scholarly journals Fluid pressure heterogeneity during fluid flow in rocks: new laboratory measurement device and method

2021 ◽  
Vol 225 (2) ◽  
pp. 968-983
Author(s):  
Nicolas Brantut ◽  
Franciscus M Aben
Keyword(s):  
Pore Pressure ◽  
Transient Flow ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Dead Volume ◽  
Low Permeability ◽  
Pore Fluid Pressure ◽  
Working Pressure ◽  
New Type ◽  
Internal Pore

SUMMARY We present a new type of transducer capable of measuring local pore fluid pressure in jacketed rock samples under elevated confining pressure conditions. The transducers are passive (strain-gauge based), of small size (7 mm in diameter at the contact with the rock and around 10 mm in length), and have minimal dead volume (a few mm3). The transducers measure the differential pressure between the confining fluid and the internal pore pressure. The design is easily adaptable to tune the sensitivity and working pressure range up to several hundred megapascals. An array of four such transducers was tested during hydrostatic pressurization cycles on Darley Dale sandstone and Westerly granite. The prototypes show very good linearity up to 80 MPa with maximum deviations of the order of 0.25 MPa, regardless of the combination of pore and confining pressure. Multiple internal pore pressure measurements allow us to quantify the local decrease in permeability associated with faulting in Darley Dale sandstone, and also prove useful in tracking the development of pore pressure fronts during transient flow in low permeability Westerly granite.

Laboratory Measurement of Low-Permeability Rocks With a New Flow Pump System

1997 ◽  
Vol 506 ◽  
Author(s):  
Ming Zhang ◽  
Manabu Takahashi ◽  
Tetsuro Esaki
Keyword(s):  
Pore Pressure ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Test System ◽  
Low Permeability ◽  
Specific Storage ◽  
Pump System ◽  
The Difference ◽  
Host Rocks ◽  
Flow Pump

ABSTRACTNearly impermeable host rocks have been recognized as favorable media for many kinds of underground utilization such as radioactive nuclear waste disposal, storage of oil and LP gas, and CAES. To properly evaluate the ability of a geologic medium to retard transmission of fluids, it is necessary to accurately measure its hydraulic properties, most notably the permeability and specific storage. This paper presents a new flow pump permeability test system capable of testing low-permeability rocks under high confining and high pore pressure conditions, which simulate ground pressures at large depths. The new system was used to test the Inada Granite from Japan. The results of present study show that: 1) both permeability and specific storage of the rock are dependent not only on the confining pressure but also on the pore pressure. They decrease with the increment of the effective confining pressure, i.e., the difference between confining and pore pressures; 2) the permeability and specific storage of Inada Granite range from 10−11 to 10−12 cm/s and 10−6 to 10−7 1/cm, respectively. The flow pump technique with its rigorous theoretical analysis can be used to effectively obtain such low permeabilities within several tens of hours; 3) the storage capacity of flow pump system itself decreases with the increment of fluid pressure within the permeating system.

Fluid pressure response to undrained compression in saturated sedimentary rock

Geophysics ◽  
1986 ◽  
Vol 51 (4) ◽  
pp. 948-956 ◽  
Author(s):  
Douglas H. Green ◽  
Herbert F. Wang
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Wave Attenuation ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Pressure Response ◽  
Additional Parameter ◽  
Seismic Wave Velocity ◽  
Specific Storage ◽  
Order Of Magnitude

The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, [Formula: see text] where [Formula: see text] is fluid pressure, [Formula: see text] is confining pressure, and [Formula: see text] is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite arc near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high‐compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure results. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure characterized by low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer‐reservoir systems at very low effective stress states, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.

Deformation of Chalk Under Confining Pressure and Pore Pressure

1981 ◽  
Vol 21 (01) ◽  
pp. 43-50 ◽  
Author(s):  
Thomas Lindsay Blanton
Keyword(s):  
Mechanical Behavior ◽  
Pore Pressure ◽  
Hydrostatic Stress ◽  
Confining Pressure ◽  
Low Permeability ◽  
High Porosity ◽  
Pore Collapse ◽  
Soft Matrix ◽  
Pore Pressures ◽  
Ductile Behavior

Abstract Compression tests with and without pore pressure have been run on Danian and Austin chalks. The rocks yielded under increasing hydrostatic stress by pore collapse. The same effect was produced by holding a constant hydrostatic stress and reducing the pore pressure. This pore collapse reduced the permeability. The ultimate strength of the chalks increased with increasing confining pressure. The yield strength increased initially, but at higher confining pressures it decreased until it yielded under hydrostatic stress. Relatively high pore-pressure gradients developed when the chalks. were compressed. In these situations, the mechanical behavior tended to be a function of the average effective stresses. Introduction Hydrocarbons have been found in chalks in the North Sea, the Middle East, the Gulf Coast and midcontinent regions of the U.S., and the Scotian Shelf of Canada1; however, problems have been encountered in developing these reservoirs efficiently because of the unusual mechanical behavior of chalk. Chalks have three characteristics that interact to differentiate their behavior from most reservoir rocks. High Porosity. Porosities may be as high as 80070.1,2 Effects of burial and pore-water chemistry can reduce this porosity to less than 1%, but notable exceptions occur in areas of early oil placement and overpressuring where porosities in excess of 40% have been reported.2,3 Low Permeability Regardless of porosity, chalks have low permeabilities, usually around 1 to 10 md. Soft Matrix. Chalks are predominantly calcite, which has a hardness of 3 on Mohr's scale. These properties create problems in the following areas of reservoir development. Drilling. High porosity combined with a soft matrix material makes for a relatively weak and ductile rock. Efficient drilling involves chipping the rock and ductile behavior inhibits this process. Stimulation. The combination of high porosity and low permeability makes chalks prime candidates for stimulation by hydraulic fracturing or acid fracturing. The best production often is associated with natural fractures.2,3 Man-made fractures could open up new areas to production, but again ductile behavior inhibits the fracturing process. Production. In many cases permeabilities are low enough to trap pore fluids and cause abnormally high pore pressures.2 These high pore pressures help maintain the high porosities at depth by supporting some of the weight of the overburden. As the field is produced and the pore pressure lowered, some of the weight will shift to the soft matrix. The result may be pore collapse and reduction of an already low permeability. These problems indicate a need for basic information on the mechanical behavior of chalks. Determining methods of enhancing brittle behavior could lead to improved drilling and stimulation techniques. The ability to predict and prevent pore collapse could increase ultimate recovery. The approach taken in this study was experimental. Specimens of chalk were subjected to different combinations of stress and pore pressure in the laboratory, and the resulting deformations were measured.

On the loading conditions for pore fluid stabilization of failure in crustal rock

2020 ◽  
Author(s):  
Franciscus Aben ◽  
Nicolas Brantut
Keyword(s):  
Pressure Drop ◽  
Shear Failure ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Pore Fluid ◽  
Failure Zone ◽  
Effective Pressure ◽  
Loading Conditions ◽  
Dynamic Failure ◽  
Pore Fluid Pressure

<p>During shear failure in rock, fracture damage created within the failure zone causes localized dilation, which, under partially drained conditions, results in a localized pore fluid pressure drop. The effective normal stress within the failure zone therefore increases, and with it the fracture and frictional strengths. This effect is known as dilatancy hardening. Dilatancy hardening may suppress rupture propagation and slip rates sufficiently to stabilize the rupture and postpone or prevent dynamic failure. Here, we study the loading conditions at which the rate of dilatancy hardening is sufficiently high to stabilize failure. We do so by measuring the local pore fluid pressure during failure and the rate of dilatancy with slip at a range of confining and pore fluid pressures.</p><p>We performed shear failure experiments on thermally treated intact Westerly granite under triaxial loading conditions. The samples were saturated with water and contained notches to force the location of the shear failure zone. For each experiment, we imposed a different combination of confining pressure and pore fluid pressure, so that the overall effective pressure was either 40 MPa or 80 MPa prior to axial deformation at 10<sup>-6</sup> s<sup>-1</sup> strain rate. Dynamic shear failure was recognized by a sudden audible stress drop, whereas the stress drop during stabilized shear failure took longer and was inaudible. Local pore fluid pressure was measured with in-house developed pressure transducers placed on the trajectory of the prospective failure.</p><p>At effective pressures of 40 MPa and 80 MPa, we observe stabilized failure for a ratio λ (imposed pore fluid pressure over confining pressure) > 0.5. For λ < 0.5, we observe dynamic failure. Of two experiments performed at λ = 0.5 and 80 MPa effective pressure, one showed stabilized failure and one failed dynamically. For λ > 0.5, we observe pore fluid pressure drops in the failure zone of 30-45 MPa for 40 MPa effective pressure, and 60 MPa for 80 MPa confining pressure. The local pore fluid pressure during dynamic failure (λ < 0.5) is 0 MPa, strongly suggesting local fluid vaporization. Of the two experiments at λ = 0.5, the dilation rate with slip is higher for the dynamic failure relative to the stabilized failure.</p><p>We show that with increasing effective pressure, dilatancy hardening increases as the local pore fluid pressure drop during failure becomes larger. For λ < 0.5, dilatancy hardening is insufficient to stabilize failure because the local pore fluid pressure drop is larger than the absolute imposed pore fluid pressure. Near λ = 0.5, small variations in dilatancy control rupture stability. For λ > 0.5, dilatancy hardening is sufficient to suppress dynamic failure.</p>

Low-frequency seismic properties of thermally cracked and argon-saturated granite

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. F147-F159 ◽  
Author(s):  
Cao Lu ◽  
Ian Jackson
Keyword(s):  
Shear Modulus ◽  
Transient Flow ◽  
Crack Density ◽  
Fluid Pressure ◽  
Low Frequency ◽  
Confining Pressure ◽  
Seismic Properties ◽  
Aspect Ratios ◽  
In Situ Permeability

Torsional forced-oscillation techniques have been used to measure the shear modulus and strain-energy dissipation on cylindrical specimens of a fine-grained granite, Delegate aplite. The specimens were subjected to thermal cycling and associated microcracking under varying conditions of confining pressure [Formula: see text] and argon pore-fluid pressure [Formula: see text] within the low-frequency saturated isobaric regime. Complementary transient-flow studies of in-situ permeability and volumetric measurements of connected crack porosity allowed the modulus measurements to be interpreted in terms of the density and interconnectivity of the thermally generated cracks. The modulus measurements indicate that newly generated thermal cracks are closed by a differential pressure, [Formula: see text], which ranges from [Formula: see text] for temperatures of [Formula: see text]. This suggests crack aspect ratios on the order of [Formula: see text]. The covariation of in-situ permeability [Formula: see text] and thermal crack density [Formula: see text] that we infer from the modulus deficit is consistent with percolation theory. There is a well-defined threshold at [Formula: see text], beyond which [Formula: see text] increases markedly as [Formula: see text], with [Formula: see text]. At lower crack densities, it is difficult to measure the sensitivity of shear modulus to variations of confining and pore pressures because pore-pressure equilibrium is approached so sluggishly. At temperatures beyond the percolation threshold, the modulus variation is a function of the effective pressure, [Formula: see text], with the value of [Formula: see text] increasing toward one with increasing crack connectivity.

scholarly journals Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

2021 ◽  
Author(s):  
Stefano Aretusini ◽  
Francesca Meneghini ◽  
Elena Spagnuolo ◽  
Christopher Harbord ◽  
Giulio Di Toro
Keyword(s):  
Shear Strength ◽  
Pore Pressure ◽  
Subduction Zone ◽  
Subduction Zones ◽  
Fluid Pressure ◽  
Low Permeability ◽  
Earthquake Rupture ◽  
Fault Rocks ◽  
Seismic Slip ◽  
Saturated Clay

<p>In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible of the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Pāpaku thrust (Hikurangi subduction zone, New Zealand). Here we show that seismic slip is characterized by an initial decrease followed by an increase of pore pressure. The initial pore pressure decrease is indicative of dilatant behavior. The following pore pressure increase, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that thermal and mechanical pressurisation of fluids facilitates seismic slip in the Hikurangi subduction zone, which was tsunamigenic about 70 years ago. Fluid-saturated clay-rich sediments, occurring at shallow depth in subduction zones, can promote earthquake rupture propagation and slip because of their low permeability and tendency to pressurise when sheared at seismic slip velocities.</p>

Response of Pore Pressure and Effective Stress in Seabed to Waves Around a Permeable Submerged Breakwater

2015 ◽  
Author(s):  
Hao Chen ◽  
Jinhai Zheng ◽  
Qianzhen Li ◽  
Naiyu Zhang ◽  
Hanyi Chen ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Fluid Pressure ◽  
Wave Model ◽  
Pore Fluid Pressure ◽  
Submerged Breakwater ◽  
Great Ability ◽  
Wave Induced ◽  
Foundation Failure ◽  
Dynamic Wave

As the unexpected wave-induced seabed instability may cause foundation failure, the evaluation of wave-induced pore pressure and effective stress in seabed plays an important role in the design of the foundation of marine structures. In this study, a two-dimensional integrated mathematical model, based on COBRAS wave model and SWANDYNE seabed model is developed to numerically investigate the mechanism of wave-induced seabed response in the vicinity of a permeable submerged breakwaters. Numerical results indicate that this model has a great ability in predicting the dynamic response of the pore pressure and effective stress around the breakwater. Both the pore fluid pressure and effective stress in seabed largely changes with an increasing water depth. It is also found that the responses of the pore pressure and effective stress of different locations to the dynamic wave loading are significantly different in the cases with variable top width of the breakwater.

2D finite-element modelling of the interaction between poroelastic effects and viscoelastic relaxation during the seismic cycle

2021 ◽  
Author(s):  
Jill Peikert ◽  
Andrea Hampel ◽  
Meike Bagge
Keyword(s):  
Pore Pressure ◽  
Lower Crust ◽  
Fluid Pressure ◽  
Coulomb Stress ◽  
Viscoelastic Relaxation ◽  
Pore Fluid Pressure ◽  
Coseismic Slip ◽  
Coulomb Stress Changes ◽  
Stress Changes ◽  
Pressure Changes

<p>The analysis of the Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay next earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip, earthquake-induced poroelastic effects as well as transient postseismic processes such as viscoelastic relaxation. In this study, we investigate the spatial and temporal evolution of pore fluid pressure changes and fluid flow during the seismic cycle, their dependency on the permeability in the crust and the interaction with postseismic viscoelastic relaxation. To achieve this, we use 2D finite-element models for intra-continental normal and thrust faults, which include coseismic slip, poroelastic effects, postseismic viscoelastic relaxation and interseismic stress accumulation. In different experiments, we vary (1) the permeability of the upper and lower crust while keeping the viscosity structure constant and (2) the viscosity of the lower crust and lithospheric mantle, while we keep the permeabilities constant. (1) The modelling results show that the highest changes in pore fluid pressure during and after the earthquake occur within a distance of ~ 1 km around the lower fault tip at the transition between upper and lower crust. The evolution of pore pressure and fluid flow depends primarily on the permeability in the upper crust. With decreasing permeability, the possibility of the pore fluids to flow decreases and thus, in the postseismic phase, the duration of the poroelastic relaxation increases, from a few days to several years, until the pore pressure reaches the initial pressure of the preseismic phase. In contrast, the influence of variations of the permeability in the lower crust on the pore pressure changes is negligible. For high upper-crustal permeabilities, postseismic vertical velocities are high and decreases rapidly with time, from around 120 mm/a after the first year by two orders of magnitude after 10 years, whereas for low permeabilities they remain consistently low over the years after the earthquake. (2) Models with low viscosity of the lower crust show that the timescales of poroelastic effects and viscoelastic relaxation overlap and affect the postseismic velocity already in the early postseismic phase and that both processes decay within a few years after the earthquake. For higher viscosities, the velocity is initially dominated by pore pressure changes during the first few years, whereas viscoelastic relaxation lasts for decades. Both processes also show differences in their spatial scale. Poroelastic effects occur within a few kilometers around the fault, whereas viscoelastic relaxation acts on tens to hundreds of kilometers. As both processes can cause Coulomb stress changes on faults in the vicinity of the earthquake source fault, it is important to understand the spatial and temporal evolution, the effects on the individual faults and the interaction of both processes during the earthquake cycle. Future work will therefore include the calculation and examination of Coulomb stress changes on intra-continental normal and thrust faults using 3D models that include poroelastic effects and viscoelastic relaxation.</p>

Fluid Initiation of Fracture in Dry and Water Saturated Rocks

2021 ◽  
Author(s):  
Tatiana Kartseva ◽  
Vladimir Smirnov ◽  
Alexander Ponomarev ◽  
Andrey Patonin ◽  
Anna Isaeva ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Induced Seismicity ◽  
Pore Space ◽  
Water Injection ◽  
Fluid Pressure ◽  
Constant Strain Rate ◽  
Confining Pressure ◽  
B Value ◽  
Laboratory Studies ◽  
Uniform Compression

<p>We present the results of the laboratory studies on fluid-initiated fracture in the samples of porous-fractured rocks that have been initially saturated with a pressure-injected fluid and then tested under increasing fluid pressure in saturated rocks. The tests were conducted at the Geophysical observatory “Borok” of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The laboratory is equipped with electrohydraulic press INOVA-1000. The experiments were conducted on the rock samples with substantially different porosity. The tested samples were made of Buffalo sandstones, granites from the well drilled in the area of Koyna-Warna induced seismicity, and of granites from the well in the Voronezh crystalline massif. The permeability of granite samples was varied by their controlled artificial cracking by successive heating and cooling. A preliminarily dried sample was initially subjected to uniaxial loading in uniform compression (confining pressure). Loading was performed at a constant strain rate until the moment when the growth rate of acoustic emission (AE) activity began to accelerate which indicated that the stress level approaches ultimate strength. Since that, the loading rate was decreased by an order of magnitude, and water was infused into a sample from its top face. The bottom end of a sample was tightly sealed and impermeable to water. After this, the pore pressure in the sample that had got saturated with water to that moment was raised in steps whose amplitudes were varied. The obtained results of the laboratory studies show that the character and intensity of fluid initiation of fracture markedly differ under primary fluid injection into the dry porous-fractured samples and under the subsequent increases of the pore pressure in the saturated samples. The time delay of acoustic response relative to fluid initiation and the amplitude of the response proved to be larger in the case of water injection into dry samples than in the case of raising the pore pressure in saturated samples. The theoretical analysis of fluid propagation in a pore space of an air-filled sample in the model with piston-type air displacement has shown that in the case of water injection into a dry sample, the fluid pressure front propagates more slowly than in the saturated sample.</p><p>Investigation of the acoustic activity and GR b-value responses to the cyclic variations of the pore pressure in the fluid saturated rocks was studied in addition. The changes of b-value were found both for increasing and decreasing of the pore pressure. Obtained laboratory results are similar to results from the investigations of the seasonal variations of the induced seismicity in the area of Koyna-Warna water reservoirs.</p><p>The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the Interdisciplinary Scientific and Educational School of Moscow University «Fundamental and Applied Space Research».</p>

DESTRUCTION OF POROSITY THROUGH PRESSURE SOLUTION

Geophysics ◽  
1977 ◽  
Vol 42 (4) ◽  
pp. 726-741 ◽  
Author(s):  
Eve S. Sprunt ◽  
Amos Nur
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Pressure Solution ◽  
Porous System ◽  
Point Counts ◽  
Grain Crushing ◽  
Porosity Reduction ◽  
Confining Pressures

A stressed fluid‐filled porous system was modeled by hollow cylinders of St. Peter sandstone subjected to various combinations of pore and confining pressure at 270° to 280°C for up to four weeks. Large reductions in porosity, up to more than 50 percent, were produced purely by pressure solution without grain crushing. Most of the porosity reduction occurred early in the experiments and in samples with the finer of two grain sizes. Experiments with the same pore pressure, but different confining pressures, and experiments with the same effective stress, but different stress magnitudes showed that a simple effective stress law does not hold for pressure solution, and that the amount of porosity reduction depends on pore fluid pressure. However, nonhydrostatic stress appears to be necessary for rapid porosity reduction because experiments with hydrostatic pressure produced very little change in porosity. Also, experiments with the same confining pressure but different pore pressures showed that the amount of porosity loss is dependent on both pore pressure and effective stress. Pore pressure appears to place an upper limit on the rate of porosity reduction, while nonhydrostatic stress appears to be necessary for rapid porosity reduction. A dry control experiment showed that fluid must be present for porosity reduction at the temperatures and pressures in our study. The porosities of many of the samples in this study were determined both gravimetrically and by point counts on cathodoluminescent micrographs. Cathodoluminescence is useful in studying pressure solution because the intergranular relationships and pore spaces are very distinct. However, in examining natural samples caution is required when relying solely on the luminescence to determine pressure solution, because luminescent characteristics change with time.

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