Pore Pressure Method Research Articles

Experimental study on the failure process and modes of loess spoil slope induced by rainfall and engineering disturbance.

In this paper, indoor model tests were conducted using image analysis, pore pressure, and displacement measurement methods to investigate the failure evolution process and modes of loess spoil slopes with various components under the influence of rainfall and artificial excavation. The results of the experiments reveal that, under the action of rainfall, there are two types of cracks-to-failure modes for pure loess spoil slopes. One involves the formation of a large gully through the dominant channel, while the other is characterized by step-by-step retreating soil damage between cracks. The failure exhibits three distinct stages, and after failure, the slope angle is relatively large (>45°). The process of rainfall-induced destruction affecting loess spoil containing 25% coarse-grained content similarly unfolds in three stages, ultimately resulting in the formation of a regional landslide. This landslide typically encompasses a broader damage range compared to pure loess spoil, albeit with a shallower depth of damage. After the landslide stops and stabilizes, a tiny slope (45°) is created (<45°). The excavation at the toe of the slope induces loess spoil damage in a progressive multi-stage receding manner. This study provides a reference and basis for disaster prevention and warning of spoiled ground in loess areas.

Estimation of Undrained Shear Strength of Overconsolidated Clay Using a Maximum Excess Pore Pressure Method Based on Piezocone Penetration Test (CPTU)

Abstract Many methods can be used to evaluate the undrained shear strength (su) in unconsolidated or normally consolidated clay. However, among the many methods, few methods can be used to evaluate the su value in overconsolidated clay. In this paper, based on the piezocone penetration test (CPTU), the maximum excess pore pressure method (Δumax) was adopted to estimate the su in overconsolidated clay. In addition, three different test sites (in China) with overconsolidated clays were selected to verify the feasibility of this method. The test results show that the maximum pore pressure cone factor (NΔumax) significantly affected the evaluation of su. Moreover, the NΔumax values (8.10, 8.70, and 8.44) for 3 test sites in Yancheng, Nanjing, and Changzhou were obtained and can provide a valuable reference for evaluating su for in situ tests with overconsolidated clay.

Study on pore pressure and fluidization evaluation method of unsaturated loess in vibration process

Study on pore pressure and fluidization evaluation method of unsaturated loess in vibration process

Evidence for the Heterogeneity of the Pore Structure of Rocks from Comparing the Results of Various Techniques for Measuring Hydraulic Properties

We applied three oscillatory methods, the previously presented axial pore-pressure and pore-flow methods, and the laboratory application of the radial oscillatory pore-flow method, and performed steady-state flow-through experiments (Darcy tests), for comparison, in experiments on samples of Westerly granite and Wilkeson sandstone. The granite and the sandstone exhibit pore spaces dominated by micro-fractures and by the granular-medium character with a connected porosity of about 1 and 10 %, respectively. Permeability determined by the axial pore-pressure method shows the closest agreement with the results of the Darcy tests. Apparent porosity and drained modulus derived from specific storage capacity deviate from measured connected porosity and reference values, respectively. The observed deviations of the hydraulic properties between methods suggest that they bear information about the structure of the pore space. Only for the sandstone, anisotropy in hydraulic properties appears to contribute to differences between the results of the various methods. We argue that oscillatory testing provides three indicators for heterogeneity, period dependence, the relation between apparent and connected porosity, and the relation between amplitude ratio and apparent penetration depth, calculated from the simple scaling law for homogeneous materials. These indicators consistently classify the samples of Wilkeson sandstone as hydraulically homogeneous and those of Westerly granite as heterogeneous.

Numerical Simulation of Oscillating Pore Pressure Experiments and Inversion for Permeability

AbstractAccurate knowledge of permeability is crucial for successful management of groundwater, petroleum resources, and subsurface contaminant remediation. The oscillating pore pressure method is a popular laboratory technique for permeability measurements of porous rock samples. We develop a novel data processing approach that utilizes a broad, multifrequency range of data and inverts it for permeability. We reprocess published data and demonstrate that our methodology outperforms traditional data reduction techniques, as our inversion results show a better fit to pressure trends. To better understand the effect of frequency on phase and amplitude data and to verify our inversion approach, we numerically simulate oscillating pore pressure experiments for four rock samples. We document a strong deviation of experimentally obtained phase data starting at 0.3 Hz oscillation frequency. A possible explanation for this deviation is poroelastic coupling during pressure diffusion. Our method can be used for robust determination of permeability and rapid prediction of experimental results using numerical simulation, ultimately improving the analysis of experimental permeability measurements.

Dynamic threshold pressure gradient in tight gas reservoir and its influence on well productivity

Flow in tight gas reservoirs shows significant non-Darcy behavior and threshold pressure gradient (TPG) due to complex geological conditions. The conventional TPG test always leads to a remarkable error because of gas slippage effect, especially for tight cores. In this paper, experimental approaches were carried out using specially designed core-flooding system to determine the accurate TPG under reservoir conditions. TPG variations of tight core at different developmental stages (different pore pressure) were investigated by using new experimental method. Results indicate that TPG of tight gas reservoir is highly effected by pore pressure and conventional experimental method will lead to a significant error: TPG obtained from conventional experimental method are much higher. The experimentally observed TPG under different pore pressure suggest that TPG is not constant during the development of reservoir, but varies with the change of pore pressure, we call it “dynamic threshold pressure gradient (DTPG)”: TPG exhibits a substantial linear increase with decreasing pore pressure. New productivity model of tight gas reservoir with DTPG effect and stress sensitivity being taken into account was established and case studied on the basis of experiments. Results show that gas well productivity with consideration of DTPG is lower than productivity obtained from conventional model which considering constant TPG.

Evaluation of Undrained Shear Strength of Clay Using the CPTU Pore Pressure Method

Difficulties in obtaining high-quality undisturbed soil samples have necessitated the increased use of cone penetration test (CPT) or piezocone penetration test (CPTU) to determine soil properties at geotechnical engineering sites. Undrained shear strength (a key parameter for most clay investigations) traditionally is evaluated by cone tip resistance or effective cone tip resistance. In this paper a method of using the clay's excess pore pressure was used to estimate the undrained shear strength. A correlation of pore pressure ratio and cone factor was discovered and compared with results from the published literature and with those from field vane tests.

Simultaneous measurements of transport and poroelastic properties of rocks.

A novel laboratory apparatus has been developed for simultaneous measurements of transport and poroelastic rock properties. These transport and poroelastic properties at reservoir pressure and temperature conditions are required inputs for various geoscience applications, such as reservoir simulation, basin modeling, or modeling of pore pressure generation. Traditionally, the transport and poroelastic properties are measured separately using, for example, the oscillating pore pressure method to measure hydraulic transport properties, static strain measurements for elastic properties, and pore volumometry for storage capacity. In addition to time, the separate set of measurements require either aliquot cores or subjecting the same core to multiple pressure tests. We modified the oscillating pore pressure method to build an experimental setup, capable of measuring permeability, storage capacity, and pseudo-bulk modulus of rocks simultaneously. We present here the test method, calibration measurements (capillary tube), and sample measurements (sandstone) of permeability and storage capacity at reservoir conditions. We establish that hydraulically measured storage capacities were overestimated by an order of magnitude when compared to elastically derived ones. Our concurrent measurement of elastic properties during the hydraulic experiment provides an independent constraint on storage capacity.

Two innovative pore pressure calculation methods for shallow deep-water formations

There are many geological hazards in shallow formations associated with oil and gas exploration and development in deep-water settings. Abnormal pore pressure can lead to water flow and gas and gas hydrate accumulations, which may affect drilling safety. Therefore, it is of great importance to accurately predict pore pressure in shallow deep-water formations. Experience over previous decades has shown, however, that there are not appropriate pressure calculation methods for these shallow formations. Pore pressure change is reflected closely in log data, particularly for mudstone formations. In this paper, pore pressure calculations for shallow formations are highlighted, and two concrete methods using log data are presented. The first method is modified from an E. Philips test in which a linear-exponential overburden pressure model is used. The second method is a new pore pressure method based on P-wave velocity that accounts for the effect of shallow gas and shallow water flow. Afterwards, the two methods are validated using case studies from two wells in the Yingqiong basin. Calculated results are compared with those obtained by the Eaton method, which demonstrates that the multi-regression method is more suitable for quick prediction of geological hazards in shallow layers.

Microstructural controls on the pressure-dependent permeability of Whitby mudstone

Abstract A combination of permeability and ultrasonic velocity measurements allied with image analysis is used to distinguish the primary microstructural controls on effective-pressure-dependent permeability. Permeabilities of cylindrical samples of Whitby mudstone were measured using the oscillating pore-pressure method at confining pressures ranging between 30 and 95 MPa, and at pore pressures ranging between 1 and 80 MPa. The permeability–effective pressure relationship is empirically described using a modified effective pressure law in terms of confining pressure, pore pressure and a Klinkenberg effect. Measured permeability ranges between 3×10 −21 and 2×10 −19 m 2 (3 and 200 nd), and decreases by approximately one order of magnitude across the applied effective pressure range. Permeability is shown to be less sensitive to changes in pore pressure than changes in confining pressure, yielding permeability effective pressure coefficients (χ) between 0.42 and 0.97. Based on a pore-conductivity model, which considers the measured changes in acoustic wave velocity and pore volume with pressure, the observed loss of permeability with increasing effective pressure is attributed dominantly to the progressive closure of bedding-parallel, crack-like pores associated with grain boundaries. Despite only constituting a fraction of the total porosity, these pores form an interconnected network that significantly enhances permeability at low effective pressures. Supplementary material: A CSV file containing all experimental conditions and a tabulation of results is available at https://doi.org/10.6084/m9.figshare.c.3785741

A review of the shale wellbore stability mechanism based on mechanical–chemical coupling theories

Wellbore instability in hard brittle shale is a critical topic related to the effective exploitation of shale gas resources. This review first introduces the physical–chemical coupling theories applied in shale wellbore stability research, including total water absorption method, equivalent pore pressure method, elasticity incremental method of total water potential and non-equilibrium thermodynamic method. Second, the influences of water activity, membrane efficiency, clay content and drilling fluid on shale wellbore instability are summarized. Results demonstrate that shale and drilling fluid interactions can be the critical factors affecting shale wellbore stability. The effects of thermodynamics and electrochemistry may also be considered in the future, especially the microscopic reaction of shale and drilling fluid interactions. An example of this reaction is the chemical reaction between shale components and drilling fluid.

Temperature influence on permeability of Sioux quartzite containing mixtures of water and carbon dioxide

CO2 sequestration in aquifers and depleted hydrocarbon reservoirs is a promising option for reducing atmospheric CO2. However, its implementation requires understanding how CO2 propagates and how it is stored in the reservoir. CO2 mixed with water exists in two forms, an immiscible phase (gas or liquid depending on the conditions of temperature and pressure) and a dissolved phase (a minor part of which reacts with H2O to form carbonic acid, H2CO3). The motion of CO2 in the reservoir is therefore a two-phase flow problem. Fluid/rock chemical reactions may also be important, but they are not considered here. When several immiscible fluids occupy the pore space, each phase may act as an obstacle to the motion of the others. Indeed, experimental evidence from oil production, vadose zone or CO2 sequestration studies [1] demonstrates that permeability is affected by the presence of a second phase in the pore space. The motion and spatial distribution of pore fluids (like water and carbon dioxide) are controlled not only by pore geometry and fluid pressure, but also by the viscosity, solubility, surface tension and wetting characteristics of each fluid phase [2], properties that usually vary with temperature. In the case of carbon dioxide and water, the solubility of CO2 is particularly sensitive to temperature. Thus, temperature variations may also affect the distribution and flow of the phases in the pore space. In this work, we measured the effect of temperature variations on the permeability of a rock saturated with a mixture of carbon dioxide and water. We used the oscillating pore pressure method (OPPM) developed by Kranz et al. [3] and Fischer et al. [4], because it minimizes fluid flux and, therefore, is not expected to disturb the phase distribution in the pore space as much as a steadily flowing fluid would. Although the method was devised to measure permeability of rocks with a single fluid phase, we can assess the effect of the second phase by comparing the permeability measurements made using a single fluid to those with a two-phase mixture.

Dynamic threshold pressure gradient in tight gas reservoir

Experimental approach was performed in this paper to study the threshold pressure gradient (TPG) in tight gas reservoir. The nonlinear flow behavior of tight gas reservoir was investigated at first, three zones of flow curve are divided to describe the nonlinear behavior of gas flow in tight formation: solid–gas viscous force zone, slippage zone and low-speed turbulent zone. On this basis, experiments using modified procedure were made in order to clarify the influence of pore pressure on TPG, results show that TPG of tight gas reservoir is highly effected by pore pressure and conventional experimental methods will lead to a significant error. And then, TPG during the development of tight gas reservoir was tested. The results we obtained demonstrated that there exists ‘Dynamic TPG Effect’ during the development process of tight gas reservoir: TPG is increasing along with the decrease of reservoir pressure (pore pressure). New concepts of ‘TPG Rising Cone’ and ‘TPG Sensitivity Coefficient’ are defined to characterize the dynamic TPG of tight gas reservoir. Finally, experiments were carried out to investigate the influence of permeability and water saturation on dynamic TPG. Results indicate that ‘TPG Rising Cone’ is more significant in reservoir with low-permeability or high water saturation.

Predrill prediction of subsalt pore pressure from seismic impedance

A target-oriented, predrill, pore-pressure prediction method using seismically estimated acoustic impedance is applied to recently acquired and processed dual-coil data in the Green Canyon and Walker Ridge areas of the Gulf of Mexico. Three deepwater subsalt wells are used in the study. One was used as a calibration well, and the other two were used as blind wells for comparing prediction results with the measured pore-pressure and mud-weight data. All three wells are in the Green Canyon area, penetrating thick salt bodies. A new seismic-inversion method is used for inversion of seismic impedance. The pore-pressure method uses a direct transform of the inverse of acoustic impedance, with two adjustable parameters. The optimization of the parameters is done through an iterative process to match the pore-pressure gradient obtained from the well acoustic impedance with those of pore-pressure measurements and mud weights within a tolerable range of those data for the calibration well. The optimized parameters are then used to transform the seismic acoustic-impedance volume to pore-pressure gradient volume. The predicted pore pressure at the blind wells from the well impedance and seismic impedance match reasonably with well data.

Finite volume coupling strategies for the solution of a Biot consolidation model

In this paper a finite volume (FV) numerical method is implemented to solve a Biot consolidation model with discontinuous coefficients. Our studies show that the FV scheme leads to a locally mass conservative approach which removes pressure oscillations especially along the interface between materials with different properties and yields higher accuracy for the flow and mechanics parameters. Then this numerical discretization is utilized to investigate different sequential strategies with various degrees of coupling including: iteratively, explicitly and loosely coupled methods. A comprehensive study is performed on the stability, accuracy and rate of convergence of all of these sequential methods. In the iterative and explicit solutions four splits of drained, undrained, fixed-stress and fixed-strain are studied. In loosely coupled methods three techniques of the local error method, the pore pressure method, and constant step size are considered and results are compared with other types of coupling methods. It is shown that the fixed-stress method is the best operator split in comparison with other sequential methods because of its unconditional stability, accuracy and the rate of convergence. Among loosely coupled schemes, the pore pressure and local error methods which are, respectively, based on variation of pressure and displacement, show consistency with the physics of the problem. In these methods with low number of total mechanical iterations, errors within acceptance range can be achieved. As in the pore pressure method mechanics time step increases more uniformly, this method would be less costly in comparison with the local error method. These results are likely to be useful in decision making regarding choice of solution schemes. Moreover, the stability of the FV method in multilayered media is verified using a numerical example.

A comparison of adaptive time stepping methods for coupled flow and deformation modeling

Many subsurface reservoirs compact or subside due to production-induced pressure changes. Numerical simulation of this compaction process is important for predicting and preventing well-failure in deforming hydrocarbon reservoirs. However, development of sophisticated numerical simulators for coupled fluid flow and mechanical deformation modeling requires a considerable manpower investment. This development time can be shortened by loosely coupling pre-existing flow and deformation codes via an interface. These codes have an additional advantage over fully-coupled simulators in that fewer flow and mechanics time steps need to be taken to achieve a desired solution accuracy. Specifically, the length of time before a mechanics step is taken can be adapted to the rate of change in output parameters (pressure or displacement) for the particular application problem being studied. Comparing two adaptive methods (the local error method—a variant of Runge–Kutta–Fehlberg for solving ode’s—and the pore pressure method) to a constant step size scheme illustrates the considerable cost savings of adaptive time stepping for loose coupling. The methods are tested on a simple loosely-coupled simulator modeling single-phase flow and linear elastic deformation. For the Terzaghi consolidation problem, the local error method achieves similar accuracy to the constant step size solution with only one third as many mechanics solves. The pore pressure method is an inexpensive adaptive method whose behavior closely follows the physics of the problem. The local error method, while a more general technique and therefore more expensive per time step, is able to achieve excellent solution accuracy overall.

間隙圧振動法および定差圧流量法による岩石の気体浸透率測定

間隙圧振動法および定差圧流量法による岩石の気体浸透率測定

A poroelastic solution of the oscillating pore pressure method to measure permeabilities of “Tight” rocks

A poroelastic solution of the oscillating pore pressure method to measure permeabilities of “Tight” rocks

Real-Time Pore Pressure and Fracture-Pressure Determination in All Sedimentary Lithologies

Summary Pore pressure and fracture gradient are the two natural limits that exert the greatest influence on drilling costs and safety. Traditional empirical "pore pressure" models are limited to one lithology type (shale) and rely on incorporating petrophysical or drilling data vs. depth trend lines. We describe a new method that quantifies the effective-stress law, p = S − σv. This method uses petrophysical data (gamma ray, resistivity, density) and mineralogic stress/strain relationships to calculate pore pressure and fracture gradient, on a foot-by-foot basis, through all sedimentary rock types. Cretaceous marls and limestones have proven to be an obstacle for traditional pore-pressure evaluation methods. With numbers of high-pressure exploration wells in the North Sea Central graben increasing, there is a need for a better understanding of pressures through the Cretaceous and into the pressured formations below. This effective-stress-law method, which is used to determine pore pressure/fracture gradient, has been tested successfully in the complex limestone/shale and sandstone-shale sequences of the North Sea as an aid to well planning and real-time drilling operations decision making by use of measurement while drilling (MWD) petrophysical data. This method has recently been used successfully on two Central graben wells. BP drilled a high pressure/high temperature (HPHT) well in the second quarter of 1993 and used on-site pore pressure and fracture gradient (PP/FG) calculations, along with other more traditional pore-pressure methods, to help set an intermediate casing string at an optimum depth. We discuss results of these case studies and technical content of this pore-pressure method.

Hydraulic diffusivity measurements on laboratory rock samples using an oscillating pore pressure method : Kranz, R L Int J Rock Mech Min SciV27, N5, Oct 1990, P345–352

Hydraulic diffusivity measurements on laboratory rock samples using an oscillating pore pressure method : Kranz, R L Int J Rock Mech Min SciV27, N5, Oct 1990, P345–352