Action mechanisms of abnormal fluid pressure on physical properties of deep reservoirs: A case study on Jurassic Toutunhe Formation in the southern margin of Junggar Basin, NW China

Considering the action mechanisms of overpressure on physical changes in skeleton particles of deep reservoir rocks and the differences in physical changes of skeleton particles under overpressure and hydrostatic pressure, the sandstone of the Jurassic Toutunhe Formation in the southern margin of Junggar Basin was taken as an example for physical modeling experiment to analyze the action mechanisms of overpressure on the physical properties of deep reservoirs. (1) In the simulated ultra-deep layer with a burial depth of 6000-8000 m, the mechanical compaction under overpressure reduces the remaining primary pores by about a half that under hydrostatic pressure. Overpressure can effectively suppress the mechanical compaction to allow the preservation of intergranular primary pores. (2) The linear contact length ratio under overpressure is always smaller than the linear contact length ratio under hydrostatic pressure at the same depth. In deep reservoirs, the difference between the mechanical compaction degree under overpressure and hydrostatic pressure shows a decreasing trend, the effect of abnormally high pressure to resist the increase of effective stress is weakened, and the degree of mechanical compaction is gradually close to that under hydrostatic pressure. (3) The microfractures in skeleton particles of deep reservoirs under overpressure are thin and long, while the microfractures in skeleton particles of deep reservoirs under hydrostatic pressure are short and wide. This difference is attributed to the probable presence of tension fractures in the rocks containing abnormally high pressure fluid. (4) The microfractures in skeleton particles under overpressure were mainly formed later than that under hydrostatic pressure, and the development degree and length of microfractures both extend deeper. (5) The development stages of microfractures under overpressure are mainly controlled by the development stages of abnormally high pressure and the magnitude of effective stress acting on the skeleton particles. Moreover, the development stages of microfractures in skeleton particles are more than those under hydrostatic pressure in deep reservoir. The multi-stage abnormally high pressure plays an important role in improving the physical properties of deep reservoirs.

Fundamentals and Processes of Fluid Pressure Forming Technology for Complex Thin-Walled Components

A new generation of fluid pressure forming technology has been developed for the three typical structures of tubes,sheets,and shells,and hard-to-deform material components that are urgently needed for aerospace,aircraft,automobile,and high-speed train industries.in this paper,an over all review is introduced on the state of the art in fundamentals and processes for lower-pressure hydroforming of tubular components,double-sided pressure hydroforming of sheet components,die-less hydroforming of ellipsoidai shells,and dual hardening hot medium forming af hard-to-deform materiais Particular attention is paid to deformation behavior,stress state adjustment,defect prevention,and typical applications.In addition,future development directions of fluid pressure forming technology are discussed,including hyper lower-loading forming for ultra-large non-uniform components,precision for ming for intermetallic compound and high-entropy alloy components,intelligent process and equipment,and precise finite element simulation of inhomogeneous and strong anisotropic thin shells.

COMPUTATIONAL FLUID DYNAMICS RESEARCH ON PRESSURE LOSS OF CROSS-FLOW PERFORATED MUFFLER

The pressure loss of cross-flow perforated of physical modeling, simulation and data processing. muffler has been computed with the procedure Three-dimensional computational fluid dynamics （CFD） has been used to investigate the relations of porosities, flow velocity and diameter of the holes with the pressure loss. Accordingly, some preliminary results have been obtained that pressure loss increases with porosity descent as nearly a hyperbolic trend, rising flow velocity of the input makes the pressure loss increasing with parabola trend, diameter of holes affects little about pressure loss of the muffler. Otherwise, the holes on the perforated pipes make the air flow gently and meanly, which decreases the air impact to the wall and pipes in the muffler. A practical perforated muffler is used to illustrate the available of this method for pressure loss computation, and the comparison shows that the computation results with the method of CFD has reference value for muffler design.

Pore pressure built-up in hydrate-bearing sediments during phase transition: A poromechanical approach

Due to the density contrast between the hydrate and methane gas,the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate.This accumulation of pore pressure decreases the magnitude of effective stress,further triggering potential geological disasters such as landslide.This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process.This model consists of two parts:an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation.Compared with the existing studies,the present work incorporates the effect of pore volume change into the pressure built-up model.In addition,the capillary effect is considered,which plays a nontrivial role in fine-grained sediments.Based on this model,the evolution of fluid pressure is investigated in undrained conditions.It is shown that four mechanisms mainly contribute to the pressure built-up:the density contrast between decomposing hydrate and producing fluid,the variation of pore volume,the compaction of hydrate due to variation of capillary pressure,and the thermal deformation of pore constituents induced by temperature change.Among these mechanisms,the density contrast dominates the pore pressure accumulation.Under the combined effect of these contributions,the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal.Nevertheless,this high-level pressure built-up results in a significant tensile strain,yielding potential fracturing of the sediment.

Characteristics and Genetic Mechanisms of Overpressure in the Depressions of Bohai Bay Basin, China

The distribution and genetic mechanisms of abnormal pressures in the Bohai Bay Basin were systematically analyzed. Abnormal pressures are widely developed in the Bohai Bay Basin, primarily in the Paleogene E2s4, E2s3, Es1, and Ed formations. From the onshore area of the Bohai Bay Basin to the center of the Bozhong area, the top depth of the overpressured zone in each depression increases gradually, the overpressured strata in each depression gradually move to younger formations, and the pressure structure successively alters from single-bottom- overpressure to double-bottom-overpressure and finally to double-top-overpressure. The distribution of overpressured area is consistent with the sedimentary migration controlled by the tectonic evolution of the Bohai Bay Basin, which is closely related to the hydrocarbon-generation capability of active source rocks. The overpressured strata are consistent with the source-rock intervals in each depression; the top of the overpressured zone is synchronous with the hydrocarbon generation threshold in each depression; the hydrocarbon generation capability is positively correlated with the overpressure magnitude in each formation. Undercompaction was the main mechanism of overpressure for depressions with fluid pressure coefficients less than 1.2, whereas hydrocarbon generation was the main mechanism for depressions with fluid pressure coefficients greater than 1.5.

Prediction of Flowing Bottomhole Pressures for Two-Phase Coalbed Methane Wells

A method is proposed to predict the flowing bottomhole pressures （FBHPs） for two-phase coalbed methane （CBM） wells. The mathematical models for both gas column pressure and two-phase fluid column pressure were developed based on the well liquid flow equation. FBHPs during the production were predicted by considering the effect of entrained liquid on gravitational gradients. Comparison of calculated BHPs by Cullender-Smith and proposed method was also studied. The results show that the proposed algorithm gives the desired accuracy of calculating BHPs in the low- productivity and low-pressure CBM wells. FBHP is resulted from the combined action of wellhead pressure, gas column pressure and fluid column pressure. Variation of kinetic energy term, compressibility and friction factors with depth increments and liquid holdup with velocity should be considered to simulate the real BHPs adequately. BHP is a function of depth of each column segment. The small errors of less than 1.5% between the calculated and measured values are obtained with each segment within 25 m. Adjusting BHPs can effectively increase production pressure drop, which is beneficial to CBM desorption and enhances reservoir productivity. The increment of pressure drop from 5.37 MPa2 to 8.66 MPa2 leads to an increase of CBM production from 3270 m3/d to 6700 m3/d and is attributed to a decrease in BHP from 2.25 MPa to 1.33 MPa.

The effect of increased intra-abdominal pressure on orbital subarachnoid space width and intraocular pressure

In accordance with the trans-lamina cribrosa pressure difference theory, decreasing the trans-lamina cribrosa pressure difference can re- lieve glaucomatous optic neuropathy. Increased intracranial pressure can also reduce optic nerve damage in glaucoma patients, and a safe, effective and noninvasive way to achieve this is by increasing the intra-abdominal pressure. The purpose of this study was to observe the changes in orbital subarachnoid space width and intraocular pressure at elevated intra-abdominal pressure. An inflatable abdominal belt was tied to each of 15 healthy volunteers, aged 22-30 years （12 females and 3 males）, at the navel level, without applying pressure to the abdomen, before they laid in the magnetic resonance imaging machine. The baseline orbital subarachnoid space width around the optic nerve was measured by magnetic resonance imaging at 1, 3, 9, and 15 mm behind the globe. The abdominal belt was inflated to increase the pressure to 40 mmHg （1 mmHg = 0.133 kPa）, then the orbital subarachnoid space width was measured every 10 minutes for 2 hours. After removal of the pressure, the measurement was repeated 10 and 20 minutes later. In a separate trial, the intraocular pressure was measured for all the subjects at the same time points, before, during and after elevated intra-abdominal pressure. Results showed that the baseline mean orbital subarachnoid space width was 0.88 ＋ 0.1 mm （range： 0.77-1.05 mm）, 0.77 ＋ 0.11 mm （range： 0.60-0.94 mm）, 0.70 ＋ 0.08 mm （range： 0.62-0.80 ram）, and 0.68 _＋ 0.08 mm （range： 0.57-0.77 mm） at 1, 3, 9, and 15 mm behind the globe, respectively. During the elevated intra-abdominal pressure, the orbital subarachnoid space width increased from the baseline and dilation of the optic nerve sheath was significant at 1, 3 and 9 mm behind the globe. After decompression of the abdominal pressure, the orbital subarachnoid space width normalized and returned to the baseline value. There was no significant difference in the intraocular pressure before, during and after the intra-abdominal pressure elevation. These results verified that the increased intra-abdominal pressure widens the orbital subarachnoid space in this acute trial, but does not alter the intraocular pressure, indicating that intraocular pressure is not affected by rapid increased in- tra-abdominal pressure. This study was registered in the Chinese Clinical Trial Registry （registration number： ChiCTR-ONRC-14004947）.

Acoustoelastic effects on mode waves in a fluid-filled pressurized borehole in triaxially stressed formations

Based on the nonlinear theory of acoustoelasticity, considering the triaxial terrestrial stress, the fluid static pressure in the borehole and the fluid nonlinear effect jointly, the dispersion curves of the monopole Stoneley wave and dipole flexural wave prop- agating along the borehole axis in a homogeneous isotropic formation are investigated by using the perturbation method. The relation of the sensitivity coefficient and the velocity-stress coefficient to frequency are also analyzed. The results show that variations of the phase velocity dispersion curve are mainly affected by three sensitivity coefficients related to third-order elastic constant. The borehole stress concentration causes a split of the flexural waves and an intersection of the dispersion curves of the flexural waves polarized in directions parallel and normal to the uniaxial horizontal stress direction. The stress-induced formation anisotropy is only dependent on the horizontal deviatoric terrestrial stress and independent of the horizontal mean terrestrial stress, the superimposed stress and the fluid static pressure. The horizontal terrestrial stress ratio ranging from 0 to 1 reduces the stress-induced formation anisotropy. This makes the intersection of flexural wave dispersion curves not distinguishable. The effect of the fluid nonlinearity on the dispersion curve of the mode wave is small and can be ignored.

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure （FIP） either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near- wellbore region, rather than by mudcake strength. Fluid loss pressure （FLP） should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

Quantitative evaluation of geological fluid evolution and accumulated mechanism:in case of tight sandstone gas field in central Sichuan Basin

Tight gas exploration plays an important part in China’s unconventional energy strategy.The tight gas reservoirs in the Jurassic Shaximiao Formation in the Qiulin and Jinhua Gas Fields of central Sichuan Basin are characterized by shallow burial depths and large reserves.The evolution of the fluid phases is a key element in understanding the accumulation of hydrocarbons in tight gas reservoirs.This study investigates the fluid accumulation mechanisms and the indicators of reservoir properties preservation and degradation in a tight gas reservoir.Based on petrographic observations and micro-Raman spectroscopy,pure CH4 inclusions,pure CO2 inclusions,hybrid CH4–CO2 gas inclusions,and N2-rich gas inclusions were studied in quartz grains.The pressure–volume–temperature–composition properties(PVT-x)of the CH4 and CO2 bearing inclusions were determined using quantitative Raman analysis and thermodynamic models,while the density of pure CO2 inclusions was calculated based on the separation of Fermi diad.Two stages of CO2 fluid accumulation were observed:primary CO2 inclusions,characterized by higher densities(0.874–1.020 g/cm3)and higher homogenization temperatures(>210°C)and secondary CO2 inclusions,characterized by lower densities(0.514–0.715 g/cm3)and lower homogenization temperatures:~180–200°C).CO2 inclusions with abnormally high homogenization temperatures are thought to be the result of deep hydrothermal fluid activity.The pore fluid pressure(44.0–58.5 MPa)calculated from the Raman shift of C–H symmetric stretching(v1)band of methane inclusions is key to understanding the development of overpressure.PT entrapment conditions and simulation of burial history can be used to constrain the timing of paleo-fluid emplacement.Methane accumulated in the late Cretaceous(~75–65 Ma),close to the maximum burial depth during the early stages of the Himalayan tectonic event while maximum overpressure occurred at~70 Ma,just before uplift.Later,hydrocarbon gas migrated through the faults and gradually displaced the early emplaced CO2 in the reservoirs accompanied by a continuous decrease in overpressure during and after the Himalayan event,which has led to a decrease in the reservoir sealing capabilities.The continuous release of overpressure to present-day conditions indicates that the tectonic movement after the Himalayan period has led to a decline in reservoir conditions and sealing properties.

Computational analysis of the flow of pseudoplastic power-law fluids in a microchannel plate

The flow of pseudoplastic power-law fluids with different flow indexes at a microchannel plate was studied using computational fluid dynamic simulation.The velocity distribution along the microchannel plate and especially in the microchannel slits,flow pattern along the outlet arc and the pressure drop through the whole of microchannel plate were investigated at different power-law flow indexes.The results showed that the velocity profile in the microchannel slits for low flow index fluids was similar to the plug flow and had uniform pattern.Also the power-law fluids with lower flow indexes had lower stagnation zones near the outlet of the microchannel plate.The pressure drop through the microchannel plate showed huge differences between the fluids.The most interesting result was that the pressure drops for power-law fluids were very smaller than that of Newtonian fluids.In addition,the heat transfer of the fluids through the microchannel with different channel numbers in a wide range of Reynolds number was investigated.For power-law fluid with flow index(n=0.4),the Nusselt number increases continuously as the number of channels increases.The results highlight the potential use of using pseudoplastic fluids in the microheat exchangers which can lower the pressure drop and increase the heat transfer efficiency.

Sensitivity Analysis and Optimization of Geometric Parameters of a New Fluid Bag Buffer Mechanism on Buffering Performance

A new fluid bag buffer mechanism,which can provide large axial stiffness under the small displacement,is designed.The dynamic change laws of the mechanism stiffness and the internal pressure of the fluid bag are studied when it is subjected to impact load.According to the protection performance for the flexible joint and the pressure change in the fluid bag during the impact process,the sensitivity of the geometric parameters of the fluid bag to the axial stiffness is analyzed by using the orthogonal experimental method,and the optimal parameter combination of the geometric parameters of the fluid bag under impact is obtained,leading to the displacement of the inner shell reduce by 41.4%.The results show that the internal pressure of the fluid bag is a rising process of oscillation and fluctuation.The sensitivity of the geometric parameters of the fluid bag to the displacement of the inner shell from high to low is as follows:Height H,radius r,wall thickness t,chamfer A.The correlation between the geometric parameters of the fluid bag and its internal pressure is:H is negatively correlated with the internal pressure,while the r,t,and A are positively correlated with the internal pressure.

Stability of rat models of fluid percussion-induced traumatic brain injury: comparison of three different impact forces

Fluid percussion-induced traumatic brain injury models have been widely used in experimental research for years. In an experiment, the stability of impaction is inevitably affected by factors such as the appearance of liquid spikes. Management of impact pressure is a crucial factor that determines the stability of these models, and direction of impact control is another basic element. To improve experimental stability, we calculated a pressure curve by generating repeated impacts using a fluid percussion device at different pendulum angles. A stereotactic frame was used to control the direction of impact. We produced stable and reproducible models, including mild, moderate, and severe traumatic brain injury, using the MODEL01-B device at pendulum angles of 6°, 11° and 13°, with corresponding impact force values of 1.0 ± 0.11 atm（101.32 ± 11.16 k Pa）, 2.6 ± 0.16 atm（263.44 ± 16.21 k Pa）, and 3.6 ± 0.16 atm（364.77 ± 16.21 k Pa）, respectively. Behavioral tests, hematoxylin-eosin staining, and magnetic resonance imaging revealed that models for different degrees of injury were consistent with the clinical properties of mild, moderate, and severe craniocerebral injuries. Using this method, we established fluid percussion models for different degrees of injury and stabilized pathological features based on precise power and direction control.

Development of a 100 MPa water–gas two-phase fluid pressurization device

This paper introduces a 100 MPa water gas twophase fluid pressurization device.The device can provide 100 MPa gas pressure and 200 MPa liquid pressure for small volume(<20 mL)high-pressure experimental devices.This device can make the pressure control independent of the temperature control without changing the material components of the system.The resolution of this device in adjusting the pressure is±0.2 MPa in the process of boosting and depressurizing.This pressure boosting device generates very little vibration during work and it can be used in experiments with strict requirements on vibration.As a thermodynamic parameter,pressure has a great influence on matter.In the field of experimental geochemistry,pressure is not only an experimental method and an extreme condition but an important physical parameter independent of temperature and chemical composition.

The association of cerebrospinal fluid pressure with optic nerve head and macular vessel density

The present study aims to investigate the effect of temporary cerebrospinal fluid pressure(CSFP) reduction on optic nerve head(ONH) and macular vessel density(VD) using optical coherence tomography angiography.Forty-four eyes of 44 adults with diagnostic lumbar puncture and CSFP reduction were recruited.Thirty-two eyes of 32 healthy volunteers were controls.ONH and macular VD images were evaluated differences between baseline and after CSFP reduction.The results showed that the mean CSFP decreased from(11.6±2.1) mm Hg to(8.2±3.4) mm Hg(P<0.001).VD in the macular regions decreased significantly after CSFP reduction in the study group(all P<0.05).The control group showed no significant changes in macular VD(all P>0.05).In the study group,decreased VD in the macular parainferior region was associated with CSFP reduction(R^(2)=0.192,P=0.003),the reduction of macular VD in parafoveal(R^(2)=0.098,P=0.018),parainferior(R^(2)=0.104,P=0.021),parasuperior(R^(2)=0.059,P=0.058),paranasal(R^(2)=0.057,P=0.042),paratemporal(R^(2)=0.079,P=0.026) was associated with mean ocular perfusion pressure decrease following CSFP reduction.ONH vessel density did not differ after CSFP reduction(all P>0.05).In conclusion,macular vessel density decreased in association with CSFP reduction.Retinal vessel density in the macular region is more sensitive than that in peripapillary region after CSFP reduction.

Changes of cerebrospinal fluid pressure after thoracic endovascular aortic repair

Background Decreasing the intracranial pressure has been advocated as one of the major protective strategies to prevent spinal cord ischemia after endovascular aortic repair. However, the actual changes of cerebrospinal fluid （CSF） pressure and its relation with spinal cord ischemia have been poorly understood. We performed CSF pressure measurements and provisional CSF withdrawal after thoracic endovascular aortic repair, and compared the changes of CSF pressure in high risk patients and in patients with new onset paraplegia and paraparesis.

Assessment effect of central venous pressure in fluid resuscitation in the patients with shock： a multi-center retrospective research

Background Central venous pressure （CVP） and intrathoracic blood volume index （ITBVI） were used to assess the fluid status. It has previously been shown that CVP is not as accurate as ITBVI for all the shock patients. We therefore hypothesized that the change of CVP has the ability to predict fluid responsiveness in some clinical cases of shock. Methods From September 1st 2009 to September 1st 2011, sixty-three patients with shock from different Intensive Care Unit （ICU） were collected into this retrospective study. All the patients received fluid challenge strategy （infusing 300 ml hydroxyethyl starch in 20 minutes）, were monitored with CVP and pulse-indicated continuous cardiac output （PICCO）. The correlation between changes in cardiac index （ACI）, CVP （ACVP） and ITBVI （AITBVI） were analyzed. Fluid responsiveness was defined as an increase in CI 〉10%. Receiver operating characteristic （ROC） curves were generated for ACVP and AITBVI. Results For all the patients, there was no correlation between ACI and ACVP （P=0.073）, but in the subgroup analysis, the correlation between ACI and ACVP was significant in those younger than 60 years old （P=0.018） and those with hypovolemic shock （P=0.001）. The difference of areas under the ROC curves of ACVP and AITBVI were not statistically significant in the group younger than 60 years old or hypovolemic shock group （P 〉0.05, respectively）. However, no similar results can be found in the group older than 60 years old and the other two shock type groups from ROC curves of ACVP and AITBVI. Conclusions ACVP is not suitable for evaluating the volume status of the shock patients with fluid resuscitation regardless of their condition. However, in some ways, ACVP have the ability to predict fluid responsiveness in the younger shock patients or in the hypovolemic shock patients.

Factors affecting cerebrospinal fluid opening pressure in patients with spontaneous intracranial hypotension

Objective： Spontaneous intracranial hypotension （SIH） is recognized far more commonly than ever before. Though usually characterized by low cerebrospinal fluid （CSF） pressure, some patients with SIH are observed to have normal pressure values. In this study, we aimed to confirm the proportion of patients with normal CSF opening pressure （CSF OP） and explore the factors affecting CSF OP in SIH patients. Methods： We retrospectively reviewed 206 consecutive SIH patients and analyzed their clinical and imaging variables （including demographic data, body mass index （BMI）, duration of symptoms, and brain imaging findings）. Univariate and multiVariate analyses were performed to identify the potential factors affecting CSF OP. Results： In a total of 114 （55.3%） cases the CSF OP was 〈60 mmH2O （1 mmH2O=9.80665 Pa）, in 90 （43.7%） cases it was between 60 and 200 mmH2O, and in 2 （1.0%） cases it was 〉200 mmH2O. Univadate analysis showed that the duration of symptoms （P〈0.001）, BMI （P〈0.001）, and age （P=0.024） were positively correlated with CSF OP. However, multivariate analysis suggested that only the duration of symptoms （P〈0.001） and BMI （P〈0.001） were strongly correlated with CSF OP. A relatively high R2 of 0.681 was obtained for the multivariate model. Conclusions： Our study indicated that in patients without a low CSF OP, a diagnosis of SIH should not be excluded. BMI and the duration of symptoms can influence CSF OP in SIH patients, and other potential factors need further investigation.

Flow field and pressure loss analysis of junction and its structure optimization of aircraft hydraulic pipe system

The flow field in junction is complicated due to the ripple property of oil flow velocity and different frequencies of two pumps in aircraft. In this study, the flow fields of T-junction and Y-junction were analyzed using shear stress transport （SST） model in ANSYS/CFX software. The simulation results identified the variation rule of velocity peak in T-junction with different frequencies and phase-differences, meanwhile, the eddy and velocity shock existed in the corner of the T-junction, and the limit working state was obtained. Although the eddy disappeared in Y-junction, the velocity shock and pressure loss were still too big. To address these faults, an arc-junction was designed. Based on the flow fields of arc-junction, the eddy in the junction corner disappeared and the maximum of velocity peak declined compared to T-and Y-junction. Additionally, 8 series of arc-junction with different radiuses were tested to get the variation rule of velocity peak. Through the computation of the pressure loss of three junctions, the arc-junction had a lowest loss value, and its pressure loss reached the minimum value when the curvature radius is 35.42 mm, meanwhile, the velocity shock has decreased in a low phase.

Recovering Period of Postseismic Fluid Pressure in Fault Valve

The present study aims to reveal the recovering period of the postseismic fluid pressure in fault zone, offering an insight into earthquake recurrence. Numerical modeling is performed based on a 2D simple layered fault-valve model to simulate the fluid activities within the earthquake fault. In order to demonstrate the features of postseismic fluid pressure in natural state, the interference of tectonic movements is not considered. The recovering period of postseismic fluid pressure includes a sudden- changing period and a much longer fluctuating period. Modeling results show that fault permeability and porosity are sensitive parameters and reversely proportional to the recovering period of the fluid pressure in earthquake fault zone. When the permeability reduces from 10^-15 to 10^-18 m^2, the recovering period increases from 400 to 2 000 yrs, correspondently. The upper and lower fluid pressures are sepa- rated by the valve seal, causing their fluctuations in opposite tendencies.