Permeability Saturation Research Articles

Analysis and Peak Value Prediction of Rainfall Component Factors Influencing Seepage Flow for Homogeneous Earth Dams

The rainfall component of seepage flow is an important indicator for monitoring earth dam safety. Compared with previous practice of only analyzing rainfall, this paper divides the analysis dimensions into three categories according to the seepage process in the process of rainfall: the saturation permeability coefficient of the soil receiving the rainfall, the rainfall intensity and rainfall duration. This paper conducted FE simulation according to the principle of unsaturated seepage flow, and it is concluded that: the influence of different rainfall intensity on seepage flow is roughly linear and can be expressed by a first power factor; the influence of different saturation permeability coefficient and rainfall duration on seepage flow is obviously nonlinear, and should be expressed by multiple powers. Based on the above law, the model expression of peak seepage flow prediction was established, and both the fitting effect and prediction accuracy were good. The results show that the law of influence factors has been summarized correctly, and the research result can provide effective theoretical support for the seepage flow monitoring and early warning and safety monitoring for homogeneous earth dams.

Investigating the mechanism of pH effect on saturated permeability of remolded loess

The pollution of remolded loess caused by acids and bases greatly affects its engineering performance, and thereby seriously threaten the service life of loess buildings or structures. To explore the changes in saturated permeability of the remolded loess polluted by hydrochloric acid or sodium hydroxide solutions, a series of the saturated seepage tests were designed in presence of different pH values. In addition, the hydrochemical components in the leachate along with the zeta potentials in water-soil solutions under different pH were measured. The changes in mineralogical, chemical, and microstructural properties of the loess samples before and after the seepage were also investigated. The results demonstrate gradual increase in the permeability of the loess samples with the decrease of pH of the solution. When acid enters the loess, the dissolution of carbonate cement and chemical cement (Al 2 O 3 and Fe 2 O 3 ) lays the foundation for the enlargement of pores. The decrease in thickness of the diffuse double layer results in aggregation of clay particles and enlarges the space of gravity water moving in the free layer. Hence, the significant increase in the permeability takes place over time during the seepage at pH = 3. The change in permeability at pH = 7 shows similar behavior to the acidic condition, and this is mainly responsible for the leaching of carbonate. However, the alkaline (pH = 11) condition inhibits the dissolution of carbonate minerals, and calcite is precipitated due to the incongruous dissolution of dolomite and dedolomitization. Al(OH) 3 , Fe(OH) 3 , and H 2 SiO 3 colloids are formed and attached to the particles. These processes plug the pores during the seepage. Moreover, the development of the diffuse double layer results in wide distribution of the clay particles with highly cementation, which greatly reduces the effective connectivity of the pores. Therefore, its permeability decreases with the seepage time. These findings will offer reference for the practice of geological engineering in the loess areas. • The change laws of saturated permeability of remolded loess are investigated at different pH. • The geochemical process, chemical behavior, and electrokinetic property are explored during the seepage. • The similarities and differences of physicochemical interactions between soil and water are revealed at different pH. • The mechanisms of pH effect on saturated permeability of remolded loess are disclosed.

Regression Committee Machine and Petrophysical Model Jointly Driven Parameters Prediction From Wireline Logs in Tight Sandstone Reservoirs

Tight sandstone reservoirs are characterized by low porosity, extra-low permeability, and diverse mineral compositions for which previously established log interpretation methods are not suitable. Therefore, it is necessary to explore new log interpretation methods. The committee machine is a recently developed composite expert network. The regression committee machine (RCM) is constructed by combining a backpropagation neural network (BPNN), extreme learning machine (ELM), wavelet neural network (WNN), and using two different weight calculators for decision-making. Petrophysical models for tight sandstone reservoirs are integrated together with the RCM. The RCM and petrophysical model hybrid intelligent system for log interpretation is developed. The data process flow and specific implementation method of the log interpretation are designed. The tenfold cross-validation method is used to train and optimize the parameters of the RCM in the hybrid intelligent system. The hybrid intelligent system is applied to the Chang 8 tight sandstone of Yanchang Formation in the Ordos Basin, China. The case studies show that the predicted porosity, permeability, and water saturation by the RCM with petrophysical models are all consistent with the core measurement. The data and models jointly driven intelligent system is more accurate than petrophysical models and individual expert networks. This study effectively improves the formation evaluation for tight sandstone reservoirs.

Bio-improved hydraulic properties of sand treated by soybean urease induced carbonate precipitation and its application Part 2: Sand-geotextile capillary barrier effect

Nonwoven-geotextile is used to construct soil-geotextile capillary barrier for minimizing water infiltration. However, the sand-geotextile capillary barrier breakthrough can occur due to the low water retention ability and the high saturated permeability coefficient of sand. To address this, enzyme induced carbonate precipitation (EICP) technology was applied to the overlying sand layer to improve its hydraulic properties and enhance the sand-geotextile capillary barrier. The precipitated calcium carbonate has the potential to enhance the capillary barrier effect by coating soil particles and filling the soil macro-pores. To investigate the effect of EICP treatment on the sand-geotextile capillary barrier, three column tests were conducted, including one untreated sand-geotextile column and two sand-geotextile columns treated with 0.5 and 1 mol/L urea-calcium chloride solutions and soybean urease liquid, respectively. Transient seepage analysis was carried out based on the column tests to calibrate the hydraulic parameters. A numerical parametric analysis was conducted to assess the effect of EICP-enhancement on the hydraulic behavior and slope stability of a sand-geotextile capillary barrier retaining wall during exposure to rainfall with a 100-year return period. The results of the column infiltration tests show that the occurrence of breakthrough in the sand-geotextile capillary barrier is significantly delayed with increasing concentration of the urea-calcium chloride solutions. The enhanced capillary barrier effect is attributed to the increased water retention ability and reduced saturated permeability coefficient. The results of the parametric analysis show that the matric suction within the backfill is maintained and the factor of safety remains almost unchanged due to the EICP-enhanced sand-geotextile capillary barrier effect.

Influence of aging on hydro-mechanical behavior of unsaturated ballast

To better understand adverse impacts of aging on track maintenance, the hydro-mechanical behavior of unsaturated fresh and aged ballasts was investigated by means of saturated permeability tests, water retention tests, and monotonic loading triaxial compression tests. Firstly, the aged ballast was prepared through the Los Angeles abrasion (LAA) test to reproduce the gradation and particle shape for in-situ aged ballast extracted from actual railway tracks in Japan. Moreover, the results of saturated permeability tests and water retention tests prove that aging induces a remarkable decrease in the saturated coefficient of permeability and a significant increase in water retentivity compared with fresh ballast, due to the presence of fine fraction content. For the shear strength, the effective shear strength parameters of fresh ballast under fully drained conditions (CD test) are close to those under fully undrained conditions (CU test), regardless of water content. However, for the aged ballast, due to the low permeability, it is difficult to simulate fully drained conditions with laboratory tests, even at a very low axial strain rate (0.01%/min). In this case, the effective shear strength parameters of aged ballast are more suitable to be determined by the CU test. Besides, the results of triaxial compression tests prove that aging reduces the effective internal friction angle of ballast, while it increases the total cohesion compared with fresh ballast, especially under the high saturation condition. Lastly, the shear strength of both materials has a decreasing tendency with increasing water content, and the declining trend of shear strength is more noticeable for aged ballast. Therefore, the effects of aging and moisture content on the hydro-mechanical behavior of the railroad ballast should be synergistically considered in the railway track design.

Exploring the geochemical mechanism for the saturated permeability change of remolded loess

Abstract Loess has been remolded as the foundation for various construction projects with a long period of history. However, the operation of loess construction engineering is affected by the continuous change of loess permeability during the seepage. Physical processes have been found to be responsible for the change of permeability, while neglecting the geochemical processes. In this study, the saturated seepage test was carried out, and the inverse geochemical model based on the PHREEQC software was established for better understanding the change in the saturated permeability of remolded loess and its underlying geochemical mechanism. Results show a negative log correlation of the mean saturated hydraulic conductivity (Ksat) with the dry density of the loess samples. The Ksat of the samples with various dry densities increases with the seepage time as a whole. Taking the loess sample with dry density of 1.45 g·cm−3 as an example, the time dependent change of Ksat can be divided into two stages: relatively stable type in the early stage and significantly increasing type in the later stage. The relatively stable permeability is due to the comprehensive effect of the dissolution of halite, gypsum, and dolomite, the precipitation of calcite, and ion exchange. Dissolution of calcite and dolomite is responsible for the increasing tendency of the permeability of the remolded loess. The cation exchange and the reaction of the clay minerals also need to be considered. In general, the carbonate leaching is the dominant factor for the increase in the saturated permeability of remolded loess. Secondly, the cation exchange and the reaction of the clay minerals lead to the change of the saturated permeability by influencing the enlargement of pore space.

An investigation into the relationship between saturated permeability and microstructure of remolded loess: A case study from Chinese Loess Plateau

Understanding the permeability law of remolded loess is important for engineering construction in the loess area. To gain insights into the changes of saturated permeability and microstructural evolution of remolded loess for different dry densities, the loess from the Chinese Loess Plateau was used as a test material and was remolded as samples with different dry densities at the optimum water content state. The saturated seepage test and scanning electron microscopy (SEM) imaging were conducted on the remolded loess samples. The microscopic properties of the loess pores in the SEM images were identified using Image-Pro Plus software. The relationship between the micro-parameters of the pores and permeability were explored using multivariate statistical analysis. Results from this study highlight that the dry density of samples can be classified into three categories: Group I (1.30 and 1.35 g·cm−3) shows permeability decreasing with seepage time, Group II (1.40 g·cm−3) shows permeability initially increasing and then decreasing with seepage time, and Group III (1.45–1.65 g·cm−3) shows permeability increasing with seepage time. Correspondingly, the dry densities of these groups were found to be unsuitable, doubtful, and suitable, respectively, for laboratory study on the saturated permeability of remolded loess. In addition, the evolution mode of remolded loess structure is basically overhead-interlocking-matrix with increasing compactness. In this process, the macropores transform into mesopores and then to small pores, while the change in micropore content is weak. Also, the directional distribution of the pores weakens, while the pore morphology changes little. Furthermore, the contents of macropores and mesopores, as well as the pore diameters, play a positive role in permeability, while the contents of small pores and micropores, and directional probability entropy have a negative impact on permeability.

Microstructure and permeability evolution of remolded loess with different dry densities under saturated seepage

Understanding water seepage in loess is important for engineering constructions and geo-hazard prevention in loess regions. However, the saturated seepage behavior of remolded loess with different dry densities has been found to be inconsistent. In order to gain a deeper insight into the changes of saturated permeability for remolded loess with different dry densities and the underlying microscopic mechanisms, the saturated seepage tests were designed for loess samples, and scanning electron microscope (SEM) imaging was carried out to obverse the changes of loess microstructure before and after the seepage test. The results of this study highlight that the water seepage in loess samples with dry densities smaller than or equal to 1.35 g·cm−3 would lead to the movement of particles within the sample, thus blocking the pores and even causing the collapse of the loess structure. In this process, the pores in the sample transform from macropores to mesopores and small pores, and from mesopores to small pores, such that the saturated permeability of remolded loess decreases with the seepage time. While, for loess samples with dry densities greater than or equal to 1.45 g·cm−3, the saturated permeability of loess increases with the seepage time. The microstructure obtained by SEM revealed that the transition from small pores and micropores to mesopores after seepage. The enlargement of the pores is mainly caused by chemical reactions during the seepage, namely, the dissolution of soluble salt and the leaching of calcium cement. For the loess samples with dry density of 1.40 g·cm−3, the changes of saturated permeability over seepage time can be defined as a transition stage from the decreasing to increasing. Leaching enlarged the pores of soil samples and enhanced their connectivity. At the same time, it also formed the space for particle migration which could lead to the blocking of pores, and caused the reduction of soil strength which may result in the structural collapse of the loess. Therefore, the saturated permeability of the sample first increases and then decreases with seepage time. These findings have practical significance for the construction and protection of loess engineering in loess area.

DIGITAL ROCK ANALYSIS: AN ALTERNATIVE METHOD TO PREDICT PETROPHYSICAL PROPERTIES, CASE STUDY FROM MISHRIF FORMATION

The digital core analysis of petrophysical properties replace the use of conventional core analysis by reducing the required time for investigation. Also, the ability to capture pore geometries and fluid behavior at the pore-scale improves the understanding of complex reservoir structures. In this work, 53 samples of 2D thin section petrographic images were used for analyses from the core plugs taken from the Buzurgan oil field. Each sample was impregnated with blue-dyed epoxy, thin sectioned and then was stained for discrimination of carbonate minerals. Each thin section has been described in detail and illustrated by photomicrographs. The studied samples include a variety of rock types. Packstone is the most common rock type observed followed by grainstone and packstone – wackestone. Floatstone and dolostone are noted rarely in the studied interval. However, the samples of thin section images are processed and digitized, utilizing MATLAB programming and image analysis software. The entire workflow of digital core analysis from image segmentation to petrophysical rock properties determination was performed. A focused has been made on determining effective and total porosity, absolute permeability, and irreducible water saturation. Absolute permeability is estimated with the Kozeny-Carman permeability correlation model and Timur-Coates permeability correlation model. Irreducible water saturation simply is derived from total and effective porosity. Also, some pore void characteristics, such as area and perimeter, were calculated. The results of Digital 2D image analysis have been compared to laboratory core measurements to investigate the reliability and restrictions of the digital image interpretation techniques.

Saturated hydraulic conductivity of problematic soils measured by a newly developed low-compliance triaxial permeameter

Research on the reliable characterization of saturated permeability has received great yet necessary attention globally. However, most studies were carried out on soils with a volumetric tendency being almost insensitive to water saturation or at least with this hidden assumption. This is clearly not the case when dealing with the two broad categories of problematic soils, including collapsible and expansive soils. As confirmed by previous research, at least one-fifth of the planet comprises of these soils with a relatively fair distribution. Therefore, the main objective of this study is to introduce a low-compliance double cell/burette permeameter to track all volumetric changes during the whole pre- and post-saturation testing stages with distinct enhancement of accuracy. In order to examine the performance of the device, two typical types of collapsible and expansive soils are synthesized and tested for a wide variety of void ratios. Comparison of results with four standardized and conventional testing protocols in the literature reveals that current laboratory methods fail in reliable quantification of saturated permeability of problematic soils due to inaccurate evaluation of void ratio, imperfect saturation, or both. Eventually, a discussion will be presented on the performance of some empirical models in capturing the new experimental trends.

Analysis of CO2 and brine relative permeability of in-situ core samples with employing double separator

The relative permeability of CO2 and brine has a significant impact on CO2 saturation, migration, and injectivity for CO2 geological storage. This study presents a new experimental apparatus consisting of a double separator that can reduce uncertainties in measuring fluid saturation. It was applied to measure relative permeabilities of in-situ cores samples of Berea, Gorae, and Paaratte sandstone. The wettability criteria and Brook-Corey model were applied to explain the characteristics in relative permeability of CO2 and brine. The result for Berea sandstone using the developed apparatus was consistent with results obtained in literatures, and they indicate high residual brine saturation and low end-point relative permeability of CO2. The results for Gorae sandstone show that the residual brine saturation is higher and the shape of the relative permeability curves are more concave compared to those of Paaratte sandstone; these results relate to the water-wetting tendency and dominant capillary flow, respectively. It means that injection of CO2 into Gorae sandstone is expected to be less efficient than into Paaratte sandstone. Therefore, it is reasonable to estimate that less amount of CO2 should be injected into Gorae sandstone compared to Paaratte sandstone.

Investigation into microscopic mechanisms of anisotropic saturated permeability of undisturbed Q2 loess

The anisotropic permeability of loess is an important engineering property. To explore the microscopic mechanisms of anisotropic saturated permeability of undisturbed loess, a series of laboratory tests, including constant-head saturated permeability test, particle size analysis, and scanning electron microscope imaging, were carried out for twelve Q2 loess samples. The results show that with increasing burial depth, the saturated permeability decreases. Increasing the hydraulic gradient leads to an increase in permeability without changing the anisotropy of permeability. The spatial variability of the vertical permeability is more obvious than that in the horizontal direction. In addition, the permeability is negatively correlated with the clay particle content, and the uniformity coefficient of the particles. The area of the pores is the dominant factor controlling the permeability, especially the extra-large pores. Moreover, the direction and morphology of pores also affect the permeability of undisturbed loess. These findings are helpful for better understanding the water infiltration process in undisturbed loess and for investigating the stability of loess slopes.

Stochastic reservoir modelling for prospect mapping: a case study of ''bright'' field, Niger delta

Deterministic reservoir modeling using geostatistical approach is inherently ambiguous because of the uncertainties contained in the generated reservoir models. Stochastic reservoir modelling using sequential gaussian simulation algorithm can resolve this problem by generating various realizations of petrophysical property models in order to map this uncertainties caused by subsurface heterogeneity. Suites of well logs for four wells with seismic data in SEG-Y format were used for this analysis. The wells were correlated and a reservoir was mapped across them in other to map their lateral extent, synthetic seismogram was generated in other to match the event on the seismic with that of the synthetic after carrying out a shift of -12ms. Seismic to well tie was done to ensure that the horizons were mapped accurately. The structural maps generated and the wells were input that goes into the stochastic modelling process. Five realizations each of facies(lithology), effective porosity, total porosity, net to gross, volume of shale and one realization for permeability and water saturation were generated. The facies models showed the distribution of sand and shale with sand at the existing well locations and the effective porosity, total porosity, net to gross, volume of shale models showed excellent values around the well location. Permeability and water saturation models showed that the existing wells were drilled at the flank of the anticlinal structure. Two drillable points (prospects) were proposed by considering all the initial petrophysical property models and the parameters of the two points named P1 and P2 showed that they contain hydrocarbon in commercial quantity. Stochastic reservoir modelling has proved effective in mapping uncertainties and detecting bypassed hydrocarbons. Â

Stability analysis of vegetated residual soil slope in Thailand under rainfall conditions

The effects of climate change including intense rainfalls have resulted in many slope failures. The bioengineering method using vegetation is an environmentally friendly alternative for slope stabilisation. This paper presents the effect of root reinforcement on the stability of vegetated slope under rainfall condition. A transient seepage and slope stability analysis was conducted based on the finite-element and limit equilibrium methods. Field monitoring on a residual soil slope in Thailand was numerically modelled. In transient seepage analysis, the soil-water characteristic curve and the saturated permeability measured from laboratory and in situ tests were used, and the results were comparable with the monitored values from the site. For slope stability analysis, the effective shear strength parameters from direct shear tests were used to model the well-compacted and non-compacted soil slopes. The additional shear strength of soil from the root cohesion was modelled for vegetated soil slope. The results show that the contribution of root cohesion to slope stability was more significant for the non-compacted soil condition than for the well-compacted one. Both modelled and monitored pore-water pressure reached the highest values about 1 d after the daily rainfall had reached its peak. This finding has a significant implication for landslide warning based on rainfall.

WELL LOGS DATA PREDICTION OF THE NAHR UMR AND MISHRIF FORMATIONS IN THE WELL NOOR-10, SOUTHERN IRAQ

Well logs data are the most widely used data to evaluate subsurface rocks, their petrophysical properties include porosity, permeability and fluid saturation. They are essential for the hydrocarbon reserves estimations and perforation zones determination for production purposes and fields development. Well logging operations of the targeted reservoirs could not be done in NO-10 Well, Noor Oilfield, Southern Iraq due to some problems related to the well condition. The gamma-ray and sonic logs were the only recorded logs, while neutron, density and deep resistivity logs are missed. The missing neutron, density and deep resistivity logs of the Early Cretaceous Nahr Umr Sandstone and the Late Cretaceous Mishrif formations of the well NO-10 were produced and compared together using the Artificial Neural Network ANN in Petrel software. The results show that the total correlation of the ANN Nahr Umr model for the neutron, density and deep resistivity logs are 0.81, 0.49 and 0.51 respectively. Interestingly, the ANN Mishrif Formation model recorded 0.88, 0.92 and 0.81 for neutron, density and resistivity logs respectively. The results show excellent relationships between the original and the predicted logs in the Mishrif model, unlike the Nahr Umr model expect in ANN of the neutron log. It was expected that the total relationships are low in Nahr Umr due to the lithology variation that includes interbedded consolidated and unconsolidated sandstone interbedded with the shale. It is also observed that the gamma log shows low values and the caliper logs is smoothed in the Mishrif. In contrast, the Nahr Umr sandstone logs show that many washouts have occurred. Therefore, logs’ responses highly possible to be affected in the Nahr Umr Formation which leads to a decreasing in the coefficient of determinations.

Effect of small-scale heterogeneity on biopolymer performance in carbonates

Polymer flooding is a well-established chemical method for enhancing oil recovery in sandstones; however, it has a limited application in carbonates. This is due to the harsh reservoir conditions in carbonates including high temperature, high salinity, and high heterogeneity with low permeability. This paper numerically investigates the effect of Schizophyllan biopolymer on oil recovery from carbonates. The effect of biopolymer on oil recovery was predicted by running several 1D simulations. Biopolymer flow behavior was modeled based on experimental data. The results showed that the effect of the investigated biopolymer on oil recovery was not much pronounced compared to conventional waterflooding. This is due to small-scale heterogeneity, which increased effective shear rate and hence, decreased in-situ polymer viscosity. Formation permeability, polymer viscosity, and oil saturation maps were consistent in justifying this observation. The findings of this study were supported by fractional flow and mobility ratio analyses. This work highlights the importance of small-scale heterogeneity of the core in modeling polymer flooding, particularly the shear effect on polymer viscosity.

Environmental consequence analysis of oil spills from onshore pipelines with parametric uncertainty

Due to spatial heterogeneities, measurement errors and inherent errors of parameter estimation, hydrogeology parameters are not exactly known. Therefore, the accuracy of environmental consequence prediction for oil spills from onshore pipeline accidents is subject to the uncertainty of soil parameters. However, due to the time-consuming calculation of uncertainty analysis and the difficulty of modeling the cross-correlation among soil parameters, the correlativity is often overlooked in the current researches involving the impacts of uncertain soil parameters on the environmental consequence estimation. To address above concerns, this study introduces a novel methodological framework for investigating the contribution of correlated soil parameters on the environmental consequence caused by spilled oil from onshore oil pipeline accidents. The proposed methodological framework combines a polynomial chaos expansion(PCE) surrogate model and methods from Uncertainty Analysis (UA) and PAWN-based Global Sensitivity Analysis (GSA). In addition, the practical application of this framework is illustrated with a case study for estimating the contaminated range of spilled oil from onshore pipeline accidents.First of all, the correlation of soil parameters is modeled by the copula function, and a PCE surrogate model has been constructed in order to alleviate the computational burden of the following UA and PAWN-based GSA. Then, UA is performed to quantify the impacts of correlativity by means of Monte Carlo (MC) method. Results reveal the correlativity of soil parameters introduces a substantial variation in the estimation of contaminated depth of spilled oil. In addition, the UA results show that selecting different copula functions to describe the correlation of soil parameters may lead to different variations of contaminated depth. Moreover, the uncertainty of the calculation results has been reduced with the consideration of correlated soil parameters, which may make the calculation results more reliable. Finally, the PAWN-based GSA is performed to identify the most influential parameter on the variation of contaminated depth. Results of the GSA show that saturated permeability and scale parameter α are primarily responsible for the variability of the model output, while the porosity exerts a smaller influence. This finding also suggests the possibility of replacing the less influential parameters with its average value, thus appreciably reducing the computation cost of the problem. The results of GSA not only offer a better understanding of correlativity to the contaminated range of spilled oil, but also identify the parameters for which additional effort needs to be invested to reduce their uncertainty and, as a result, the uncertainty associated with environmental consequence estimation of onshore pipeline accidents.

Characterization of effective permeability of prepreg fibers under autoclave molding process conditions using process model simulations

During autoclave molding process of prepreg composites, resin flow occurs through the fibers in the transverse direction of the layup due to applied external pressure. The extent of resin flow depends largely on the permeability of the reinforcement used in prepreg, and hence, the permeability is an important parameter for process modeling and simulations of composites fabrication processes. Most of the existing methodologies for measurement of transverse permeability of the reinforcements described in the literature are based on experimental techniques. However, the permeability measured through these experiments differs greatly from the actual permeability of the prepreg during actual fabrication process. This article focuses on this aspect and a methodology for the prediction and measurement of saturated transverse permeability of fiber preform of prepregs as encountered during fabrication is reported here. The methodology takes into account the actual process conditions and simulates them to predict permeability of the reinforcement. The permeability thus obtained will be the ‘effective permeability’ of the reinforcement. The methodology is applicable for prepregs containing any kind of resin, reinforcement and its weaving architecture.

Tight gas production model considering TPG as a function of pore pressure, permeability and water saturation

Threshold pressure gradient has great importance in efficient tight gas field development as well as for research and laboratory experiments. This experimental study is carried out to investigate the threshold pressure gradient in detail. Experiments are carried out with and without back pressure so that the effect of pore pressure on threshold pressure gradient may be observed. The trend of increasing or decreasing the threshold pressure gradient is totally opposite in the cases of considering and not considering the pore pressure. The results demonstrate that the pore pressure of tight gas reservoirs has great influence on threshold pressure gradient. The effects of other parameters like permeability and water saturation, in the presence of pore pressure, on threshold pressure gradient are also examined which show that the threshold pressure gradient increases with either a decrease in permeability or an increase in water saturation. Two new correlations of threshold pressure gradient on the basis of pore pressure and permeability, and pore pressure and water saturation, are also introduced. Based on these equations, new models for tight gas production are proposed. The gas slip correction factor is also considered during derivation of this proposed tight gas production models. Inflow performance relationship curves based on these proposed models show that production rates and absolute open flow potential are always be overestimated while ignoring the threshold pressure gradients.

Correlation of the reservoir characteristics from the well-logging data and core measurements in QASR field, north Western Desert, Egypt

Petrophysical well-logging data and core measurements were integrated in order to assess the Lower Safa reservoir characteristics of the QASR gas field, northern part of the Western Desert, Egypt. The study mainly depends on the relation between the core and log data of the hydrocarbon-bearing reservoir Jurassic (Lower Safa) using four productive wells QASR-3, QASR-5, QASR-6, and QASR-12; the linear regression is then used to establish a respective relationship between core and log data to estimate some of the reservoir parameters such as permeability (K), porosity (Φ), shale volume (Sh), clay content (Vcl), and fluid saturation (Sf), which are variables that determine reservoir quality. Average permeability values of the reservoir was found to be above 100 md, while porosity values ranged between 10 and 15%, reflecting well-sorted coarse grained sandstone reservoirs with minimal cementation, indicating very excellent reservoir quality. Low water saturation (10–30%) in the Lower Safa reservoir of the QASR wells indicated that the proportion of void spaces occupied by water is low, thus, showing high hydrocarbon saturation. Quantitative porosity fulfillment using regression equations revealed significant similarity in the porosity values obtained from petrophysical well logs and core data. Plots on scatter diagrams using porosity values derived from both core and log data of the Lower Safa reservoir reflecting good linear relationships. Plots of values of water saturation derived from core analysis and that from petrophysical log for the Lower Safa reservoir also showed linear trends. Porosity against permeability plots represents a linear relationship between both variables in Lower Safa reservoir, indicating that Lower Safa reservoir are permeable and have pores that are strongly communicated. The petrophysical properties of the Lower Safa reservoir in the QASR field are sufficient to permit hydrocarbon production.