pore pressure is the fracture propagation pressure. When this is exceeded, hydraulic fracturing can occur and the fluids escape, allowing the fracture to close and pressure to build again. In this environment, primary hydrocarbon migration may be largely dependent on the rate of the hydraulic fracturing formed as a consequence of the extreme fluid pressures generated by fluid expansion. An understanding of these pressure generating mechanisms, together with improved porosity determinations, has led to more accurate pore and fracture pressure determinations. Implementing the results into well planning and drilling can help avoid many of the costly pressure related problems inherent to HPHT wells.