Calculated Pore Pressure Research Articles

Calculation of foam injection ratio and regulation method of muck compressibility under shield soil chamber pressure conditions

The proper compressibility of shield muck is crucial for ensuring the safety and efficiency of shield tunneling. Foam conditioning is an effective method for improving the compressibility of muck. However, the amount of foam injected in practical construction is mostly based on experience or subjective judgment. A novel foam injection ratio calculation formula was derived based on Boyle’s law and the pore pressure calculation model. This formula comprehensively considers the impact of chamber pressure, pore water, soil gradation, and existing muck conditioning parameters on foam consumption. Starting with the premise that pore foam lifts the soil skeleton, reducing the effective stress in the soil to zero, a method for regulating the compressibility of muck in shield tunnel construction sites was proposed. This method presents the advantage of accurately quantifying the amount of foam injection. Verification results from field applications confirm that optimizing the muck conditioning parameters through this new method leads to improvements in workability, permeability, and muck compressibility, meeting the requirements for efficient shield tunneling. The fluctuation of chamber pressure is reduced, lowering the risk of tool wear. Additionally, there is a reduction of 29.96 % in average total thrust and 29.82 % in cutterhead torque, with a corresponding increase in tunneling speed by 24.42 %. Furthermore, the advantages and limitations of the proposed method were discussed.

Numerical investigation of heat extraction performance affected by stimulated reservoir volume and permeability evolution in the enhanced geothermal system

The high-permeability stimulated reservoir volume (SRV) containing artificial fractures is the main region for deep geothermal extraction. Evaluating the SRV distribution and the contributions of fluid pressure and thermal stress to permeability evolution is essential to accurately quantify heat extraction. Here, we employ the numerically calculated pore pressure and the field-measured rock failure as evaluation indicators to reasonably identify the SRV and investigate the effect of rock and fracture permeability evolution on heat extraction performance. We find that simply assuming the entire model as the SRV would significantly overestimate the heat extraction, and scientifically defining the SRV based on hydraulic fracturing is crucial. The variable rock permeability model extracts marginally less heat as the working fluid squeezes and cools surrounding rocks causing their lower permeability. In regions around the injection well, both the high fluid pressure and the thermal stress increase the fracture aperture and permeability, whereas the fluid pressure shows the opposite effect around the production well as it is lower than the initial pore pressure. When the main fractures connect injection and production wells, the widened fractures accelerate fluid flow, causing higher heat extraction for a short time (first 4 years); however, after 4 years’ operation, the heat extraction remains at a lower value for a long time as the increased fracture permeability causes more fluids to flow directly from main fractures to the production well without playing participating in heat exchange. When the main fractures do not connect injection and production wells, the increased fracture permeability slightly enhances the heat extraction over the geothermal extraction process with a relative error less than 0.025. This study provides guidance for the appropriate selection of geothermal extraction simulation assumptions.

Features of the numerical modeling of the hydrogeological conditions of the soil massive during the excavation of the pit in the Plaxis software complex

The construction of multi-storey civilian buildings, especially in the central districts of cities, is often accompanied by a developed underground part, which requires deep pits. The construction of such pits often leads to a change in the hydrogeological condition at and near the construction site.
 This paper presents a comparison of the stress-strain state of the " base - pit enclosure" system, depending on the method of modeling hydrogeological conditions.
 The study is divided into two stages: determination of the main features of the Plaxis software complex toolkit for modeling and determining pore water pressures; creating of numerical models and performing calculations. . The first part of the study describes the basic principle of determining stresses in saturated soils, pore pressure calculation types, hydraulic boundary conditions, and types of soil drainage.
 In the second part of the study, the numerical models were developed in Plaxis 2D, based on the real object and hydrogeological conditions at the construction site. The two most popular methods for modeling hydrogeological conditions during dry excavation in the pit are described. A step-by-step calculation is performed taking into account the stages of excavation in the pit and dewatering and the construction of retaining structures according to the scheme provided by the design solutions.
 The results of the calculations demonstrate the difference in the obtained values of forces and horizontal displacements of the retaining wall for models created using different methods of setting the hydrogeological conditions of the base.
 In order to improve the accuracy of calculations and determine the impact of soil permeability, the impact of the stratigraphy of the base was excluded by solving theoretical problems with a homogeneous soil massif composed of clay or sandy soil with groundwater filtration rates in a wide range (kx=ky=0.001...6m/day). Based on the results obtained, it was found that both methods correlate with each other in the case of the presence of poorly filtering soils with a filtration coefficient of k=0.001...0.05 m/day, the calculations demonstrated similarity in the displacements and forces of the retaining wall in such schemes up to 97.9-99.9%. Instead, in the case of soils with high water permeability (k=0.4...6 m/day), the difference in results is up to 56.5%.

Paleo fluid system change from deep burial to exhumation of the Precambrian petroleum reservoirs in the Sichuan Basin, China: Evidence from P-T-X records

Economically viable accumulations of hydrocarbons hosted in the Precambrian rocks are globally rare, as a protracted history available for them to escape or be destroyed due to various processes including thermal and bacterial degradations and tectonic disruption. A recent gas discovery in the late Ediacaran dolomites of the Sichuan Basin (SW China) provides an opportunity to understand the formation and preservation of ancient petroleum systems. Paleo fluid pressure-temperature-composition (P-T-X) conditions from deep burial to exhumation are evaluated from microthermometry and Raman data obtained from fluid inclusions.We identified three texturally distinct generations of gas-bearing fluid inclusion assemblages (FIAs), which delineate an operating petroleum system where gas remigrated during exhumation that followed oil cracking during deep burial. The older FIAs (types 1 and 2) show a petrographic association with bitumen and are interpreted to be the result of thermal cracking. Type 1 FIAs formed through in-situ cracking of pre-existing oil inclusions and have recorded the paleo oil charge during the middle Triassic to early Jurassic. Type 2 FIAs were trapped during the prolonged intra-reservoir oil cracking that occurred between the late Jurassic and late Cretaceous. Our pore pressure calculations indicate overpressured conditions, a result of active gas generation during thermal cracking. Lithostatic overpressures accumulated and were sustained under weak tectonic compression prior to peak burial.Type 3 FIAs have been interpreted as reflective of the gas-bearing system reactivation during exhumation. These FIAs indicate the relative reduction in the proportion of aqueous to gas inclusions, which may be due to a decrease in water saturation following gas emplacement. Uplift processes during early exhumation likely facilitated breaching of the system and remobilization of fluids, resulting in gas escape and overpressure release. The drastic increase in formation water salinity, which was documented in the fluid inclusion dataset, points to the arrival of hypersaline brine during the Eocene to Oligocene. This brine could have originated either from the lower Cambrian evaporated CaCl2 seawater or from the dissolution of the lower-middle Triassic halite. From the late Miocene to present, progressively exhumed reservoirs may have received an influx of relatively fresh water from shallow aquifers, as shown by the correspondence between P-T-X records and present-day reservoir conditions.

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

Pore pressure is a critical parameter for the exploration and development of hydrocarbons, and it is difficult to predict in unconventional basins that have been unloaded by erosion. We predict pore pressure from mudrock velocities in the Delaware Basin, which is an unconventional basin with significant late-stage erosion. We use an elastoplastic model to describe mudrock decompaction due to unloading, resulting in a velocity-effective stress relationship that differs from a mudrock's normal compaction trend. We establish this velocity-effective stress relationship using well log and measured pore pressure data in a well with a known erosional value. We next use this relationship to estimate the amount of erosion at each new prediction well, and then predict vertical effective stress with depth. We calculate pore pressure as the lithostatic stress less the vertical effective stress. We demonstrate the predictive capability of our approach in four wells spanning the eastern portion of the basin where the amount of erosion differs, and show that our approach better matches the measured pore pressures than the ones predicted by two conventional methods.

Vertical Stress Prediction for Zubair Oil Field/ Case Study

Predicting vertical stress was indeed useful for controlling geomechanical issues since it allowed for the computation of pore pressure for the formation and the classification of fault regimes. This study provides an in-depth observation of vertical stress prediction utilizing numerous approaches using the Techlog 2015 software. Gardner's method results in incorrect vertical stress values with a problem that this method doesn't start from the surface and instead relies only on sound log data. Whereas the Amoco, Wendt non-acoustic, Traugott, average technique simply needed density log as input and used a straight line as the observed density, this was incorrect for vertical computing stress. The results of these methods show that extrapolated density measurement used an average for the real density. The gradient of an extrapolated method is much better in shallow depth into the vertical stress calculations. The Miller density method had an excellent fit with the real density in deep depth. It has been crucial to calculate vertical stress for the past 40 years because calculating pore pressure and geomechanical building models have employed vertical stress as input. The strongest predictor of vertical stress may have been bulk density. According to these results, the miller and extrapolated techniques may be the best two methods for determining vertical stress. Still, the gradient of an extrapolated method is much more excellent in shallow depth than the miller method. Extrapolated density approach may produce satisfactory results for vertical stress, while miller values are lower than those obtained by extrapolating. This may be due to the poor gradient of this method at shallow depths. Gardner's approach incorrectly displays minimum values of about 4000 psi at great depths. While other methods provide numbers that are similar because these methods use constant bulk density values that start at the surface and continue to the desired depth, this is incorrect.

ОПРЕДЕЛЕНИЕ ПОРОВОГО ДАВЛЕНИЯ В СЛАБО ФИЛЬТРУЮЩИХ ГРУНТАХ МЕТОДОМ ЛИНЕЙНОЙ ИНТЕРПОЛЯЦИИ

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

Pre-drill pore pressure prediction using seismic interval velocity and wireline log: Case studies for some wells in Cuu Long and Song Hong basins

Pore pressure can be obtained from seismic interval velocity by the velocity to pore pressure transform technique. In this paper, the authors present the Eaton experimental method to calculate pore pressure for some wellbores in the Cuu Long and Song Hong basins, where complex processes of subsidence, burial, geological transformation, and geothermal activity took place, causing abnormal pressure zones.The obtained results show that the pore pressures calculated from the seismic interval velocity data are closely correlated with the values measured by well logging methods and the density of the drilling fluids. Therefore, using seismic interval velocities to calculate pore pressure values, identify and predict abnormal zones by the Eaton method can be effectively applied in frontier areas to improve safety, reduce risks while drilling.

Pore pressure prediction using seismic acoustic impedance in an overpressure carbonate reservoir

The drilling engineers favor a quantifiable understanding of the subsurface overpressure zones to avoid drilling hazards. The conventional pore pressure estimation techniques in carbonate reservoirs are prone to uncertainties that affect the calculated pore pressure model resolution and are still far from satisfactory. Basically, in carbonate reservoirs, the effect of chemical process and cementation on porosity is more important than the mechanical compaction, so the conventional pore pressure prediction methods based on the normal compaction trend mostly do not provide acceptable results. Using the conventional methods for carbonate reservoirs can yield large errors, even suggesting a reduction in abnormal pressure in overpressure zones where considerable attention must be paid. Conventional methods need to model density and velocity to calculate the effective and overburden pressures. Converting acoustic impedance to density and velocity is always associated with errors and generally provides low resolution, which adds substantial uncertainties to the pressure prediction. Although pore pressure measurements are usually associated with low resolution, additional error-prone steps can be dropped if used directly. This research outlines the pore pressure estimation of a famous Iranian carbonate reservoir using direct acoustic impedance without inverting it to density and velocity. Finally, this method gives acceptable results in carbonate formations compared to the results of the Repeat Formation Test (RFT) in this region. The results show a zone of overpressure between the two low-pressure intervals of the carbonate reservoir. This result can be of great help in determining reservoir boundaries as well as in planning for drilling trajectory for new wells. Furthermore, the pore pressure estimation results also show pressure reduction in the central part of the seismic section. The proposed approach is a viable alternative to the conventional method and is in line with the geological field report, where the ratio of hydrocarbon potential of total rock on the reservoir sides is higher than its middle part. In this study, we want to emphasize that the calibrated function obtained in our area can be used in similar basins with carbonate reservoirs.

A Calculation Method of Thermal Pore Water Pressure Considering Overconsolidation Effect for Saturated Clay

With the increase of soil consolidation degree, the pore water pressure induced by thermal loading drops dramatically. To conveniently and quickly calculate the thermal pore water pressure inside the soil under different overconsolidation states and quantify overconsolidation effect on thermal pore water pressure, a calculation method of thermal pore water pressure considering overconsolidation effect for saturated clay is proposed. The method is verified by the relevant experimental data, and good agreements were achieved. Through analyzing the influence mechanism of OCR on the thermal pore water pressure, three important findings were captured. (1) For overconsolidated clay, thermal pore water pressure decreases nonlinearly with the increase of OCR. (2) There is a critical threshold of OCR 4.3; when 1 < OCR ≤ 4.3 (slightly overconsolidated state), the ratio of compression line slope to recompression line slope (Λ) of overconsolidated clay is consistent with that of the normally consolidated clay, while when OCR > 4.3 (highly overconsolidated state), the value of Λ is smaller than that of normally consolidated clay. (3) For highly overconsolidated clay (OCR > 4.3), considering the reducing of Λ with OCR, the prediction accuracy of the thermal pore pressure calculation method has been greatly improved; especially when OCR equals 30, the prediction accuracy improves by 92.7% as temperature change achieves 35 °C.

Pore pressure prediction based on rock stress applied to Marmousi seismic data

This paper focuses on the modeling of a sedimentary basin where the exploration of oil and gas takes place. In the modeling, we calculate the vertical variation of pore and rock pressure, which serve as natural pumps for the accumulation of fluids, where we use post-migration surface seismic data and information as necessary. We compare two methods of pore pressure calculation. In the first case, we calculate stress due to vertical loading created by gravitational geological overburden. In the second case, we propose a more complex method to calculate the stress distribution based on the mechanics of solids under gravity loading, using the concept of the first tensor invariant and the linear behavior of Hooke’s law. We prove the use of the P and S velocities and density information to calculate rock, pore, and effective pressure distribution, useful for characterizing potential zones for oil and gas accumulation. The proposed method allows formulation of rock pressure from different calculations instead of as a simple overburden pressure value. The joint analysis of the calculated sections can be used to identify patterns and correlations, outline and characterize the target zones, and make practical geological conclusions.

Evaluation of Mud Weight Using Safe Mud Window Concept Based on Well Log Data: A Case Study of Well OP-002 in the North Sumatra Basin Area, Indonesia

In the drilling operation of well OP-002 which is located in the North Sumatra Basin at a depth interval of 2887 - 3186 m occurred partial loss, and caving at a depth interval of 500 - 1650 m, where the drilling problem is caused by the use of inappropriate mud weight. Safe mud window analysis is carried out by processing well log data to build PPFG (Pore Pressure Fracture Gradient) and 1D Geomechanics model using several calculation methods. Furthermore, the results of the calculation of pore pressure and fracture gradient are validated with well test data from the well OP-002, so the safe mud window can be determined, and can be used as a basis in the analysis of the drilling problems that occur. The optimum mud weight can minimize wellbore instability, with a limit value that must be greater than the collapse pressure, but not exceeding the minimum insitu stress limit. From the results of the mud safe window analysis, it can be concluded that at a depth interval of 500 - 1650 m caving occurs, because the density value used is smaller than the shear failure gradient, and at a depth interval of 1619 - 2829 m, the density value used is greater than Shmin. To overcome this problem, a mud wight with a safe mud window concept is recommended, namely the selection of the optimum mud weight to be used must be greater than the pore pressure and shear failure gradient and does not exceed the minimum horizontal stress and fracture gradient values.

A New Method to Estimate Formation Pore Pressure in Fractured Areas of Shale Gas Reservoir

The matrix of shale gas reservoir has the characteristics of low porosity and ultra-low permeability, but also develop natural and hydraulic fractures. Many field operations observed that the formation pore pressure after hydraulic fracturing is higher than the original formation pore pressure. Drilling in the fractured areas of shale gas reservoir, including natural fractures and artificial fractures, always faces greater overflow risk. The pore pressure test after fracturing also shows that the test value is higher than the formation initial pore pressure. However the conventional formation pore pressure predicting methods are difficult to obtain the accurate pore pressure in the fractured areas of shale gas reservoir.In this paper, the causes of formation pore pressure changing and fracturing pressurization effect in the fractured zones of shale gas reservoir are analyzed. Based on the assumption of closed and constant volume material balance and the gas equation of state, the internal mechanism of the increase in pore pressure caused by fracturing operation in low-permeability shale gas formation is theoretically analyzed. A new method to predict pore pressure changing of shale gas reservoir during drilling and hydrofracturing operations is proposed. The pore connectivity of shale gas reservoir is poor, and the permeability is tens of nano-darcy. The permeability is usually between 0.01-0.20 millidarcy when natural fractures are developed. After fracturing, the permeability can be increased by 2-3 times. Based on the principle of effective stress, a pore pressure calculation model with the influence of permeability change is established by using logging data. Three influence terms are considered in the model: the first one is the overburden pressure term, the second one is the skeleton stress term, and the last one is the influence term of permeability change rate on local formation pore pressure. The proposed method shows simple principle, clear mechanism and easy to use in the field. Field applications show that this method could quickly and accurately predict dynamic formation pore pressure changing of shale gas reservoir. It offsets the shortcomings of traditional formation pore pressure calculation methods.

Overpressure-generating mechanisms in the Blok F3, North Sea, Netherland

Block F3 North Sea is a block with pore pressure values that vary over time due to complex geological conditions such as burial and various sedimentation zones. Pore pressure is one of the important aspects that need to be analyzed as a basis for the identification of zones and overpressure mechanisms. Overpressure is a greater pore pressure condition than normal pressure and may cause drilling problems, such as kicks, blowouts, etc. This study calculated pore pressure values using the eaton method approach with well data and seismic data. Both data are integrated for generating pore pressure values in 1D and 3D. 1D Modelling uses Interactive Petrophysics 3.5, while 3D modeling uses Petrel software. In 3D modeling, the variables used are interval velocity and inversion velocity obtained by acoustic impedance inversion. The sub-variables used are the inversion density and the regression density obtained from well density acoustic impedance inversion. The existence of a 1D overpressure zone at a depth of 1,100 – 1,800 m with an overpressure value of 3,836 – 18,975 kPa. In addition, the overpressure value based on the 3D model is 8,000 – 18,000 kPa. The overpressure zone is validated using an acoustic impedance inversion model with a high value of 5,200 – 5,380 (m/s)*(gr/cc). Overpressure in Block F3 is predicted to occur from disequilibrium compaction..

Undrained compressibility characteristics and pore pressure calculation model of foam-conditioned sand

Soil conditioning with foam is widely used to transform natural soil into the desired state suitable for mechanised tunnelling. Since the excavated soil is rapidly discharged out with little water drained during earth pressure balance (EPB) shield tunnelling, it is necessary to investigate the undrained compressibility characteristics of foam-conditioned soil under chamber pressure. In this study, a series of compression tests were carried out in a large-scale, self-designed compression device equipped with pore pressure and lateral earth pressure sensors. The results demonstrated that the foam considerably enhanced the compressibility, pore pressure and lateral pressure of the conditioned sand. The workability of foam-conditioned sand may be well conducted in slump tests under the atmospheric condition, but the compressibility of foam-conditioned probably cannot satisfy its requirement for EPB shield tunnelling due to compression of foam under high chamber pressure. In addition, the total and effective lateral pressure coefficients of the foam-conditioned sand increased and decreased, respectively with increasing foam injection ratio and total vertical pressure. Then, the mechanism of the compressibility of foam-conditioned sand is explained with an existing two-dimensional (2-D) model. Finally, according to Boyle’s law, an analytical model is proposed to predict the pore pressure of the foam-conditioned sand under compression pressure, and a satisfactory agreement between the calculated values and the test results was found.

Calculation of Average Reservoir Pore Pressure Based on Surface Displacement Using Image-To-Image Convolutional Neural Network Model

The average pore pressure during oil formation is an important parameter for measuring the energy required for the oil formation and the capacity of injection–production wells. In past studies, the average pore pressure has been derived mainly from pressure build-up test results. However, such tests are expensive and time-consuming. The surface displacement of an oilfield is the result of change in the formation pore pressure, but no method is available for calculating the formation pore pressure based on the surface displacement. Therefore, in this study, the vertical displacement of the Earth’s surface was used to calculate changes in reservoir pore pressure. We employed marker-stakes to measure ground displacement. We used an improved image-to-image convolutional neural network (CNN) that does not include pooling layers or full-connection layers and uses a new loss function. We used the forward evolution method to produce training samples with labels. The CNN completed self-training using these samples. Then, machine learning was used to invert the surface vertical displacement to change the pore pressure in the oil reservoir. The method was tested in a block of the Sazhong X development zone in the Daqing Oilfield in China. The results showed that the variation in the formation pore pressure was 83.12%, in accordance with the results of 20 groups of pressure build-up tests within the range of the marker-stake measurements. Thus, the proposed method is less expensive, and faster than existing methods.

Research on Integrated Liquefaction Assessment for Stone Column Applied in Coral Sand Area within High Earthquake Magnitude Zone

For deep and thick coral reef sand with high fine contents in high magnitude zone where the traditional vibro-compaction is not sufficient to compact, the stone column shall be applied as liquefaction mitigation measurement. Based on the analysis of the three major anti liquefaction criterion of stone column, an integrated evaluation method combined densification, shear stress distribution and drainage criterion for liquefaction mitigation was proposed, further a field test with 10.1% replacement ratio was launched. According to the analysis of the test result, the loose reclamation coral sand could be improved to an average 4.3 times SPT N value comparing. While for the medium dense coral sand, the densification among columns is limited, SPT N value is only increased by 1.3 times, and the densification decreases with fines content increasing. Further, the safety factor of liquefaction is only 0.70 and 1.01 when only considering densification criterion, or combined with shear stress redistribution criterion. However, if assessed by integrated assessment method which is mainly based on drainage criterion together with densification criterion and shear stress redistribution criterion, the calculated pore pressure ratio is still far less than the design value of 0.6 and that indicated the risk of liquefaction is still low. Moreover, when the bottom feed vibro-replacement process is adopted, the permeability coefficient of stone gravel is expected to reduce by half after stone column construction completion.

Pore Pressure Prediction and Its Relationship with Rock Strength Parameters and Weight on Bit in Carbonate Reservoirs (A Case Study, South Pars Gas Field)

Accurate pore pressure knowledge is particularly critical in drilling, such as drilling safety (avoidance of blowouts and well control lost-time incidents). Pore pressure prediction in carbonate reservoir due to the fact that the pore system and fractures are more complex than the clastic reservoir, also the effectiveness of pore pressure on drilling parameters like weight on bit that has direct effects on drilling rate is very important. As well as, the pore pressure and rock strength parameters have a special place in geomechanical studies. In this study, pore pressure was calculated using the porosity and effective stress in the carbonate reservoir. The porosity was calculated with three methods: neutron–density, neutron–sonic and sonic. The rock strength parameters (Bulk and Shear modulus) were calculated using P and S waves obtained from dipole shear sonic imager log. The calculated pore pressure was compared with modular formation dynamics tester (MDT), WOB and rock strength parameters. The result of the study showed that among available methods, predicted pore pressure with sonic porosity has the best matched to MDT data. Moreover, pore pressure has an inverse relationship to WOB and rock strength parameters, so that with increasing pore pressure of both WOB and rock strength parameters were decreasing. The comparison among predicted pore pressure, mud weight and MDT data was showing that the predicted pore pressure has high accuracy and can use it for geomechanical studies and having a safe drilling operation.

Study on Electroosmosis Consolidation of Punctiform Electrode Unit

In the electroosmosis method, when the distance between the opposite electrode and the same electrode is equal, the two‐dimensional effect of electroosmotic consolidation is significant, and the use of one‐dimensional model will overestimate the potential gradient, making the calculated pore pressure value too large. Aiming at this problem, according to the electrode arrangement rule and the minimum composition, a punctiform electrode unit model is proposed, and electroosmotic experiments are carried out on the symmetric and asymmetric unit models. The two‐dimensional electroosmotic consolidation governing equation of the punctiform electrode unit is established. The electric potential field of the electrode unit and the finite element form of the electroosmotic consolidation equation are given by the Galerkin method. The PyEcFem finite‐element numerical library is developed using Python programming to calculate. The research results show the following: (1) The two‐dimensional effect of the potential field distribution of the punctiform electrode unit is significant. The reduction of spacing of the same nature electrode in the symmetrical unit can make the potential distribution close to a uniform electric field. The asymmetry prevents the electric potential field distribution from being reduced to a one‐dimensional model. (2) The number of anodes will affect the electroosmosis effect of the soil. The more the anodes, the better the electroosmosis reinforcement effect of the soil, and the distribution of negative excess pore water pressure will be more uniform. (3) In the early stage of electroosmosis, the more the drainage boundaries, the faster the generation of negative pore pressure, but in the middle and late stages of electroosmosis, the potential value becomes the decisive factor, and the amplitude of negative pore water pressure in asymmetric units is higher than that in symmetric units. The potential distribution will not affect the degree of consolidation but will affect the extreme pore water pressure.

A new quantitative model and application for overpressure prediction in carbonate formation

The prediction of pore pressure in subsurface formations is important for the estimation of hydrocarbon accumulation and for safe drilling activities. Typically, pore pressure is estimated from either well logs (e.g., P-wave velocity, resistivity) or seismic data (seismic velocities, acoustic impedance) using empirical relations based on compaction laws. Due to the high density and extremely heterogeneous physical properties, it's not feasible to calculated pore pressure in carbonate rocks with these empirical methods. The result of ultrasonic velocity measurements in carbonate rocks shows that velocity is affected by multiple factors, e.g., stress condition, porosity, rock composition, fluid composition, and other rock properties. It's practicable to predict pore pressure with velocity when other factors are kept constant or evaluated quantitatively.In this paper, a quantitative model to compute the elastic attributes as a function of pressure and petrophysical properties is proposed. The model as a rock physical model is based on the poroelasticity theory and describes the effect on the elastic property of stress conditions. The model is validated using laboratory measurements. A case study for this model application from the northeastern Sichuan Basin, including a set of well logs and borehole measurements is also presented. The pore pressure gradient in carbonate reservoirs is ranging from 0.99 to 2.13 MPa/100 m. The predicted result shows the relative error between model predictions and observations measured while drilling is from 0.60% to 10.22%. The method proposed in this study is effective for pore pressure prediction in carbonate formation.