Showing papers on "Effective porosity published in 2018"

Silica-Encapsulated DNA-Based Tracers for Aquifer Characterization

Abstract: Environmental tracing is a direct way to characterize aquifers, evaluate the solute transfer parameter in underground reservoirs, and track contamination. By performing multitracer tests, and translating the tracer breakthrough times into tomographic maps, key parameters such as a reservoir's effective porosity and permeability field may be obtained. DNA, with its modular design, allows the generation of a virtually unlimited number of distinguishable tracers. To overcome the insufficient DNA stability due to microbial activity, heat, and chemical stress, we present a method to encapsulated DNA into silica with control over the particle size. The reliability of DNA quantification is improved by the sample preservation with NaN3 and particle redispersion strategies. In both sand column and unconsolidated aquifer experiments, DNA-based particle tracers exhibited slightly earlier and sharper breakthrough than the traditional solute tracer uranine. The reason behind this observation is the size exclusion effect, whereby larger tracer particles are excluded from small pores, and are therefore transported with higher average velocity, which is pore size-dependent. Identical surface properties, and thus flow behavior, makes the new material an attractive tracer to characterize sandy groundwater reservoirs or to track multiple sources of contaminants with high spatial resolution.

Push-pull tests for estimating effective porosity: expanded analytical solution and in situ application

Abstract: The analytical solution describing the one-dimensional displacement of the center of mass of a tracer during an injection, drift, and extraction test (push-pull test) was expanded to account for displacement during the injection phase. The solution was expanded to improve the in situ estimation of effective porosity. The truncated equation assumed displacement during the injection phase was negligible, which may theoretically lead to an underestimation of the true value of effective porosity. To experimentally compare the expanded and truncated equations, single-well push-pull tests were conducted across six test wells located in a shallow, unconfined aquifer comprised of unconsolidated and heterogeneous silty and clayey fill materials. The push-pull tests were conducted by injection of bromide tracer, followed by a non-pumping period, and subsequent extraction of groundwater. The values of effective porosity from the expanded equation (0.6–5.0%) were substantially greater than from the truncated equation (0.1–1.3%). The expanded and truncated equations were compared to data from previous push-pull studies in the literature and demonstrated that displacement during the injection phase may or may not be negligible, depending on the aquifer properties and the push-pull test parameters. The results presented here also demonstrated the spatial variability of effective porosity within a relatively small study site can be substantial, and the error-propagated uncertainty of effective porosity can be mitigated to a reasonable level (< ± 0.5%). The tests presented here are also the first that the authors are aware of that estimate, in situ, the effective porosity of fine-grained fill material.

Petrophysical evaluation of well log data and rock physics modeling for characterization of Eocene reservoir in Chandmari oil field of Assam-Arakan basin, India

Abstract: Petrophysical evaluation of well log data has always been crucial for identification and assessment of hydrocarbon bearing zones. In present paper, petrophysical evaluation of well log data from cluster of six wells in the study area is carried out in combination with rock physics modeling for qualitative and quantitative characterization of Eocene reservoir in Chandmari oil field of Assam-Arakan basin, India. Petrophysical evaluation has provided the estimation of fluid and mineral types, rock/pore fabric type and fluid and mineral volumes for invaded and virgin zones. Calibrations are made where core data were available. Rock physics study is carried out for analyzing the influence of porosity, mineral compositions and saturation variations on the elastic properties of the subsurface. The rock physics modeling allowed quantitative prediction of relationship between porosity, saturation (gas, oil and water), clay volume and the elastic properties. Cross-plots of different elastic parameters are generated to identify the lithology variability and pore-fluid type, and to establish likely distinction between the hydrocarbon bearing sands, brine sands and shale. The Eocene reservoirs are found to be primarily sandstone intermixed with incidental clay matrix and some calcareous cementation. Sands are interpreted to be continuous in most of the blocks. The effective porosity for these sands varies from 15 to 22% in most of the wells. A wide range of variation is observed in water saturation with lowest being 5%. Finally, a numerical rock physics model is prepared to predict the elastic properties of the rock from petrophysical properties.

Investigating long-term effects of subsurface drainage on soil structure in paddy fields

Abstract: Subsurface drainage systems provide suitable conditions for annual cropping and crop diversification in northern Iran’s paddy fields. Such structural and managerial changes affect soil structure through influencing saturated hydraulic conductivity and effective porosity of the soil. The aim of this research was to investigate the long-term effects of subsurface drainage systems (D0.90L30: drainage system with 0.90 m depth and 30 m spacing, D0.65L30: drainage system with 0.65 m depth and 30 m spacing and D0.65L15: drainage system with 0.65 m depth and 15 m spacing) on the soil saturated hydraulic conductivity and drainable porosity. Water table and drain discharge were measured daily during different growing seasons (2011–2015) following installation of the subsurface drainage systems in the paddy fields of Sari Agricultural Sciences and Natural Resources University. The saturated hydraulic conductivity was calculated using Hooghoudt and Glover-Dumm equations and the drainable porosity was calculated with Taylor's method. Computed hydraulic conductivities by the Hooghoudt and Glover-Dumm equations were in the range of 0.002 to 1.95 m day−1 and 0.003 to 2.5 m day−1, respectively, that highest value was related to D0.65L15 and lowest value was related to D0.90L30. So, the shallow drainage systems were more effective in improving the soil hydraulic condition compared with the deep ones. The drainable porosity in the D0.90L30, D0.65L30 and D0.65L15 systems ranged from 0.002 to 0.314 m3 m−3, 0.003 to 0.13 m3 m−3 and 0.004 to 0.126 m3 m−3, respectively. In conclusion, the installation of subsurface drainage especially at the shallower depth will improve paddy soil structure during time.

The occurrence state of moisture in coal and its influence model on pore seepage

Abstract: Moisture is one of the most important factors that influences coal seepage and coal-bed methane (CBM) extraction. To obtain the water occurrence state and dynamic processes of water change in coal, a series of microscopic observation experiments of Wei Jiagou coal by using field-emission environmental scanning-electron microscopy (ESEM) was conducted under the condition of a fixed point. Afterwards, a mathematical model to explain the influence of water on porosity and permeability was proposed based on the ESEM observations. It was found that there were three main types of water occurrence state: a crescent shape, a full filled shape and an annulus shape, which can provide powerful evidence to explain the influence of water on porosity and starting pressure gradient. As well as this, the box counting reached a minimum at a chamber pressure of 520 Pa and the box counting reduced after water wetting. Based on the mathematical model analysis, the water-occupied area of crescent shapes would reach a peak value with an increase of the contact angle, which has a critical impact on the effective porosity. The influence model that we built matched well with experimental data, which in turn demonstrated the validity of the mathematical model. The prominent combined effect of strain and water saturation appeared on the ridge of the permeability contour, while strains have little influence on permeability at a large initial porosity. Furthermore, a model for contact angle and wetting height was proposed and discussed, and contact angles with different improving fluids were tested. It also can be shown that using better wettability improving-fluid can save the cost of volume and have a good performance on the results of hydraulic technology based on model and experimental tests.

Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and 14 C dating

Abstract: . Effective porosity plays an important role in contaminant management. However, the effective porosity is often assumed to be constant in space and hence heterogeneity is either neglected or simplified in transport model calibration. Based on a calibrated highly parametrized flow model, a three-dimensional advective transport model (MODPATH) of a 1300 km 2 coastal area of southern Denmark and northern Germany is presented. A detailed voxel model represents the highly heterogeneous geological composition of the area. Inverse modelling of advective transport is used to estimate the effective porosity of 7 spatially distributed units based on apparent groundwater ages inferred from 11 14C measurements in Pleistocene and Miocene aquifers, corrected for the effects of diffusion and geochemical reactions. By calibration of the seven effective porosity units, the match between the observed and simulated ages is improved significantly, resulting in a reduction of ME of 99 % and RMS of 82 % compared to a uniform porosity approach. Groundwater ages range from a few hundred years in the Pleistocene to several thousand years in Miocene aquifers. The advective age distributions derived from particle tracking at each sampling well show unimodal (for younger ages) to multimodal (for older ages) shapes and thus reflect the heterogeneity that particles encounter along their travel path. The estimated effective porosity field, with values ranging between 4.3 % in clay and 45 % in sand formations, is used in a direct simulation of distributed mean groundwater ages. Although the absolute ages are affected by various uncertainties, a unique insight into the complex three-dimensional age distribution pattern and potential advance of young contaminated groundwater in the investigated regional aquifer system is provided, highlighting the importance of estimating effective porosity in groundwater transport modelling and the implications for groundwater quantity and quality assessment and management.

A hypoplastic model for gas hydrate‐bearing sandy sediments

Abstract: Key Laboratory for Mechanics in Fluid Solid Coupling Systems, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049, China University of Natural Resources and Life Sciences, Vienna, Austria Qingdao Institute of Marine Geology, Qingdao, China State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China State Key laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China

A new method for calculation of water saturation in shale gas reservoirs using V P -to-V S ratio and porosity

Abstract: Total water saturation is an important parameter for calculating the free gas content of shale gas reservoirs. Owing to the limitations of the Archie formula and its extended solutions in zones rich in organic or conductive minerals, a new method was proposed to estimate total water saturation according to the relationship between total water saturation, V P -to-V S ratio and total porosity. Firstly, the ranges of the relevant parameters in the viscoelastic BISQ model in shale gas reservoirs were estimated. Then, the effects of relevant parameters on the V P -to-V S ratio were simulated based on the partially saturated viscoelastic BISQ model. These parameters were total water saturation, total porosity, permeability, characteristic squirt-flow length, fluid viscosity and sonic frequency. The simulation results showed that the main factors influencing V P -to-V S ratio were total porosity and total water saturation. When the permeability and the characteristic squirt-flow length changed slightly for a particular shale gas reservoir, their influences could be neglected. Then an empirical equation for total water saturation with respect to total porosity and V P -to-V S ratio was obtained according to the experimental data. Finally, the new method was successfully applied to estimate total water saturation in a sequence formation of shale gas reservoirs. Practical applications have shown good agreement with the results calculated by the Archie model.

Microscale Numerical Simulation of Non-Darcy Flow of Coalbed Methane

Abstract: To obtain the real physical model characterizing the pore structure of coal rocks and further explore the rules governing the flow of coalbed methane (CBM) using numerical simulation, we proposed a sequential method from real coal model to computer-aided designed (CAD) coal model to finite element coal model. First, six different coal samples were scanned using $$\upmu $$ CT225kVFCB CT system. The obtained CT data were subject to threshold segmentation using the compensated digital terrain model and a coal model in STL format was established. Then, we elaborated a reverse engineering method to convert the coal model into a CAD coal model. Lastly, we obtained the finite element coal model after setting the boundary conditions with CFX software, numerically simulated CBM flow at 30 different pressure gradients based on the finite element coal model and analyzed the impacts of effective porosity on permeability and seepage velocity of CBM. The results showed that (1) at the microscale ( $${<}100\,\upmu \hbox {m}$$ ), the seepage velocity and pressure gradient were in agreement with the Forchheimer law of high-speed nonlinear seepage; (2) permeability of CBM shows a fluctuating, but not absolute increase with effective porosity increasing; (3) at the same pressure gradient, the overall seepage velocity of CBM increases with the effective porosity increasing, but declines with non-Darcy flow coefficient increasing; (4) the non-Darcy flow coefficient reduces with both effective porosity and permeability increasing. Fitting to power function analysis shows that compared with permeability, the effective porosity has more significant impact on the non-Darcy flow coefficient.

Reservoir Properties and Facies Distribution of Mishrif Formation in Ratawi Oilfield, Southern Iraq

Abstract: The M. Cenomanian-E. Turonian Mishrif Formation is considered as the most extended reservoir of south Iraq. The present paper reveals the facies of this formation in Ratawi oilfield by the analysis of seven wireline logs. According to reservoir properties, i.e. effective porosity, permeability, and water saturation, the Mishrif Formation is divided into two main units, the upper (mA) and the lower (mB), separated by a shale member. These reservoir units are almost capped by two shale intervals located in the lower and upper contacts with Rumaila and Khasib formations, respectively. The reservoir properties, in terms of porosity and permeability, decrease (porosity less than 5%, permeability less than 0.001 md) to the south of Ratawi structure due to the dominance of deep marine facies, whereas the formation predicted to have good reservoir properties (porosity more than 10%, permeability more than 5 md) to other directions especially to the east, contrary to what was previously expected.

Simulation Analysis on Water’s Micro Seepage Laws under Different Pressure Gradients Using Computed Tomography Method

Abstract: The aim of this paper was to develop a model that can characterize the actual micropore structures in coal and gain an in-depth insight into water’s seepage rules in coal pores under different pressure gradients from a microscopic perspective. To achieve this goal, long-flame coals were first scanned by an X-ray 3D microscope; then, through a representative elementary volume (REV) analysis, the optimal side length was determined to be 60 μm; subsequently, by using Avizo software, the coal’s micropore structures were acquired. Considering that the porosity varies in the same coal sample, this study selected four regions in the sample for an in-depth analysis. Moreover, numerical simulations on water’s seepage behaviors in coal under 30 different pressure gradients were performed. The results show that (1) the variation of the simulated seepage velocity and pressure gradient accorded with Forchheimer’s high-velocity nonlinear seepage rules; (2) the permeability did not necessarily increase with the increase of the effective porosity; (3) in the same model, under different pressure gradients, the average seepage pressure decreased gradually, while the average seepage velocity and average mass flow varied greatly with the increase of the seepage length; and (4) under the same pressure gradient, the increase of the average mass flow from the inlet to the outlet became more significant under a higher inlet pressure.

Formation MicroScanner Providing Better Answers for Carbonate Secondary Porosity in Alamein Dolomite Formation, NW Desert, Egypt

Abstract: The use of borehole imaging tools has become widespread in recent years with more specialized studies of reservoir properties, particularly in highly-porous and fractured carbonate systems. In this study, the Formation MicroScanner (FMS) borehole imaging tool and conventional well log data have been used to study the secondary porosity of the dolomitic Alamein Formation in the Alamein Field, north Western Desert, Egypt. Based on well log analyses of the formation from Tourmaline-1X and N.Alamein-6X wells, we show that secondary porosity occurs across the formation, and is filled mostly with hydrocarbon. We also show that the formation has good average effective porosity and hydrocarbon saturation. FMS images of the Tourmaline-1X well confirms that the formation is intermittently vuggy with solution-filled channels from the top to its base. The vug pores are observed to be well-connected, which supports good effective porosity values interpreted from petrophysical data. An additional set of core photographs of the Alamein Formation from N.Alamein-5X well confirms the presence of secondary pores, which are filled by hydrocarbon, and exhibit intense fluorescence under UV light. Our results show that the abundance of secondary porosity in Alamein Formation would play a key role in evaluating its reservoir quality and reservoir performance.

The quantification of total and effective porosities in travertines using PIA and saturation-buoyancy methods and the implication for strength and durability

Abstract: The porosity of travertine controls its strength characters and durability. In this study, seven travertine samples were collected from mines in Iran. The saturation-buoyancy (SB) method was used to determine density, water absorption by weight, effective porosity, and total porosity. Petrographic image analysis (PIA) was applied to measure total porosity. Uniaxial compressive strength (UCS) of travertine samples was obtained under both oven-dried and saturated conditions. The results showed that mean PIA total porosity of samples is 2–4 times more than total porosity obtained from the SB method. The underestimation of total porosity by SB method was attributed to pores type and pores size. According to regression analyses, the UCS shows a better correlation with SB effective porosity than SB total porosity. Based on results, water absorption by weight has a large impact on UCS of travertines. The SB effective porosity shows high correlation with water absorption by weight. There is a good power relation between PIA total porosity and UCS of travertines.

An Analytical Method for Assessing Recharge Using Groundwater Travel Time in Dupuit‐Forchheimer Aquifers

Abstract: An analytical solution to calculate the recharge of unconfined aquifers with Dupuit-Forchheimer type flow conditions is proposed. This solution is derived from an existing closed-form analytical solution initially developed to determine groundwater travel time when the recharge of the aquifer is known. This existing solution has been modified to determine recharge when groundwater travel time is known. An illustration is given with a field case example for the Bonifacio aquifer of the island of Corsica (France), in the Mediterranean. In this aquifer, previously established differences in groundwater residence time between two water samples were determined from anthropogenic atmospheric gas (chlorofluorocarbons and sulfur hexafluoride) measurements. The time difference is entered into the new analytical solution to determine recharge. The calculations yield a value of average recharge that agrees with the results obtained by several other methods that were presented in previous studies to assess the recharge of the Bonifacio aquifer. Also presented in this study is a sensitivity analysis of the new analytical solution, to quantify the influence of different parameters that control recharge: hydraulic conductivity, effective porosity and the groundwater travel time. This study illustrates how geochemical data can be combined with physical models to measure recharge. Such an approach could be adopted in other homogeneous aquifers worldwide that satisfy Dupuit-Forchheimer type flow conditions.

Free LNAPL Volume Estimation by Pancake Model and Vertical Equilibrium Model: Comparison of Results, Limitations, and Critical Points

Abstract: Light nonaqueous phase liquids (LNAPLs), due to their low solubility, dissolve slowly, acting as a long-term source of water contamination, and consequently they represent an important environmental issue. In the subsoil, more than 99% of spilled LNAPL remains as adsorbed and free phase; therefore, the volume estimation of free phase, obtained in this case through two different conceptual models (Pancake Model and Vertical Equilibrium Model), is considered a fundamental step for a correct site remediation. According to the first model, the LNAPL floating on the water table and its saturation is up to 100%; instead, according to the second one, the LNAPL can penetrate below the water table and the coexistence of LNAPL, water, and air in the pore fraction, leads to a lower LNAPL saturation, variable with the depth. Actually, in subsoil LNAPL and water saturations vary with depth due to the influence of capillarity, leading to the inaccuracy of Pancake Model assumption. Despite the evident limitation of Pancake Model, both models were applied, coupled with area calculations with Thiessen polygons and grid at regular mesh, to roughly estimate the free LNAPL volume existing in a contaminated site. The volume estimation carried out, considering the LNAPL type and its features, the soil type, and relative effective porosity, provides estimates of volumes having differences up to thousands of cubic meters. The results analysis shows that this estimation has several critical points such as area definition and the lack of site-specific data (e.g., porosity). Indeed, the sensitivity analysis for porosity shows that a reduction of this parameter provides a 20% reduction of estimated volume.

Application of seismic post-stack inversion for gas reservoir delineation: A case study of Talang Akar Formation, South Sumatera Basin

Abstract: Nauli Field is one of the marginal fields that is located in the western region of Indonesia and has been proven to produce oil and gas hydrocarbons. The field is located in the South Sumatra Basin with reservoir target is sandstone from Talang Akar Formation. The purpose of this study is to integrate petrophysical interpretation and seismic data analysis. Reservoir characterization was performed based on porosity and water saturation for each layer within the wells. Seismic inversion methods chosen to estimate the attributes of P-impedance (Zp), S-impedance (Zs) and density (ρ). Amplitude Versus Offset (AVO) inversion through Lambda-Mu-Rho (LMR) attribute can provide information on lithology and fluids content in the reservoirs. The connection between seismic inversion and AVO inversion was to validate the gas content from the seismic data and its lateral dispersion. Data processing and analysis shows that the effective porosity for Nauli Field is between 10 to 20 porosity unit (p.u) with 20-70% water saturation. The thickness of net pay is 2-8 meters. Post-stack inversion shows impedance anomaly around 1580-1590 ms with value 22.000-25.000 (m/s)*(g/cc). Petrophysical analysis estimated this anomaly as layer-x sandstone with gas fluid saturated. LMR attribute confirm this fluid with lower Lambda-Rho values around 11-20 (Gpa)*(g/cc) than Lambda-Rho values around 28-32 (Gpa)*(g/cc).

Stoneley Wave Predicted Permeability and Electrofacies Correlation in the Bangestan Reservoir, Mansouri Oilfield, SW Iran

Abstract: Reservoir characterization is one of the most important goals for the development of any oilfield. Determination of permeability and rock types are of prime importance to judge reservoir quality. In this research, Stoneley waves from dipole sonic tools were used in order to discover changes in permeability in the Bangestan reservoir, Mansouri oilfield. Index (tortuosity) could be estimated by Stoneley waves. After comparing the permeability resulting from Stoneley waves, cores and the Timur method, it was concluded that all the three permeabilities were very similar. The core porosity and effective porosity from the analysis of well logs were found to match as well. Electrofacies (EF) method, as a clustering method, was utilized to find rock types in order to define reservoir and non-reservoir zones. Simultaneous with EF clustering, gamma ray, neutron porosity, density, sonic, water saturation and porosity (PHIE) data from 78 wells were also considered and interpreted. Nine clusters were defined as a result of the analysis, being reduced to only four clusters after applying PC (capillary pressure) data. Among the four clusters, clusters 1 and 2 contained more vuggy pores than the others. Fracture abundance and solution seams were observed more frequently in EF-3 as compared to other EFs. Based on the matrix type, Archie porosity classification types I and III were recognized. The pore sizes in EFs-1 and 2 were mostly of the B type while in EF-3, it was A type. The EFs generated and determined by Stoneley waves and the well log data were also compared, showing a good correlation.

Conceptual definition of porosity function for coarse granular porous media with fixed texture

Abstract: Porous media’s porosity value is commonly taken as a constant for a given granular texture free from any type of imposed loads. Although such definition holds for those media at hydrostatic equilibrium, it might not be hydrodynamically true for media subjected to the flow of fluids. This article casts light on an alternative vision describing porosity as a function of fluid velocity, though the media’s solid skeleton does not undergo any changes and remain essentially intact. Carefully planned laboratory experiments support such as hypothesis and may help reducing reported disagreements between observed and actual behaviors of nonlinear flow regimes. Findings indicate that the so-called Stephenson relationship that enables estimating actual flow velocity is a case that holds true only for the Darcian conditions. In order to investigate the relationship, an accurate permeability should be measured. An alternative relationship, therefore, has been proposed to estimate actual pore flow velocity. On the other hand, with introducing the novel concept of effective porosity, that should be determined not only based on geotechnical parameters, but also it has to be regarded as a function of the flow regime. Such a porosity may be affected by the flow regime through variations in the effective pore volume and effective shape factor. In a numerical justification of findings, it is shown that unsatisfactory results, obtained from nonlinear mathematical models of unsteady flow, may be due to unreliable porosity estimates.

Fractal dimension measuring and calculating method for effective pores of hydrate-containing sediments

Abstract: The invention belongs to the field of basic physical property testing of unconventional oil and gas reservoir engineering and geotechnical engineering and particularly relates to a fractal dimension measuring and calculating method for effective pores of hydrate-containing sediments. Firstly, X-CT gray images of the hydrate-containing sediments are acquired and the porosity is measured; then, thegray images are colored again, X-CT colored images of the hydrate-containing sediments are acquired from two groups with gray values of soil particles, hydrate particles, gas and moisture in obvious comparison, then, X-CT colored images with set thresholds are subjected to binarization processing, black and white images are obtained, the effective porosity of the acquired black and white images isthe same as that of the X-CT gray images, and white and black represent a framework material and a pore fluid respectively. The fractal dimensions of black areas in the black and white images, namely, the fractal dimensions of effective porosity of the hydrate-containing sediments, are solved. The problems that solid framework space and pore fluid space cannot be segmented effectively with existing methods are solved, and the fractal dimensions of effective porosity of the hydrate-containing sediments can be measured and calculated accurately.