Pore Pressure Profile Research Articles

Finite-Element Analysis of Inclined Piezocone Penetration Test in Clays

A three-dimensional finite-element analysis was performed to analyze the effect of soil anisotropy on the inclined piezocone penetration test in normally consolidated clay. The piezocone penetration was numerically simulated based on a large strain formulation using the commercial finite-element code ABAQUS, and the anisotropic modified cam clay model (AMCCM) was chosen and implemented into ABAQUS through the user subroutine UMAT. For verification purposes, numerical simulations were first performed on previously conducted calibration chamber tests, and the predicted results were compared with the measured values. For different initial stress conditions and different penetration angles, the cone tip resistance profile; excess pore pressure profile at the cone tip; typical stress, strain and excess pore pressure distributions around the cone; and excess pore pressure dissipation at the cone tip are provided. This study shows that when the initial stress state is anisotropic, the soil behavior is different under different angles of penetration.

Fluid expulsion and overpressure development during initial subduction at the Costa Rica convergent margin

A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment in which to examine the interrelationship of tectonically driven loading, compaction, and fluid expulsion. In this study, we investigate overpressure development and fluid expulsion using a numerical modeling method that allows fluid pressures, porosities, and permeabilities to evolve during subduction. Incoming sediments at the Costa Rica margin consist of ∼180 m of hemipelagic sediments overlying 200 m of pelagic carbonates. Results of the modeling suggest that a permeability–porosity relationship for the hemipelagic sediments derived from laboratory tests (log( k) = −22 + 7.8 × porosity) is compatible with pore pressures inferred from consolidation tests and compaction rates estimated from sediment porosity and thickness changes, in contrast to results of previous simplified calculations. Comparison of one and two-dimensional modeling indicates that lateral fluid flow modifies the pore pressure profile only moderately, unless the permeability of the pelagic carbonate sediment was an order of magnitude or more higher than indicated by laboratory tests (5 × 10 −16 m 2). Due to complete subduction of the incoming sediment column, high porosity, and thus high permeability sediments are directly below the decollement zone. Rapid dewatering of these sediments can support high rates of fluid flow along the decollement or upward through the wedge. However, within the first 1.5 km of subduction, drainage leads to a rapid decrease in simulated porosity (0.8 to 0.5) of the uppermost sediments, which would decrease permeabilities to 0.5% of initial values and inhibit further dewatering.

Modelling of reinforced polymer composites subject to thermo-mechanical loading

A coupled finite element model is developed to analyse the thermo-mechanical behaviour of a widely used polymer composite panel subject to high temperatures at service conditions. Thermo-chemical and thermo-mechanical models of previous researchers have been extended to study the thermo-chemical decomposition, internal heat and mass transfer, deformation and the stress state of the material. The phenomena of heat and mass transfer and thermo-mechanical deformation are simulated using three sets of governing equations, i.e. energy, gas mass diffusion and deformation equations. These equations are then assembled into a coupled matrix equation using the Bubnov–Galerkin finite element formulation and then solved simultaneously at each time interval. An experimentally tested 1.09 cm thick glass-fibre woven-roving/polyester resin composite panel is analysed using the numerical model. Results are presented in the form of temperature, pore pressure, deformation, strain and stress profiles and discussed. The maximum normal stress failure criterion is used in order to establish the load-bearing capability of the composite panel. Significant pore gas pressure build-ups (to 0.8 MPa and higher) have been perceived at high thermo-chemical decomposition rates where the material experiences a complex expansion/contraction phenomenon. It is found that the composite panel experiences structural instability at elevated temperatures up to 300°C but retains its integrity even under moderate external loading. Copyright © 2005 John Wiley & Sons, Ltd.

Deepwater hazard avoidance in a large top-hole section using LWD acoustic data

Drilling expensive and challenging wells in deepwater requires significant prewell planning to mitigate risks. Understanding the regional drilling hazards that nearby operators have encountered and correlating them to the current project must be done to gain a clear appreciation of potential problem hole sections. This paper describes a deepwater well drilled by Agip where significant efforts were taken to fully evaluate the potential for a shallow water flow in the top-hole section. Another operator in a nearby block had recently lost well to a shallow water flow. This heightened the concern of the Agip asset team so an extensive pore-pressure analysis of the surface seismic data was undertaken. The results of this analysis indicated that there was also a strong potential for a shallow water flow on this well. In order to build the best possible understanding of the pore pressure profile the surface seismic pressure analysis was taken much deeper than what is customary, in this case to the top of the salt. Prewell pore pressure models confirmed Agip's surface seismic pore pressure assessment. As a result, real-time pore pressure monitoring services were deemed essential to closely monitor the progress of the well and, with that information, make real-time adjustments to the drilling parameters (i.e. mud weight) as they became necessary. An extremely important and sometimes overlooked aspect of real-time interactivity is the definition of the role each team member has and how each team member interacts with the others. Assembling a team and defining each member's roles and responsibilities in the planning phase streamlines the time-critical decision-making process and allows for a decisive, unambiguous, work flowpath. Figure 1 shows the communication and workflow organization assigned by Agip before the well was drilled. Rig-based personnel are highlighted in green and office-based personnel in blue. The operations geologist was the …

Chemical–mechanical wellbore instability model for shales: accounting for solute diffusion

A model that combines chemical effects with mechanical effects and provides a quantitative tool for evaluating wellbore stability is presented. In the past, wellbore stability models have introduced chemical effects by adding an osmotic potential modified by a membrane efficiency to the pressure acting at the wellbore wall [Fonseca, C.F., 2000. Chemical–mechanical modeling of wellbore instability in shales. Proceeding of ETCE 2000 and OMAE 2000 Joint Conference: Energy for the New Millenium, Feb. 14–17, 2000, New Orleans, LA.]. In this paper, an entirely different approach is adopted. The fluxes of water and ions into and out of the shale are accounted for. The pressure profiles obtained using our model differ significantly from the error function decline in pressure that is predicted by earlier models. As a consequence of this near wellbore pore pressure profile, wellbore failure can now occur inside the shale not just at the wellbore wall (as predicted by earlier models). The onset of instability now depends not only on the activity of the water but also on the properties of the solutes.

Causes of disparity between predicted and measured pore pressure

The term pore pressure can be confusing. Pore pressure is predicted in relatively impermeable beds (shale and clay) and measured in reservoir quality rocks (sand). However, in many occasions, there is not a direct link between the pressure in the sand and in the sandwiching shale. Assuming such an immediacy can lead to serious drilling problems. The relationship between predicted (PPP) and measured (MPP) pore pressure is often complicated and establishing the relationship involves analysis of the geologic setting and expected hydrocarbons. This article will discuss the various causes for differing PPP and MPP and construction of an accurate pore pressure profile. In relatively clastic young sediments (Pleistocene-Tertiary), porosity indices (e.g., sonic slowness) are widely used to calculate PPP. Prior to drilling, seismic velocities are very helpful in estimating PPP. In reservoir type rock (sand, sandstone, oolites, etc.), pore pressure is measured using wireline tools and drilling stem gauges. MPP in wet sand usually follows the main hydrostatic gradient in the region as long as the formation water density stays the same. MPP in pay zones follows the gradient of the hydrocarbon which, in turn, depends on the density of the oil or gas column. The envelope shift in the pressure gradient (PG) between two compartments usually takes place in the seals (shale and clay). Most borehole problems (e.g., sloughing shale, enlarged and backed off-hole) take place when the PG slope changes from linear in the reservoir to exponential in the shale, especially near the seal base. Blow-outs, kicks, flow-kill-breakdown, and loss of circulation problems usually occur along the interface zone between the seal and the compartment below. The age of the deposits, rate of sedimentation versus rate of accommodation, structural setting of a prospect, and the fault plane lithology juxtaposition play a substantial role in pressure differential distribution in …

ABSTRACT: Pore Pressure Profile Predictions in the Challenging Supra / Sub-Salt Exploration Plays in Deep Water, Gulf of Mexico

ABSTRACT: Pore Pressure Profile Predictions in the Challenging Supra / Sub-Salt Exploration Plays in Deep Water, Gulf of Mexico

Fluid flow from pore pressure measurements off La Palma, Canary Islands

In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary Islands, are presented. The data obtained with a Pop Up Pore Pressure Instrument (PUPPI) provide constraints on the fluid circulation and its causes in a very special context: The sediment piles near an intraplate oceanic island built on the continental rise of the Northwest African Margin. The ambient pore pressures estimated from 2 to 4days long record are negative in almost all cases with values, at depths of a few meters below sea floor, usually on the order of −10 to −70Pa. Excess pore pressures develop only in the distal most areas. The permeabilities and compressibilities obtained respectively from the decay of the insertion pressures and the amplitude of the tidally induced pore pressure variations range between 2.5×10−18 and 6.6×10−16m2 and, 6.2×10−9 and 1.5×10−7Pa−1. According to these permeabilities fluid flow is estimated to be mostly downward and usually on the range between 0 and −0.3mmy−1. However, from the excess pore pressure profile a complex pattern of fluid circulation is inferred where horizontal fluid motion cannot be neglected. Horizontal flow is probably controlled by significant contrasts in the permeability of the different layers. The prevailing downward fluid flow is abnormal for a classical passive margin. We thus interpret these results as the superposition to the loss of fluids by sediment compaction (on the continental rise), of a large-scale flow system stimulated by thermal buoyancy (100km wide) related to the volcanic activity on the island of La Palma.

Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes

Drilling on Ocean Drilling Program (ODP) Leg 170, offshore Costa Rica indicates that the entire incoming sedimentary section is underthrust. Thus, observed changes in the thickness of underthrust sediments as they are progressively buried beneath the margin wedge provide a direct measure of the rate and magnitude of sediment dewatering. Laboratory consolidation tests indicate that in situ excess pore-fluid pressures within the underthrust section range from 1.3 MPa at the top of the section to 3.1 MPa near the base. The inferred pore pressure profile implies that fluids escape the uppermost sediments most rapidly, whereas the basal sediments remain essentially undrained. This interpretation suggests that the sedimentary and underlying ocean crustal hydrologic systems are decoupled. We use a simple model of fluid flow to demonstrate that dewatering of the underthrust sediments can occur via lateral flow only if sediment permeability is strongly anisotropic, or if flow is focused along permeable stratigraphic layers. If significant dewatering occurs by vertical fluid flow, it must occur within closely spaced, high-permeability conduits.

Dynamic Axial Load Transfer from Elastic Bar to Poroelastic Medium

The time-harmonic response of a cylindrical elastic bar (pile) partially embedded in a homogeneous poroelastic medium and subjected to a vertical load is considered. The bar is modeled using 1D elastic theory valid for long bars in the low-frequency range, and the porous medium using Biot's 3D elastodynamic theory. The bar is bonded to the surrounding medium along the contact surface. The problem is formulated by decomposing the bar/porous medium system into a fictitious bar and an extended porous medium. A Fredholm's integral equation of the second kind governs the distribution of axial force in the fictitious bar. The integral equation involves kernels that are displacement and strain influence functions of a poroelastic half-space subjected to a buried, uniform vertical patch load. The governing integral equation is solved by applying numerical quadrature. The solutions for axial displacement and axial force of the bar, and the pore pressure are also derived. Selected numerical results for vertical impedance, axial force, and pore pressure profiles are presented to portray the influence of bar stiffness and length/radius ratio, frequency of excitation, and poroelastic properties.

Pore Pressure Dissipation Behavior Surrounding Driven Piles and Cone Penetrometers

Driven piles are a common foundation used to support highway bridges. Data gathered from instrumented piles and piezocone tests in finegrained soils indicate that variable pore pressure profiles, which depend on the overconsolidation ratio of the soil, can develop during penetration. For piles driven in soft clays and silts, the pore pressure profiles developed show a monotonically decreasing response with time. However, for piles driven in heavily overconsolidated silts and clays, the pore pressure profiles show dilatory behavior, with the pore pressure behavior increasing from the initial measured value to a maximum, and then decreasing to hydrostatic values. A theoretical framework is presented that combines cavity expansion theory and critical state soil mechanics with an analytical solution to the radical consolidation equation to estimate the pore pressure response as a function of time and to evaluate the coefficient of consolidation of fine-grained soils on the basis of instrumented pile and piezocone data. The method is able to describe the pore pressure response curve in soils that demonstrate either monotonically decreasing or dilatory pore pressure behavior. Driven pile data from three sites are used to verify the estimations of the model.

Pore Pressure Estimation from Mudrock Porosities in Tertiary Basins, Southeast Asia

Porosity reduction during mechanical compaction of a sediment generally has been assumed to be controlled by the increase in vertical effective stress, which is convenient because vertical stress profiles may be readily calculated from density logs. Poroelasticity theory shows, however, that mean effective stress controls porosity reduction. According to published data, horizontal stresses increase with overpressure, as well as with depth, so mean stress and vertical stress profiles are poorly correlated in overpressured sections. We have used wireline logs to compare the pore pressures estimated in mudrocks by relating porosity to mean effective stress and to vertical effective stress for overpressured Tertiary sections in southeast Asia. Wells from three different basins were studied. Mudrock porosities were estimated from the sonic log response and sorted by lithology according to the natural gamma-log response. Two sets of normal compaction curves, relating porosity to mean effective stress and to vertical effective stress, were determined empirically by fitting data points where the pore pressure was thought to be hydrostatic. These curves were then used to estimate the minimum pore pressure corresponding to mudrock porosity values in the overpressured sections. The pore pressures inferred using the mean effective stress are consistent with direct measurements of pore pressure in the adjacent sands. In contrast, pore pressures inferred in mudrocks using the vertical effective stress are significantly lower for the overpressured sections, implying discontinuities in the pore pressure profiles at lithological boundaries, which cannot readily be explained. We conclude that the pore pressures estimated using the vertical effective stress are wrong and that empirical relationships between porosity and vertical effective stress should not be used for estimating pore pressures: porosity should be empirically related to mean effective stress instead.

Discharge and transport of nearly buoyant granular solids in liquids: Part II: An investigation of the effects of particle properties on mixture-discharge rates and pore-pressure profiles

Three different solid-liquid food analogues and one model food were used in a series of flow experiments in a model conical hopper and vertical stand-pipe system. The mixture discharge rates, the liquid fraction in the discharge and the differential pore pressures set up during flow along the hopper and stand-pipe walls were monitored. The liquid fraction in the discharge decreases from an asymptotic limit set at high mixture rates to values considerably less than 0.5 as the discharge rate is reduced. Individual solid particle properties affect both the value of the critical discharge rate, below which the discharge has a higher solid content and the limiting value of the discharge rate, above which the liquid content remains unchanged. The results obtained with near-spherical particles couple well with the tomographic observations of the corresponding flow fields presented in Part I (Faderani et al., 1998, Chem. Engng Sci. 53, 553), which indicate the onset of a settling suspension well within the conical hopper at low mixture-discharge rates but a packed-bed flow extending almost to the plane of the hopper orifice as the mixture rate is increased. With cylindrical particles, the packed-bed to settling suspension flow transition is postponed to occur within the vertical stand-pipe resulting in a high liquid content in the discharge even at small-mixture discharge rates. The profiles of pore pressure measured along the hopper and stand-pipe walls during steady discharge agree well with the observed flow regime transitions and the changes in the liquid content of the discharge. The values of pore pressures corresponding to the packed-bed flow regime are compared with the predictions based on a modified form of the Ergun equation first proposed by Mills Lamptey and Thorpe (1990, Chem. Engng Sci. 46, 2197). The interstitial fluid pressures measured in the suspension regime are also compared with the theoretical predictions corresponding to the incipient settling condition. Good agreement with experiment is reported in both regimes when the transition from packed-bed to settling suspension occurs at the vicinity of the hopper orifice. A simple, first-order theoretical treatment based on Wallis' Drift-Flux Model (1969, One-dimensional Two-phase Flow, McGraw-Hill, London) explains well the observed relationships between the mixture discharge rates, the voidage profiles within the flow field and the liquid content of the discharge.

An experimental study on bleeding and channelling of cement paste and mortar

Existing studies on bleeding are mostly confined to the study of the normal bleeding of cement paste. Channelled bleeding, which may occur under certain conditions during bleeding, has not been as well researched. Further, in most studies, these processes are treated as surface phenomena, and only surface measurements were made. In this study, the bleeding process, including channelling of cement paste and mortar, was investigated. The latter material is a step forward in the study of bleeding of concrete. Besides surface settlement, pore pressure and density profiles were also measured during the experiments to give a better insight into the mechanism of bleeding and channelling. The effect of bleeding and channelling on the strength development in a column was also studied. A difference in strength of about 30% was found between samples from upper and lower parts of a column. However, the effect of channels on the compressive strength is found to be negligible as the area of such channels is small compared to the total area. With these data, a better understanding of the channelling mechanism was achieved. A simple model was then used to estimate when channelling will occur.

Development of pore pressures by nonuniform electroosmosis in clays

A theory is presented for the development of nonuniform pore pressures during electrokinetic treatment caused by local deviations in the electric field intensity and soil surface chemistry. The electric field intensity and surface (zeta) potential are the driving parameters for electroosmosis in any local soil region; when these parameters vary with space, there is a nonuniform driving force that must be balanced by a decrease or increase in local pore pressure. The theory is derived for the case of a rigid capillary, and is based on the assumptions of incompressible flow, negligible consolidation, the Helmholtz-Smoluchowski equation, and constant hydraulic conductivity. Comparisons of the theory with laboratory measurements made by Eykholt in 1992 are presented for one-dimensional electroosmotic flow through saturated kaolinite plugs. Measurements of the net electroosmotic flow rate, pore pressure and electrical potential profiles, and hydraulic conductivity, can be used to assess, at least qualitatively, the relative contributions to electroosmosis in soil regions along the profile. The k eo profile simulations provided here did not correlate well with measured chemical profiles. Future success of the simulation strategy will likely depend on two factors: accurate electric field measurements and electrokinetic characterization of soil sections.

Modelling of bleeding of cement paste and mortar

Existing bleeding models are unable to explain fully the mechanism of bleeding as the effect of hydration reaction of cement is not considered. To understand the effect of hydration on bleeding, compressibility tests were first conducted for cement paste and mortar at different stages, but all within the first 120 min. Subsequently, bleeding tests were conducted for tall specimens, and, in addition to surface settlement, pore pressure and density profiles were also measured. A rheological model is proposed which can account for the effect of hydration on bleeding. This model is implemented in a self-weight consolidation bleeding model. A number of bleeding tests were conducted and the results were compared with the predictions. The good agreement shows that this model is able to explain the bleeding mechanism of cement paste and mortar, including the effect of hydration, better than previous models.

Pore pressure profiles in fractured and compliant rocks1

AbstractFluid permeability in fractured rocks is sensitive to pore‐pressure changes. This dependence can have large effects on the flow of fluids through rocks. We define the permeability compliance γ= 1/k(k/δpp)pc, which is the sensitivity of the permeability k to the pore pressure pp at a constant confining pressure pc, and solve the specific problems of constant pressure at the boundary of a half‐space, a cylindrical cavity and a spherical cavity. The results show that when the magnitude of permeability compliance is large relative to other compliances, diffusion is masked by a piston‐like pressure profile. We expect this phenomenon to occur in highly fractured and compliant rock systems where γ may be large. The pressure profile moves rapidly when fluids are pumped into the rock and very slowly when fluids are pumped out. Consequently, fluid pressure, its history and distribution around injection and production wells may be significantly different from pressures predicted by the linear diffusion equation. The propagation speed of the pressure profile, marked by the point where δpp/δx is a maximum, decreases with time approximately as and the amplitude of the profile also dissipates with time (or distance).The effect of permeability compliance can be important for fluid injection into and withdrawal from reservoirs. For example, excessive drawdown could cause near‐wellbore flow suffocation. Also, estimates of the storage capacity of reservoirs may be greatly modified when γ is large. The large near‐wellbore pressure gradients caused during withdrawal by large γ can cause sanding and wellbore collapse due to excessive production rates.

Analysis and application of in situ pore pressure measurements in marine sediments

Recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the Mid‐Atlantic Ridge using a Pop Up Pore Pressure Instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments. The in situ pore pressure recordings of 4–5 days duration contain three distinct components. The early part of the record is dominated by the decay of a pressure pulse associated with probe penetration. The rate of decay of this pulse is used to estimate in situ permeabilities, which range from 3.1 × 10−16 m2 to 6.5 × 10−15 m2. The recording also exhibits a low‐amplitude oscillation that is associated with tidal pressure variation on the seafloor. A study based on Biot's theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments, and thus the oscillation components are used to estimate these parameters. The frame compressibilities and the permeabilities estimated from tidal oscillations range from 1.2 × 10−8 Pa−1 to 4.1 × 10−8 Pa−1 and from 1.1 × 10−15 m2 to 4.8 × 10−15 m2, respectively. Laboratory‐measured permeabilities range from 7.6 × 10−16 m1 to 1.2 × 10−15 m2. Permeabilities determined by these three methods are comparable. The third component is the ambient equilibrium pore pressure. Negative pore pressure gradients were identified on all deployments. When combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2–7 mm/yr. The pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment. Extrapolated pore pressure at the top of the basement is about 80 kPa (0.8 bar) below the hydrostatic pressure.

Forward modelling of porosity and pore pressure evolution in sedimentary basins

ABSTRACTThe gravitational compaction of sediments is an important process in forward basin modelling. This paper presents a mathematical model for the one‐dimensional compaction of an accreting layer of argillaceous sediments. Realistic constitutive laws for the clay compressibility and the clay permeability, based on soil mechanics tests, were incorporated into the model. The governing equations were put in dimensionless form and the extent of abnormal pore fluid pressure development was found to depend on the sedimentation parameter, a dimensionless group representing the ratio of the sediment hydraulic conductivity to the sediment accumulation rate. The effects of clay compressibility were studied and highly colloidal clays such as montmorillonite developed higher overpressures than less compressible materials. The results also showed that overpressuring developed in shales for cases in which the clay permeability did not go to zero in the limit of zero porosity. Linear models based on simplifying assumptions inappropriate for sedimentary basins were found to give significantly different estimates for the conditions leading to overpressuring. Using reasonable parameters, the model adequately reproduced porosity and pore pressure profiles measured in the sand‐shale sequences of the South Caspian Sea.

NUMERICAL SIMULATION OF DRYING OF REFRACTORY CONCRETE

ABSTRACT The drying process of refractory concrete is simulated numerically using a newly-developed one-pint quadrature finite element algorithm. The effect of volumetric heat supply (e.g. simplified microwave heating) is also examined numerically. Time-dependent temperature, pore pressure and moisture profiles are presented at selected locations. Integrated water release curves are also given.