Pore Pressure Prediction Research Articles

A novel approach to pore pressure modeling based on conventional well logs using convolutional neural network

Accurate prediction of pore pressure (PP) is among the most critical concerns to the design of drilling operation because of the remarkable role of this parameter in preventing particular drilling problems such as wellbore instability, drilling pipe stuck, mud loss, kicks, and even blow outs. Given the limitations of PP measurement through in-hole well tests, a number of analytic and intelligent techniques have been developed to estimate the PP from conventionally available petrophysical logs at offset wells. In this contribution, analytic equations are combined with intelligent algorithms (IAs) in an integrated workflow for estimating the PP. For this purpose, we collected the required data from two wells (herein referred to as Wells A and B) penetrating a carbonate reservoir in two fields in southwestern Iran. The collected data included full-set petrophysical log data at a total of 2850 points as well as 15 measured PPs using the RFT tool. In order to model and validate the results, the data from Well A was used to train the model, with the Well-B data used for validation. Once finished with data collection, a noise attenuation stage was implemented through median filtering. Subsequently, PP estimation was practiced using a couple of popular analytic models, namely modified Eaton's, Bowers', and compressibility models, with the results compared to the measured PPs. Next, a feature selection phase was conducted where depth (Depth), gamma ray log (CGR), density log (RHOB), resistivity log (RT), pore compressibility (Cp), and slowness log (DT) were selected as the most effective parameters for estimating the PP out of the 8 parameters studied at Well A. Feature selection was performed using the second version of nondominated-sorting genetic algorithm (NSGA-II) combined with multilayer perceptron (MLP) neural network (NN). Next, deep learning techniques, simple form of the least square support vector machine (LSSVM) and its hybrid forms with particle swarm optimization (PSO), cuckoo optimization algorithm (COA), and genetic algorithm (GA), and multilayer extreme learning machine (MELM) hybridized with the PSO, COA, and GA were used to estimate the PP based on the data at Well A, with the results then validated using the data at Well B. Results of the training and testing phases showed that, among the 9 models considered in this research, the best results were produced by the CNN model followed by MELM-COA, and LSSVM-COA, corresponding to root-mean-square errors (RMSEs) of 0.1072, 0.1175, and 0.1237 and determination coefficients (R2) of 0.9884, 0.9860, and 0.9844, respectively, indicating the higher accuracy and generalizability of these models compared to other investigated models. Evaluation of these models on the validation data from Well B further remarked the superiority of the CNN model, as per an RMSE and R2 of 0.1066 and 0.9806, respectively. Indeed, the better performance of the CNN model than the other models in both the training and validation phases reflects the high generalizability of this model in the range of the studied data. In general, the good performance of the intelligent models in similar formation along two wells – where the analytic models rather failed to exhibit consistently good performance – proves the superiority of the IAs over conventional analytic models. This methodology is strongly recommended provided more diverse data is available at in larger amounts.

An Equation for the Bulk Modulus of Composites Derived From the Effective Medium Theory

Abstract Bulk modulus has wide applications in well engineering, seismic exploration, waste reinjection, and predicting pore pressure in carbonate reservoirs. However, there is no easy way to obtain accurate values for the effective bulk modulus of rocks. Practically, researchers use rigorous, costly, and time-consuming experiments on core samples. But, stress release and changing rock’s environment have affected the accuracy of results. Also, it is impossible to get accurate values of the effective bulk modulus from theory without accounting for the deformation of microcracks in the rock. Existing models do not consider the presence of microcracks because of the inability to define the positions of cracks relative to one another. Thus, earlier studies introduced approximations to define the upper and lower bounds of values. This study aims to overcome this limitation by accounting for the fluids in the microcracks, apart from those in stiff pores. From the product of the surface area and thickness of the fluid in the microcracks, the authors generated proportionality between the volume of fluid and that of the grain and obtained expression for the crack porosity. Then analytical and numerical techniques were applied to obtain models for the effective bulk modulus. The results show that the presence and magnitude of inclusions reduce the effective bulk modulus significantly. This was validated by a finite element analysis (FEA) using the FEATool run in matlab. In addition, higher volume of fluids in the microcracks makes the rate of change of the bulk modulus with the porosity to be higher.

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.

Elastic depth trends for siliciclastic sequences

Elastic depth trends are an integral component of reservoir characterization whether explicitly or implicitly constructed. Applications to seismic data include migration velocity model building, pore-pressure prediction, inversion, and amplitude-variation-with-offset (AVO) analysis. In practice, workflows often involve fitting well logs with smooth curves, without reference to known lithology-dependent templates that could provide realistic constraints. We have constructed porosity-depth trends for shales and sandstones with reference to reliable measurements that span mechanical compaction and chemical diagenesis domains. These porosity trends are then used as inputs to well-established rock-physics models to estimate the median and bounding trends for acoustic and shear impedance. The resulting modeling has shown an important sensitivity to the pore-pressure regime (notably in shales) and the onset of cementation (in sands), among other variables. AVO predictions made from these trends provide a useful reference frame for anomaly prediction with depth, which is critical in frontier exploration.

New understanding of overpressure responses and pore pressure prediction: Insights from the effect of clay mineral transformations on mudstone compaction

Chemical compaction dominated by clay mineral transformations, commonly develops in deeply buried mudstones, and prominently affects the petrophysical properties of mudstones and fluid flow. This paper takes the Neogene mudstones in the Ledong Slope of the Yinggehai Basin as research object, analyzes the relationship between chemical compaction and overpressure, establishes a new multisegment normal compaction trends (NCTs) and reevaluates the overpressure mechanisms. The results show obvious clay mineral transformation with increasing depth, and the top of the clay mineral transformations is approximately equal to the depth of the overpressure. The logging responses of overpressured mudstones are affected by clay mineral diagenesis, showing a 45% increase in the sonic transit time at 3500-5000 m and a 10% decrease in the density at the same interval, and the density response exhibits a relative lag. A method comprising an inversion algorithm calibrated by measured pressure data is proposed to establish multisegment NCTs. Furthermore, new loading-unloading curves comprising a mechanical loading curve and chemical loading curve are established to reevaluate the overpressure mechanisms. The loading-unloading curves show that the pressure gradient generated by mudstone compaction maybe less than 1.8. While the pressure data with pressure gradient larger than 1.8 fall on the unloading curve, indicating elevated sandstone pressures rather than in situ mudstone pressures. The multisegment NCTs were proven to provide a satisfactory overpressure estimate in the Ledong Slope as well as the relative contribution of different overpressure mechanisms. This paper provides new insight into mudstone compaction and overpressure responses, as well as a meaningful reference for the identification of multiple-overpressure generation mechanisms and the prediction of pore pressure in deeply buried formations.

Identification of Geo – Hazard Using Pore Pressure Analysis in ‘MAC’ Field, Niger Delta

Identification of geo-hazard zones using pore pressure analysis in ‘MAC’ field was carried out in this research. Suite of wireline logs from four wells and RFT pressure data from two wells were utilized. Lithologic identification was done using gamma ray log. Resistivity log was used to delineate hydrocarbon and non-hydrocarbon formations. Well log correlation helps to see the lateral continuity of the sands. Pore pressure prediction was done using integrated approaches. The general lithology identified is alternation of sand and shale units. The stratigraphy is typical of Agbada Formation. Three reservoirs delineated were laterally correlated. Crossplot of Vp against density (Rho) colour coded with depth revealed that disequilibrium compaction is the main overpressure generating mechanism in the field. Prediction of overpressure by normal compaction trend was generated and plot of interval transit time against depth show that there is normal compaction from 250m to about 1700 m on MAC-01, but at a depth of about 1800m, there was abnormal pressure build up that shows the onset of overpressure. A relatively normal compaction was observed on MAC-02 until a depth of about 2100m where overpressure was suspected. The prediction of formation pore pressure using Eaton’s and Bower’s method to determine the better of the two methods to adopt for pore pressure prediction shows that the pore pressure prediction using Eaton’s method gave a better result similar to the acquired pressure in the field. Hence Eaton’s method appears to be better suited for formation pore pressure estimation in ‘MAC’ field. The validation of the pore pressure analysis results with available acquired pressure data affirmed the confidence in the interpreted results for this study.

WITHDRAWN: Investigation of abnormal pore pressure variations by the application of seismic inversion in Norne Field, Mid-Norwegian margin Norway

The study of pore pressure in a sedimentary basin is the panacea for safe and productive exploitation of the subsurface for hydrocarbon exploration, and field optimization. Pore pressure prediction has an important application in the proper selection of the casing points and reliable mud weight. In addition, using cost-effective techniques of pore pressure prediction, which give an extensive and continuous range of data, is much more affordable than the direct measuring of pore pressure. This research aims at predicting and modeling abnormal pressure variations from the seismic volume through Seismic inversion. The abnormal pore pressure region was identified by applying a normal compaction trend line on P-sonic and density log. This reveals a change in the compaction trend from the interval of 1400 m to 2500 m in well 6608/10_4 and 1350 m to about 2600 m in well 6608/10_4, which is within the Kia formation of about 1360 m towards the end of Not formation about 2600 m before the hydrocarbon reservoir. The estimated pore pressure from the well log suggests the pore pressure from this region to be abnormal, and it complements the normal compaction trend. The predicted pressure from the well was used to calibrate the acoustic impedance in the investigated interval. The result from the seismic inverted volume justifies the predicted pressure from the logs, and the result was able to make clear areas that are not penetrated by well in the volume. The observed seismic inverted result reveals lateral and vertical variations in pore pressure which suggests inhomogeneity in the formation depositional trend.

The accuracy in pore pressure prediction via seismic and well log data: a case study

The accuracy in pore pressure prediction via seismic and well log data: a case study

An empirical model for predicting pore pressure development in artificial freeze-thaw soft clay under cyclic loading

In recent years, the artificial ground freezing (AGF) method has been widely used in subway construction, and the engineering properties of soil have changed significantly after freezing and thawing. In this paper, a series of dynamic triaxial tests were used to investigate the pore pressure development of freeze-thaw soft clay under different cyclic dynamic stress ratios, loading frequencies and freezing temperatures. The test results show that the types of normalization pore pressure development include developmental and stable. An empirical model, considering the effects of cyclic dynamic stress ratio, loading frequency, freezing temperature and number of cyclic loading, was proposed and compared with three representative models proposed by the predecessors to validate that the proposed method has the best estimation on the development of the pore pressure.

Practical workflows for monitoring saturation and pressure changes from 4D seismic data: A case study of Malay Basin

Discrimination between fluid saturation and pore pressure-related changes using 4D seismic response is a challenging issue in reservoir studies. The main objectives of this study are the estimation of pore pressure and water saturation for baseline and monitor seismic surveys as well as determination of bypassed hydrocarbons location. To achieve this goal, we propose two practical workflows. The workflows originate from the fact that changes in pressure and saturation result in different trends on a Vp/Vs ratio versus P-Impedance cross-plot. The fact that an increase in hydrocarbon saturation results in a decrease in the Vp/Vs ratio and that an increase in pore pressure causes an increase in the Vp/Vs ratio plays a key role in this study. In this approach, after rock physics modeling and seismic data conditioning, elastic seismic inversion is performed for baseline and monitor surveys. The Bayesian probability classification technique is used to differentiate pay zones from non-pay zones which is then incorporated for prediction of fluid saturation changes. In the pore pressure prediction workflow, the Bowers equation is replaced with an empirical relationship in which a consistent match is attained between the calculated effective pressure and Vp/Vs ratio at injected water zones. Pore pressure prediction reveals noticeable anomalies around the injector wells but pressure depletion around the producers remains uncertain due to the insignificant pore pressure drop and the poor quality of seismic data near the platform area. However, at most of the producers' locations, the predicted pore pressure shows a depletion trend confirmed by the field-measured pressure. In this case study, the unswept oil and injected water areas are clearly outlined, which helps to improve sweep efficiency and update EOR scenarios.

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.

Finite Element Simulation of Vacuum Preloading at Palembang – Indralaya Toll Project

Methods for the prediction of soil behavior during the application of vacuum preloading are available but have not been used precisely and have not been proven yet in Indonesia. There are two common approaches to vacuum preloading simulation, based on the application of a uniform external load to the vacuum area, and based on suddenly lowering the groundwater level to create vacuum conditions, respectively. This affects the settlement, lateral deformation, and pore pressure predictions. The objective of this research was to improve the prediction of soil behavior based on high-quality field data by using state of the art vacuum preloading simulations. The results were compared with those of a series of instrumentation equipment, i.e. a settlement plate, an extensometer, and a piezometer. This research used data from the Palembang-Indralaya Toll Road, a section of the Trans Sumatera Toll Road that is approximately 22 km long and has an embankment height of about 4 m to 9 m. It was built over a swampy soft soil area, using vacuum preloading to improve the soil. Axisymmetric analysis of vacuum preloading was conducted for a single-drain system, plane-strain analysis was conducted for single- and multiple-drain systems, and 3D analysis was conducted for single-drain, multiple-drain, and cluster-drain systems. The results show that the proposed method produced a good correlation between the predicted data and the recorded monitoring data.

The method for prediction of formation pore pressure based on mechanical specific energy theory

With traditional prediction methods, the accuracy for prediction of formation pressure is low in formations with complex lithology, such as saline aquifers, fractured formations and carbonate rocks. Therefore, in Qinghai Oilfield, in order to ensure the drilling safety, studies have been conducted in order to find the method for predicting the pore pressure of E32 formation, in which the lithology is complex. Mechanical specific energy is the total energy required for breaking and removing rock per unit volume. With the increase of depth, the confining pressure of rock and vertical effective stress increase. Therefore, more energy is required to break and remove rock per unit volume. However, in abnormal high-pressure formation, pore fluid bears some overburden pressure, and the rock effective stress is reduced, leading to less requirement for mechanical specific energy. Therefore, it is feasible to evaluate the formation pressure qualitatively based on the mechanical specific energy theory. In this study, according to the actual drilling conditions, a model for prediction of formation pressure has been constructed and optimized on the basis of mechanical specific energy theory. With this model, the prediction accuracy on formation pressure has been improved obviously. This model has been used in prediction of formation pressure of the E32 formation in several wells in Qinghai Oilfield, which are carbonate rocks with fractures developed. The application results show that this formation pressure prediction model based on the mechanical specific energy works well in prediction of formation pore pressure, with an error less than 12%, which meets the engineering requirements. It provides a new method for predicting the pore pressure in complex lithologic formations such as carbonate rocks.

Pore pressure prediction in Niger Delta high pressure, high temperature (HP/HT) domains using well logs and 3D seismic data: a case study of X-field, onshore Niger Delta

Few wells targeting high temperature, high pressure intervals in most tertiary sedimentary basins have achieved their objective in terms of technicalities and cost. Since most shallow targets have been drilled, exploration focus is drifting into deeper plays both onshore and in deep offshore areas. To ensure safe and economic drilling campaigns, pore pressure prediction methodologies used in the region needs to be improved. The research aims at generating and testing a modification of Eaton’s equation fit for high temperature, high pressure intervals on a field. The evolution of pore pressure in the field was established from offset well data by making several crossplots, and fracture gradient was computed using Mathew and Kelly’s equation. Eaton’s equation parameters were then calibrated using several wells until a desired field scale result was achieved when compared with information from already drilled intervals i.e., kicks and RFT data. Seismic velocity data resulting from high density, high resolution velocity analysis done to target deep overpressured intervals were then used to predict 1D pore pressure models at six selected prospect locations. Analyses reveal depths shallower than 3800 m TVD/MSL with geothermal gradient 3.0 °C/100 m and pressure gradient less than 1.50sg EMW are affected mainly by undercompaction; depths greater than 3800 m TVD/MSL with geothermal gradient of 4.1 °C/10 m and pressure gradients reaching 1.82–2.12sg EMW are affected by unloading with a narrow drilling margin for the deep highly pressured prospect intervals. Eaton’s n-exponent was modified to 6, and it proved accurate in predicting high overpressure in the first prospect wells drilled.

Integration of seismic methods and 3D basin modeling approach for improved predrill pore pressure prediction: an example from Malay Basin, Malaysia

Integration of seismic methods and 3D basin modeling approach for improved predrill pore pressure prediction: an example from Malay Basin, Malaysia

Wave-induced seabed momentary liquefaction in shallow water

To realize seabed behaviours under extreme waves in coastal regions, a wave-induced seabed response subjected to high-order cnoidal wave theory is studied via Biot’s consolidation theory. An analytical model is proposed for a dynamic response comprising pore pressure and effective stresses in a permeable layer of the seafloor. The seabed pressure solutions yielded by cnoidal waves up to the third order are derived as an input loading under boundary conditions, and high-power Jacobian elliptic cosine functions involved in cnoidal wave theory are approximated in terms of a Fourier series to construct the seabed response via the superposition of harmonic solutions. For the present Fourier series approximation of Jacobian elliptic cosine functions, a convergence analysis is performed, and a verification of seabed pressure compared with experimental data shows good agreement. The characteristics of the present solution are investigated, and the discrepancies among the solutions for each order and a harmonic solution for linear waves are demonstrated on the profiles of effective stresses and excess pore pressure. Compared with the third-order solution, the harmonic solution provides a better prediction at the wave peak than the other solutions, but the prediction of excess pore pressure around the wave trough shows a remarkable overestimation for every solution except the second-order solution. The impact of seabed momentary liquefaction induced by shallow water waves is evaluated, and the influence of wave parameters, including wave height and wavelength, is determined. This study reveals the significance of the nonlinear effect on seabed response under high-order cnoidal waves and provides the underlying implication to liquefaction assessment.

Caldera’s Breathing: Poroelastic Ground Deformation at Campi Flegrei (Italy)

Ground deformation at Campi Flegrei has fuelled a long-term scientific debate about its driving mechanism and its significance in hazard assessment. In an active volcanic system hosting a wide hydrothermal circulation, both magmatic and hydrothermal fluids could be responsible, to variable degrees, for the observed ground displacement. Fast and large uplifts are commonly interpreted in terms of pressure or volume changes associated with magma intrusion, while minor, slower displacement can be related to shallower sources. This work focuses on the deformation history of the last 35 years and shows that ground deformation measured at Campi Flegrei since 1985 is consistent with a poroelastic response of a shallow hydrothermal system to changes in pore pressure and fluid content. The extensive literature available for Campi Flegrei allows constraining system geometry, properties, and conditions. Changes in pore pressure and fluid content necessary to cause the observed deformation can then be calculated based on the linear theory of poroelasticity. The predicted pore pressure evolution and fluid fluxes are plausible and consistent with available measurements and independent estimates.

Pore Pressure Increase and Evaluation in Saturated Sand Based on In Situ Liquefaction Tests

Abstract Through self-developed testing technology, in situ liquefaction tests were conducted to investigate pore pressure responses of saturated sand in the field, and a new incremental model is proposed to predict pore pressure increase in saturated sand liquefaction. The pore pressure generation of saturated sand in the field is slower than that shown through the dynamic triaxial test. In situ testing results herein indicate that the effect of number-of-cycle is different from the results obtained from the dynamic triaxial test. The variation of pore pressure rise in the in situ tests initially increases then decreases with the increasing number of loading cycles rather than a monotonic decrease mode. Different from existing models, the new pore pressure model is directly expressed with in situ parameters, such as acceleration, buried depth, etc., which are commonly applied to describe soil characteristics in the field. The advantages of the proposed formula by in situ tests are that it avoids uncertain stress evaluation during establishment and application of the new model and can greatly reduce artificial errors. Comparisons between predicted and measured results indicate the reliability of the new model. The proposed model can conveniently evaluate the pore pressure increase of saturated sand in a free horizontal field.

Predictive modelling of piezometric head and seepage discharge in earth dam using soft computational models.

Predictions of pore pressure and seepage discharge are the most important parameters in the design of earth dams and assessing their safety during the operational period as well. In this research, soft computing models namely multi-layer perceptron neural network (MLPNN), support vector machine (SVM), multivariate adaptive regression splines (MARS), genetic programming (GP), M5 algorithm, and group method of data handling (GMDH) were used to predict the piezometric head in the core and the seepage discharge through the body of earth dam. For this purpose, the data recorded by the absolute instrument during the last 94 months of Shahid Kazemi Bukan Dam were used. The results showed that all of the applied models had a permissible level of accuracy in the prediction of the piezometric heads. The average error indices for the models in the training phase were R2= 0.957 and RMSE= 0.806 and in the testing phase were equal to R2= 0.949 and RMSE= 0.932, respectively. The performances of all models except the M5 and MARS in predicting seepage discharge are nearly identical; however, the best is the MARS, and the weakest is the M5 algorithm.

Analysis of wellbore stability by pore pressure prediction using seismic velocity

Borehole problems result in the wastage of huge expenditures in petroleum industry. One of the major challenges is to keep the borehole intact at a particular location and prevent it from falling in reactive shale in high- or low-pressure zones. To convert seismic velocity to pore pressure, the modified Eaton's equation is widely used. A guideline was prepared using both stochastic and non-uniform parameter distributions. Pore pressure and fracture pressure were calculated using a modified version of Eaton's and Mathew and Kelly's methods. To validate the results, geomechanical models were developed based on good correlation. Different regions in the eastern state of India at Tripura were taken into consideration and pore pressure was estimated for the regions of Kathalchari and Ambassa by plotting pressures. The actual pore pressure predicted in Tripura was calculated using seis P and seis P3, the indigenous software which is a modified version of Eaton's method and Mathew and Kelly's method, and the pore pressure from our method matches the calculated Repeat Formation Test (RFT) data from wells.