Water Saturation Changes Research Articles

A grid-search approach for 4D pressure-saturation discrimination

We develop a grid-search-based inversion method to discriminate between changes of pore pressure and water saturation. In this approach, we build a forward model by combining different rock-physics models and we predict grids of time-lapse attributes within a range of pressure and saturation. The time-lapse measurements from the data are matched with these predicted attributes to find the solution. We suggest using the independent time shifts and time-lapse attenuation to stabilize the inversion result. A synthetic example illustrates the advantages of the inversion workflow. The method helps to visualize how pressure and saturation change the elastic properties. This advantage is also useful in quality control and uncertainty analysis. We apply our method to a real underground blowout example in the North Sea area where gas has leaked into two sand layers at different conditions. We find that gas migrated according to the sand structure with the dipping layer leading to faster migration rates. The inversion results indicate that careful data preparation is crucial, not only to obtain stable inversion performance, but also to capture valuable information when interpreting the blowout development.

Modeling and analysis of the acidizing process in carbonate rocks using a two-phase thermal-hydrologic-chemical coupled model

We present a two-phase thermal-hydrologic-chemical coupled model for simulating the dissolution process during the acidization of carbonate rocks. In particular, we develop a new model to describe the change in irreducible water saturation, residual oil saturation, and the maximum relative permeability of oil and water phases with dissolution proceeding. We also present a new method for the generation of the initial porosity field with controllable correlation length. In numerical calculation, the sequential iteration approach is adopted to solve the presented model, and the operator splitting method is used to deal with the reaction relevant equations. The involved equations are discretized using the finite-volume method, where the convection term is discretized by the MINMOD scheme which can prevent overshoot/undershoot of the numerical solution. Additionally, sensitivity analysis of the dissolution process concerning rock properties, the exothermic heat of reaction, and two-phase flow, is carried out. Based on the predicted results, several recommendations for the carbonate acidizing operation are given, and the potential extensions of the current work are summarized.

BUILDING A LOW-LEVEL MODEL FOR TWO-PHASE FILTRATION

Background The article proposes a method for constructing low-level models for two-phase filtration based on a combination of a finite-difference solution and a detailed spatial approximation, which makes it possible to use approximating models for solving problems of optimal control of operational parameters of an oil reservoir. The combined motion of two phases is considered: wetting and non-wetting liquids. Aims and Objectives The two-phase filtration process is non-stationary for incompressibility of liquids and pore space. The phase saturations change and the relative permeability coefficients of the phases change as the fluids move. Therefore, at constant intensities of sources of liquids, the pressure field in the reservoir will vary with time. To construct an approximate solution based on lower-level models, the following approach is proposed. The considered area is divided into larger blocks, which allows to quickly carry out the calculation based on the finite-difference method. Spatial detailing is based on radial approximators associated with the calculated parameters of the integrated blocks. Results The distribution is obtained by calculation by the finite difference method on a 256x256 grid. It is revealed that, compared with the initial period of the process, there was a significant flooding of the reservoir. The results of the comparison are presented, from which it follows that the approximation of water saturation has detailed spatial detailing. The change in water saturation in the middle section of the region is shown. The presented method for constructing low-level models for two-phase filtration, based on a combination of a finite-difference solution and a detailed approximation, revealed the feasibility and continuity of using approximating models to solve the problems of rational experimental control of oil reservoir parameters.

Unstable spatial differences of the geomagnetic elements caused by changing water saturation of a volcanic sediment

Since the beginning of geomagnetic recording at Tihany Geophysical Observatory, baseline instability has been observed, especially in the case of the old variation house. The regularly observed annual baseline change is independent from the type of magnetometer. Common environmental effects on the instruments (for example temperature effects) are not suitable to characterize this variation. A possible reason of this instability is an unconventional geomagnetic effect on the magnetic properties of the sediment. The water saturation of a lake mud in a geyser cone shows correspondence with the variation of the base values. X-ray powder diffraction measurement detected goethite in the lake mud which, in fact makes possible the appearance of a super-ferromagnetic effect on goethite. In this study we present a possible source and the mechanism of the effect.

Quantitative prediction of oil saturation of unconsolidated sandstone reservoir based on time-lapse seismic “relative difference method”: Taking Zeta oil field in West Africa as an example

Abstract In view of the disadvantage that the absolute difference of time-lapse seismic (the difference between monitoring data and base data) is not only related to the change of oil saturation, but also closely related to the thickness of reservoir, a time-lapse seismic “relative difference method” (the ratio of monitoring data to base data) not affected by the thickness of reservoir but only related to the change of fluid saturation, is proposed through seismic forward modeling after fluid displacement simulation. Given the same change of fluid saturation, the absolute difference of time-lapse seismic conforms to the law of “tuning effect” and seismic reflection of “thin bed”, and the remaining oil prediction method based on absolute difference of time-lapse seismic is only applicable to the reservoirs with uniform thickness smaller than the tuning thickness or with thickness greater than the tuning thickness. The relative difference of time-lapse seismic is not affected by reservoir thickness, but only related to the change of fluid saturation. It is applicable to all the deep-sea unconsolidated sandstone reservoirs which can exclude the effect of pressure, temperature, pore type and porosity on seismic. Therefore, the relation between the relative difference of time-lapse seismic and the change of fluid saturation, which is obtained from seismic forward modeling after Gassmann fluid displacement simulation, can be used to quantitatively predict the change of reservoir water saturation and then the distribution of the remaining oil. The application of this method in deep sea Zeta oil field in west Africa shows that it is reasonable and effective.

English

This study is concerned with the water coning phenomenon that takes place around production wells of hydrocarbon reservoirs. In this paper, the development of artificial neural networks to predict the water saturation buildup around vertical and horizontal wells with a good level of accuracy is described. In the development of expert systems, it is assumed that water encroachment originates from an active aquifer which is located under the hydrocarbon reservoir (reservoir with bottom water drive). A high-fidelity numerical model is utilized in generating training data sets that are used in structuring and training the artificial neural networks. The artificial expert systems that are developed in this paper are universal and are capable of predicting the change of water saturation around the wellbore as a function of time and the prediction process is faster than a reservoir simulator and requires less data, which saves time and effort. With the help of these models, it will be possible to predict the position of high water saturation zones around the wellbore ahead of time so that remedial actions such as closing the perforations that produce the water can be implemented on a timely basis. Key words: Bottom water drive, water coning, neural network, water saturation, vertical well, horizontal well. &nbsp

Pulsed Neutron Capture Logging: Monitoring Co2 Breakthrough in Multiple Co2-Eor Fields

Pulsed neutron capture (PNC) well logging technology was utilized in the large-scale, long-term CO2-EOR Midwest Regional Carbon Sequestration Partnership (MRCSP) to monitor CO2 breakthrough in the Michigan Niagaran reef trend. Comprehensive tool response modeling and assessment of pre- and post-CO2 injection PNC data were performed to evaluate oil, gas, and water saturation changes and potential CO2 breakthrough in monitored reefs. An examination of the different phases of the program and monitoring data are presented.

Effect of evaporative drying on complex conductivity spectra of sandstones

Dehydration is a common process occurring in natural environments, rocks, and building materials. Suitable methods for monitoring the changing moisture content should be identified. We have investigated the impact of dehydration of sandstones on complex conductivity spectra. Spectral induced polarization measurements were performed on five sandstone samples during evaporative drying. The complex conductivity spectra indicate considerable changes with decreasing saturation. The relationship between conductivity and saturation can be described by a power law known as the second empirical Archie equation. Separate saturation exponents have been determined for the real and imaginary parts of conductivity. The imaginary part of conductivity indicates higher saturation exponents for the investigated sandstones compared to the real part. Obviously, the saturation exponents depend on the method used for changing water saturation. Evaporative drying, which is used in our experiments, causes an increase of pore-water salinity and results in lower saturation exponents for the real part of conductivity but higher exponents for the imaginary part in comparison with the classical imbibition/drainage technique. We evaluate a theoretical approach that considers the influence of pore water salinity on the saturation exponents of the real and imaginary parts of conductivity. The complex conductivity spectra are processed by a Debye decomposition procedure. The resulting integrating parameters such as direct current resistivity, total chargeability, normalized chargeability, and mean relaxation time indicate a power law dependence on saturation. Our experiments indicate that the imaginary part of conductivity and normalized chargeability have a high sensitivity related to the drying process of rocks and might be suitable indicators to monitor the changes of moisture content in sandstones.

Change of water saturation in tight sandstone gas reservoirs near wellbores

Tight sandstone gas reservoirs commonly contain water, so liquid loading often appears near wellbores, leading to production decline and even shutdown of gas wells. Therefore, the study on the change of water saturation near wellbores is of great significance to understanding the water production mechanisms of gas wells. In this paper, a set of physical simulation experiment procedures of identifying the change of water saturation near wellbores was designed according to the principle of radial well seepage of gas wells, and the production performance after vertical well fracturing in gas reservoirs was simulated by connecting tight cores with a diameter of 10.5 cm, 3.8 cm and 2.5 cm in series in a descending order of distance. According to the depressurizing production mode of gas wells, tubes with small diameters of 20, 30, 40 and 50 μm were used to simulate gas well tubing to control the gas production rate. And the change of water saturation near wellbore in the process of depletion production and its influencing factors were investigated. Finally, combined with actual data of production wells, the water saturation and water production of gas wells near wellbores and in different zones were calculated at the above four different small diameters of tubes and the changes thereof were also analyzed. The following results were obtained. First, each gas production rate corresponds to a critical water saturation. When the initial water saturation is lower than the critical value, the formation water flowing near the wellbore and in the middle zone can be carried out along with the production of gas and no liquid loading is formed. Second, when the initial water saturation is higher than the critical value, a large amount of formation water migrating from the far-wellbore zones accumulates near the wellbore, and thus liquid loading occurs at the bottom hole. Third, when the initial water saturation is equal to the critical value, the higher the gas production rate is, the more easily liquid loading tends to form near the wellbore. Fourth, for the same water saturation, water production increases and recovery factor decreases with the increase of gas production rate. In conclusion, the cumulative water production chart of a gas well generated by the physical simulation experiment method proposed in this paper agrees well with the water production behavior of the corresponding gas well. The research results are conducive to the effective prediction of gas well water production and can be used as guidance for the reasonable gas well water control.

Rapid Estimation of Volumetric Groundwater Recharge in the Vadose Zone via Ground Penetrating Radar

Saturation in the vadose zone is known to reduce the ground penetrating radar (GPR) electromagnetic wave velocities. We examine the impact of increased water content in highly-permeable beach and dune systems on GPR velocities along coastal margin of Perth, Western Australia. We acquire repeat GPR transects in May and August before and after annual high rainfall periods. The assumption of exceedingly flat water table reflector permits us to estimated change in GPR velocity at successive dates. This change in velocity can be translated to an estimated change in water saturation via the Topp relationship which is an imperial mapping of water saturation to dielectric permittivity.

Modeling formation resistivity changes due to invasion and deformation during initial leak-off test build-up

Leak-off tests (LOTs) are performed to determine the strength of a newly drilled formation below a cased interval and to characterize the upper bound of mud weight that can be safely used while drilling the next section, without risk of formation breakdown and lost circulation. In an LOT, drilling mud is pumped into the wellbore, causing the wellbore pressure to increase and exceed the formation pore pressure. During the initial LOT build up, excess pressure in the wellbore causes the surrounding rock to deform and mud filtrate to invade into the formation via porous flow. In this paper, change in formation resistivity around a wellbore during initial LOT build-up has been investigated. Invasion is modeled assuming two-phase radial Darcy flow and deformation using a 3D finite element model. Invasion may result in an exchange of conductive ions between water-based drilling mud and formation water both by diffusion in the direction favored by the concentration gradient of the ions and by convective transport. This process is incorporated into the model by solving the radial convection-diffusion equation for the aqueous phase using a finite difference method. Archie's law is used to determine the formation resistivity. Findings show that the direct effect of deformation on porosity, therefore on formation resistivity during an LOT, is negligibly small even when the formation rock is highly compressible with compressibility in the order of 10−3 psi−1. While salinity solely controls formation resistivity during an LOT conducted in a fully water-saturated interval, water saturation change and salinity change compete to produce a compound effect on formation resistivity of an oil-bearing zone where water saturation varies dynamically due to displacement of formation fluids. Unlike compressibility, the effect of permeability on formation resistivity response is found to be evident and readily observable. While analyzing the formation resistivity responses at various depths of investigation (DOIs), it is found that the effect of DOI on resistivity response can be useful in studying invasion and assessing formation damage during an LOT. In addition to this, through comparing time-lapse resistivity logs at multiple DOIs during an LOT with numerically synthesized resistivity responses, the model promises a novel approach towards determining the permeability of a freshly drilled and unaltered interval.

Amplitude versus offset (AVO) modelling and analysis for quantitative interpretation of porosity and saturation: A case study for Sawan gas field, middle Indus basin, Pakistan


 
 
 Porosity and water saturation are among the important reservoir parameters in terms of reservoir monitoring which in turn are necessary for placement of future production wells. Amplitude versus offset (AVO) modelling and analysis technique have been utilized for quantitative interpretation of porosity and water saturation. For AVO modeling, Gassmann fluid substitution model in connection with 2D seismic angle gathers were used for reservoir monitoring over Sawan-4 and Sawan-7 wells. AVO modellingresults showed significant increase of density log by substituting gas by water, moderate increase of P-wave velocity and no change in S-wave velocity. Moreover, AVO modelling proved a strong relationship between porosity and reflection amplitude for all offset ranges. The class IV anomaly was evident for all porosities down to 5%. From fluid substitution modelling it is evidentthat changes in water saturation do not change the AVO effect significantly which further substantiate by investigation of pre-stack time migration (PSTM) gathers verifying the modelling results. The amplitude relationship between Sawan-4 and Sawan-7 is close to the modelled result. The amplitude and the AVO behaviour indicate lower porosity or significantly reduced sand thickness in the vicinity of Sawan-4 and Sawan-7 wells.
 
 

Temporal scale analysis of two phase flow in fractured well

Two phase flow between two fractures commonly exists around fractured wells. Due to the fracture network, the flow velocity is extremely variable in different regions and it can be described by the temporal scale analysis. The flow velocity also changes with time during the change in water saturation, but these changes cannot be captured by the conventional temporal scale analysis. The temporal scales are obtained based on a principal equation, so it is difficult to accurately estimate the formation properties and to determine the exploitation scheme. In this study, the two phase flow model between two fractures has been proposed for the temporal scale description.The impact of reservoir heterogeneity on the temporal scales is considered in the model. Due to the tortuous flow channel shape between the fractures, the flow pressure affects the fluid flow afterwards. This is explained by the time lag effect in the model. The temporal scales are obtained by using finite element method and Laplace transformation at all time points with these considerations. Moreover, the well is soaked intermittently. The soaking duration is chosen based on temporal scales, and then discussed in terms of efficiency and recovery factor.The results showed that the water saturation profile position is reflected by the temporal scales, the flow resistance and the flow velocity are represented by their zero status response contributions, while the initial flow pressure is described by their zero input contribution. The highest zero status response contribution is different for different properties. Moreover, the time lag effect promotes the initial pressure fluctuation, and also upscales the fluid flow in tortuous channels. Finally, the soaking duration is an approximation of the time required to reduce the fracture flow pressure.

Monitoring Saturation Changes with Ambient Seismic Noise and Gravimetry in a Karst Environment

Core Ideas Saturation changes can be monitored with ambient seismic noise. We present a new and nondestructive tool to investigate deep and heterogeneous vadose zones. At the site scale, this tool fills the gap between local boreholes and global basin information. Different geophysical datasets can be assimilated for hydrological model calibration. On a heterogeneous karstic site in the Larzac plateau (France), we performed cross‐correlations of ambient seismic noise recorded at two broadband seismometers to obtain daily seismic velocity changes. Rayleigh velocity changes at the 6‐ to 8‐Hz frequency band show variations of ±0.2% over 1 yr. Assuming a simple velocity profile, changes are expected to come from depths of tens of meters. Therefore velocity changes at 6 to 8 Hz were interpreted as induced by water saturation changes. A slow infiltration rate would explain the delay of several months between the rainy season (November) and the minimum velocity (June). Superconducting gravimeter, evapotranspiration, and magnetic resonance sounding (MRS) measurements were then combined with seismic data in one‐dimensional physical simulations. Velocity changes clearly constrain hydrological parameters, like saturated hydraulic conductivity, even if the Biot–Gassmann theory does not explain all of the amplitude observed. Nevertheless, this nondestructive method demonstrates great potential in hydrological model calibration. It overcomes the lack of depth resolution of gravimetry and the lack of temporal resolution of MRS. The combination of ambient seismic noise with gravimetry and MRS could fill the instrumental gap currently existing in hydrology for the study of deep and/or complex critical zones.

Revisiting the horizontal redistribution of water in soils: Experiments and numerical modeling.

A series of experiments and related numerical simulations were carried out to study one‐dimensional water redistribution processes in an unsaturated soil. A long horizontal Plexiglas box was packed as homogenously as possible with sand. The sandbox was divided into two sections using a very thin metal plate, with one section initially fully saturated and the other section only partially saturated. Initial saturation in the dry section was set to 0.2, 0.4, or 0.6 in three different experiments. Redistribution between the wet and dry sections started as soon as the metal plate was removed. Changes in water saturation at various locations along the sandbox were measured as a function of time using a dual‐energy gamma system. Also, air and water pressures were measured using two different kinds of tensiometers at various locations as a function of time. The saturation discontinuity was found to persist during the entire experiments, while observed water pressures were found to become continuous immediately after the experiments started. Two models, the standard Richards equation and an interfacial area model, were used to simulate the experiments. Both models showed some deviations between the simulated water pressures and the measured data at early times during redistribution. The standard model could only simulate the observed saturation distributions reasonably well for the experiment with the lowest initial water saturation in the dry section. The interfacial area model could reproduce observed saturation distributions of all three experiments, albeit by fitting one of the parameters in the surface area production term.

Probing the water distribution in porous model sands with two immiscible fluids: A nuclear magnetic resonance micro-imaging study

The effect of the structural heterogeneity of porous networks on the water distribution in porous media, initially saturated with immiscible fluid followed by increasing durations of water injection, remains one of the important problems in hydrology. The relationship among convergence rates (i.e., the rate of fluid saturation with varying injection time) and the macroscopic properties and structural parameters of porous media have been anticipated. Here, we used nuclear magnetic resonance (NMR) micro-imaging to obtain images (down to ∼50μm resolution) of the distribution of water injected for varying durations into porous networks that were initially saturated with silicone oil. We then established the relationships among the convergence rates, structural parameters, and transport properties of porous networks. The volume fraction of the water phase increases as the water injection duration increases. The 3D images of the water distributions for silica gel samples are similar to those of the glass bead samples. The changes in water saturation (and the accompanying removal of silicone oil) and the variations in the volume fraction, specific surface area, and cube-counting fractal dimension of the water phase fit well with the single-exponential recovery function {f(t)=a[1-exp(-λt)]}. The asymptotic values (a, i.e., saturated value) of the properties of the volume fraction, specific surface area, and cube-counting fractal dimension of the glass bead samples were greater than those for the silica gel samples primarily because of the intrinsic differences in the porous networks and local distribution of the pore size and connectivity. The convergence rates of all of the properties are inversely proportional to the entropy length and permeability. Despite limitations of the current study, such as insufficient resolution and uncertainty for the estimated parameters due to sparsely selected short injection times, the observed trends highlight the first analyses of the cube-counting fractal dimension (and other structural properties) and convergence rates in porous networks consisting of two fluid components. These results indicate that the convergence rates correlate with the geometric factor that characterizes the porous networks and transport property of the porous networks.

Estudio de la sensibilidad a su propia deformación de escorias de alto horno activadas alcalinamente y reforzadas con fibra de carbono

The addition of carbon fibres is used in cement composite materials with a twofold objective: enhancement of their mechanical performance, and functionalisation of the composite, e.g. as strain sensor. Alkali-activated blast furnace slag pastes were reinforced with carbon fibres, which increased bending and compressive strengths, controlled drying shrinkage, and their resistivity could be related to strains during cyclic compression tests. However, in order to optimise this strain self-sensing property, changes in water saturation of samples are necessary, because fully saturated samples did not work properly as a strain sensor.

Ross scheme, Newton–Raphson iterative methods and time-stepping strategies for solving the mixed form of Richards' equation

Abstract. The solution of the mathematical model for flow in variably saturated porous media described by the Richards equation (RE) is subject to heavy numerical difficulties due to its highly nonlinear properties and remains very challenging. Two different algorithms are used in this work to solve the mixed form of RE: the traditional iterative algorithm and a time-adaptive algorithm consisting of changing the time-step magnitude within the iteration procedure while the nonlinear parameters are computed with the state variable at the previous time. The Ross method is an example of this type of scheme, and we show that it is equivalent to the Newton–Raphson method with a time-adaptive algorithm.Both algorithms are coupled to different time-stepping strategies: the standard heuristic approach based on the number of iterations and two strategies based on the time truncation error or on the change in water saturation. Three different test cases are used to evaluate the efficiency of these algorithms.The numerical results highlight the necessity of implementing an estimate of the time truncation errors.

Elastoplastic model for hydro-mechanical behavior of unsaturated soils

A coupled elastoplastic constitutive model is presented for describing the hydraulic and mechanical behaviors of unsaturated soils. The model is capable of considering the influence of the irreversible changes in water saturation on the mechanical behavior and in plastic deformation on the hydraulic behavior. The mechanical and hydraulic behaviors are captured using the subloading surface and bounding surface plasticity frameworks, respectively. The coupling between hydraulic and mechanical behaviors is established using the intergranular stress concept in addition to appropriate coupled hardening rules. Attention is also given to the movement of the soil water characteristic curve due to the plastic deformation. Model predictions for some unsaturated soil samples are compared with experimental data, and a reasonable agreement is achieved.

Impact of water blocking in fractures on the performance of hydraulically fractured horizontal wells in tight gas reservoir

Tight gas is a major gas resource which accounts for 14% of the total gas resources and 29% of unconventional gas in the world. The natural production of tight reservoir is very low, and multi-stage fracturing technology is widely used in horizontal wells. Multi-stage hydraulic fracturing for stimulating tight gas reservoirs requires a significant amount of fracturing fluid, which is usually water-based. Water blocking is considered as a potential type of damage in tight gas reservoirs. There are many factors affecting the production decline caused by the water blocking in fractures. However, the water blocking and fluid segregation in fractures are ignored. This study investigates the characteristics of water blocking in fractures and the effects of wettability, fracture fluid viscosity and fracture fluid filtration on water blocking and gas production. Numerical simulation is used to show that three stages can be used to describe the change of water saturation at the top and bottom of fractures, where the second stage indicates water blocking in hydraulic fractures. The results demonstrate that the stronger the interfacial tension, the more obvious the water wettability is, and the more water traps in the reservoir matrix. Decreasing interfacial tension improves the load recovery, and reduces the formation damage, and further enhances the gas production. A higher fracturing fluid viscosity causes more water traps at the bottom of fractures and intensifies the formation damage on tight gas wells. Results indicate that the fracturing fluid filtration has a great impact on the flowback performance and later gas production. Results suggest that increasing the filter cake thickness decreases the gas production peak value, and delays the time showing the peak of gas production, and further reduces the cumulative gas production.