Variations In Water Saturation Research Articles

Monitoring managed aquifer recharge with electrical resistivity probes

The use of managed aquifer recharge (MAR) to supplement groundwater resources can mitigate the risks to an aquifer in overdraft. However, limited information on subsurface properties and processes that control groundwater flow may lead to low levels of recapture of infiltrated water, reducing the efficacy of MAR operations. We used long 1D electrical resistivity probes to monitor the subsurface response over one diversion season at five locations beneath an operating recharge pond in northern California. The experiment demonstrated the benefits of integrating geophysical and standard hydrologic measurements. The water table response interpreted from time-lapse electrical resistivity images was in good agreement with traditional pore-pressure transducer measurements at coincident locations. Moreover, the electrical resistivity measurements were able to identify vertical variations in water saturation that would not have appeared in pore-pressure data alone. Changes in saturation estimated from electrical resistivity models indicated large hydraulic gradients at early time and suggested the presence of highly permeable conduits and baffles between the surface and the screened interval of recovery wells. The interpreted structure of these conduits and baffles would contribute to the movement of a large amount of infiltrated water beyond the capture zone of recovery wells before pumping begins, accounting in part for the low recovery rates.

Application of Bayesian decision making tool in detecting oil-water contact in a carbonate reservoir

Defining Oil-Water Contact (OWC) is essential for detail petrophysical evaluations and reservoir volumetric calculations. This paper presents Bayesian decision making tool as a sophisticated technique in OWC detecting from well log data. The proposed method is applied to data related to three wells of an oil field of the Southwestern Iran. The method’s performance is evaluated based on well testing reports and also through comparisons with the results of conventional approach based on permeability prediction. Results indicate that the proposed method is more accurate than conventional approach and may improve the results about 5% on average. In addition, using this method, any variation of water saturation (Sw) log and reservoir fluid types may be detectable.

Field examples of the joint petrophysical inversion of resistivity and nuclear measurements acquired in high-angle and horizontal wells

A recently introduced interpretation workflow has confirmed that inversion-based interpretation is more reliable than conventional well-log analysis in high-angle (HA) and horizontal (HZ) wells because the former accounts for well trajectory and shoulder-bed effects on well logs. Synthetic examples show that the inversion workflow could improve the estimation of hydrocarbon volumes by 15% and 10% in HA and HZ intervals, respectively. Using field examples of thinly interbedded calcite-cemented siltstone formations, we document results of the joint petrophysical inversion of logging-while-drilling multisector nuclear (neutron porosity, density, natural gamma ray, photoelectric factor) and multiarray propagation resistivity measurement for improved formation evaluation in HA/HZ wells. Under the assumption of multilayer formation petrophysical models, the inversion approach estimates formation properties by numerically reproducing the available measurements. Subsequently, inversion-derived hydrocarbon pore volume is calculated for assessment of reservoir pay. Application of the joint inversion-based interpretation in challenging field examples highlights petrophysical characteristics such as capillary trends or water saturation variations in a hydrocarbon column influenced by reservoir quality and formation electrical anisotropy which otherwise remain inconspicuous with conventional and quick-look interpretation of well-logs.

Fractal properties of capillary imbibition of rocks

UDC 622.276 In a series of laboratory experiments, the deterministic and fractal parameters of spontaneous capillary im- bibition of rocks have been investigated. The capillary imbibition of higher-permeability samples was charac- terized by a sharp increase in the saturation at the initial stage and a further increase that is not described by the power law. Analysis of the parameters of the multifractal spectrum of water-saturation variations of the investigated samples in the process of capillary imbibition has shown that the generalized Hurst constant tends to decrease with time in the process of imbibition. the sample preparation procedure, and the equipment used in experiments. Experiments on countercurrent capillary im- bibition were carried out with originally dry samples. The curves of the current capillary displacement coefficient are given, and the evolution of parameters of the fractal spectra obtained for these curves is shown. The obtained experi- mental results are discussed with the use of theoretical models of the fractal properties of porous media and known examples of their experimental corroborations. Some of the possible practical applications of the discovered fractal properties of countercurrent capillary imbibition of oil traps are presented. Experimental Equipment and Procedure. The countercurrent capillary imbibition parameters were deter- mined by the curves of the hydrostatic weight of cores completely submerged in the imbibition fluid. To perform ex- periments, we assembled and tested a facility incorporating a VR-4Ebalance. The measurement principle relied on the transformation of a pitch of the balance beam to an electric signal. The balance is based on a VR-4 beam balance, in which agate plates are used as support platforms for prisms. On the balance beam a pitch sensor based on a solid ac- celerometer is fixed. The electric signal from the pitch sensor arrives at an amplifier and then at a second-order low- frequency filter. The time constant of the filter is 15 s. The investigated sample was suspended by a thin thread from the beam arm and dipped into a vessel with water. On the other arm of the beam a balancing load was placed. Experiments were performed at room temperature. The increase in the sample weight was recorded automatically by means of an analog-to-digital converter with data output to a personal computer with a time discretization from 10 s to 2 min. The sensitivity to a change in the sample

Coupled thermo-hydro analysis of steam flow in a horizontal wellbore in a heavy oil reservoir

A novel model for dynamic temperature distribution in heavy oil reservoirs is derived from the principle of energy conservation. A difference equation of the model is firstly separated into radial and axial difference equations and then integrated. Taking into account the coupling of temperature and pressure in the reservoir and wellbore, models for calculating distributions of the reservoir temperature, reservoir pressure, and water saturation are also developed. The steam injected into the wellbore has a more significant effect on reservoir pressure than on reservoir temperature. Calculation results indicate that the reservoir temperature and pressure decrease exponentially with increasing distance from the horizontal wellbore. The radial variation range of the pressure field induced by steam is twice as wide as that of the temperature field, and both variation ranges decrease from the wellbore heel to the toe. Variation of water saturation induced by steam is similar to the temperature and pressure fields. The radial variation ranges of the reservoir temperature and pressure increase with steam injection time, but rate of increase diminishes gradually.

Enhancing Hydrocarbon Estimates in a Deepwater Turbidite Sequence

Thinly laminated formations can be significant hydrocarbon reservoirs, particularly in turbiditic and fluvial environments. However, often such formations exhibit low resistivities when measured with conventional resistivity tools. These are known in the literature as classical low-resistivity pay and often are anisotropic in resistivity. Over the past decade, studies have shown that electrical anisotropy greater than three in these layered formations is caused by thin water-bearing beds, such as shale layers, alternating with the oil sands. Initial laboratory studies have shown that alternating variations in water saturation is a major contributor to resistivity anisotropy, but detailed laboratory research is needed before this can be generalized to all shaly-sand reservoirs. Since their introduction in the early 1950s, wireline induction tools measured mainly the horizontal resistivity (Rh). Consequently, when confronted with a potential reservoir containing thinly laminated sand/shale sequences, it was a challenging task to decipher the thin hydrocarbon-bearing sands in the apparent low-resistivity hydrocarbon-bearing layer. The low-resistivity reading had to be corrected, usually by adding a shale-contribution term using one of the many published equations. More recently, technologies have been developed to measure the vertical dimension of this resistivity problem, or the vertical resistivity (Rv), which can be evaluated using three tools: logging-while-drilling resistivity tools when the apparent angle between the tool and the formation is high; joint inversion of array laterolog and array-induction; and the triaxial induction tool, the focus of this article. North Africa Case Study In a North Africa well, array-induction, nuclear magnetic-resonance (NMR), and nuclear (density-neutron and gamma ray) tools were run initially. The target interval was a deepwater turbiditic levee/overbank deposit, consisting of highly organized thin layers of high-quality gas-bearing sands. Layer thicknesses ranged from almost a meter to less than a centimeter, with most of the layers being in the centimeter range. The logged well encountered two gas-filled channel systems separated by a pressure barrier (Fig. 1). The lower system was a more proximal levee/overbank facies, of which the reservoir portion was made up of organized layers of high-quality coarse-grained sand interbedded with shales and some mudstones. Conversely, the upper system consisted of a levee/overbank facies having thin layers of finer-grained sands with shales and small amounts of mud. Regular array-induction logs did not show noticeable invasion profile in this formation. The porosity (including clay- bound water) was 35%, and the effective porosity (without clay-bound water) was 25%. The separation between density and neutron showed a high amount of shale except in three places (X150–X155 ft, X188–X192 ft, and X268–X271 ft), where the cross-over between density and neutron indicated gas, also confirmed there by the higher resistivity measured by the regular induction log (Fig. 2). From the deep array-induction log, water saturation (Sw) was computed with a dual-water equation to correct for clay conductivity. The data analysis, based on the array-induction tool and a dual-water shaly-sand approach, led to unreasonably high water saturations, with all resulting Sw values being close to unity, except in the three higher resistivity depths.

Role of Small-Scale Variations in Water Saturation in Optimization of Steamflood Heavy-Oil Recovery in the Midway-Sunset Field, California

Summary In recovery of heavy oil by steamflood, efficiencies can be realized by limiting the placement of steam to the portions of the reservoir with highest oil saturations, thus reducing the disproportionate loss of heat to connate water. This optimization strategy requires knowledge of water-saturation (Sw) distributions within the heavy-oil reservoir at the scale of operations. Application of this strategy has contributed to the successful reactivation of the shut-in Pru Fee property in the Midway-Sunset field 1 mile west of Taft, a U.S. Dept. of Energy (DOE) Class 3 oil-technology demonstration project. The 40 new wells drilled and logged within the 40-acre Pru Fee property, together with a single continuous core, have permitted 3D mapping of Sw within the 250- to 350-ft-thick pay zone in the Monarch sand reservoir. Water saturations are observed to vary at three different scales: A systematic vertical reduction in Sw through a 100- to 150-ft interval above the oil/water contact (OWC) caused by dominant capillary influence where the buoyancy effects are diminished by the low-density contrast of the 13°API oil and formation water. Lateral variations in Sw on the scale of 10 to 100 ft caused principally by prior oil production from the reservoir, but modified by its internal stratigraphic architecture. Bed-to-bed variations in Sw on the order of a few feet or less constrained by grain-size-controlled differences in porosity/ permeability in these crudely graded sands. Overall production efficiency in the steamflood has been improved by limiting steam injection to the upper one-half to two-thirds of the pay zone, where Sw is lowest. Knowledge of the lateral variations in Sw has permitted more accurate appraisal of the effectiveness of individual producers and nine-spot injector/ producer arrays. The recognition of the bed-to-bed variations has permitted a better petrophysical model for calibrating Sw calculated from logs.

Two-dimensional laboratory simulation of LNAPL infiltration and redistribution in the vadose zone

A quantitative two-dimensional laboratory experiment was conducted to investigate the immiscible flow of a light non-aqueous phase liquid (LNAPL) in the vadose zone. An image analysis technique was used to determine the two-dimensional saturation distribution of LNAPL, water and air during LNAPL infiltration and redistribution. Vertical water saturation variations were also continuously monitored with miniature resistivity probes. LNAPL and water pressures were measured using hydrophobic and hydrophilic tensiometers. This study is limited to homogeneous geological conditions, but the unique experimental methods developed will be used to examine more complex systems. The pressure measurements and the quantification of the saturation distribution of all the fluids in the entire flow domain under transient conditions provide quantitative data essential for testing the predictive capability of numerical models. The data are used to examine the adequacy of the constitutive pressure-saturation relations that are used in multiphase flow models. The results indicate that refinement of these commonly used hydraulic relations is needed for accurate model prediction. It is noted in particular that, in three-fluid phase systems, models should account for the existence of a residual NAPL saturation occurring after NAPL drainage. This is of notable importance because residual NAPL can act as a non negligible persistent source of contamination.

Determination of True Effective In Situ Gas Permeability in Subnormally Water-Saturated Tight Gas Reservoirs

Abstract The phenomena of subnormally saturated, very low permeability gas reservoirs, which exhibit the unique combination of very low initial water saturation (less than the expected capillary equilibrium value) combined with low to very low permeability (0.0001 - 0.1 mD) have been well-documented in the literature and exist extensively in a number of regional sedimentary basins on a worldwide basis. Previous technical papers have concentrated on the mechanism to create these types of reservoirs and how to properly measure the in situ subnormal water saturation and connate water composition. This paper provides an extension of this work, which allows the direct measurement, under in situ reservoir conditions of temperature, pressure, and overburden pressure, of the true effective permeability of gas under any possible range of porosity and water saturation conditions that can conceivably exist in the reservoir. This procedure, known as the "incremental phase trap test," provides a valuable tool in the evaluation of whether given pay zones in an ultra-tight gas reservoir situation are worthy of completion. It also provides information on how this new method can be used in the exploitation of this ever increasing area of tight gas reservoir production. Introduction The value of the initial water saturation present in gas reservoirs can strongly affect the effective in situ permeability of the gas. Many very low permeability gas reservoirs (in situ permeability of less than 0.1 mD) are prone to large reductions in effective gas permeability if normal capillary equilibrium water saturations are present in the porous media. A capillary equilibrium water saturation refers to the water saturation which would be present if the formation matrix of a given pore size distribution is in direct hydraulic contact with a free water zone at a certain height above the defined free water contact level. In many cases where these types of low permeability, gas-bearing formations are productive, theinitial water saturation exists in a subnormal capillary equilibrium condition (where the initial water saturation is lower than would be expected in a normal "equilibrium" situation. This is favourable from a reserves and productivity perspective, but makes these types of formations much more susceptible to various types of formation damage, notably phase trapping. A more detailed discussion of formation damage mechanisms in tight gas reservoirs, and the concept of subnormal initial water saturation conditions, is contained in the literature(1-14). A common problem in formations of this type is that the exact value of the initial water saturation is unknown, or, considerable variations in water saturation exist throughout the reservoir due to permeability contrasts or varying degrees of desiccation caused by the spatial location of the sediments and degree of capillary continuity that exists in the formation. Therefore, a method to accurately etermine the true effective in situ permeability of the formation at any possible combination of immobile water saturation and porosity level is very valuable.

Relating variations in water saturation of a sandstone sample to pore geometry by neutron tomography and image analysis of thin sections

Neutron tomography experiments were undertaken on a quartz arenite sample in order to determine saturation variations in the sample after the initially water-saturated sample was subjected to air flushing. The experiments were carried out in order to study the applicability of the neutron tomography facility at the Paul Scherrer Institute, Switzerland, for imaging fluid distribution in porous rocks. The variation in water saturation was related to the pore geometry of the sample as investigated by petrographic image analysis of thin sections. A variation in fluid saturation was successfully imaged. The spatial saturation distribution noted from the neutron tomography corresponds well with variations in pore geometry, indicating that the water was preferentially flushed from the coarse-grained laminae. Des mesures tomographiques neutroniques ont ete realisees sur un echantillon de gres quartzeux dans le but d'y determiner des variations de degre de saturation apres que l'echantillon initialement sature en eau ait ete soumis a un drainage force. Les experiences ont ete conduites afin d'etudier la capacite d'un dispositif de tomographie neutronique de l'Institut Paul Scheerer (Suisse) a donner une image de la repartition des fluides dans les roches poreuses. Des variations de degre de saturation ont ete identifiees avec succes, variations en relation avec la geometrie des pores de l'echantillon comme cela a pu etre identifie par des techniques d'analyse d'image de lames minces. La repartition spatiale du degre de saturation observee a partir de la tomographie neutronique correspond bien a des variations dans la geometrie des pores, indiquant que l'eau avait ete preferentiellement chassee des lamines constituees de gros grains.

Determination of Water Saturation Using Miniature Resistivity Probes During Intermediate Scale and Centrifuge Multiphase Flow Laboratory Experiments

Abstract Physical modeling is essential to the conceptual understanding of the mechanisms governing multiphase flow in porous media. However, the need for accurate data is hampered by the limited amount of appropriate instrumentation designed to measure fluid saturation in three-fluid phase flow experiments. In centrifuge testing, this is accentuated by the fact that the instruments must be small and resistant to the effects of increased gravity. Miniature resistivity probes, developed at Cambridge University Engineering Department (CUED), were used to determine water saturation variations during a centrifuge test and a 1 g two-dimensional multiphase flow experiment. These experiments were conducted to study the migration of light non-aqueous phase liquids (LNAPL) in unsaturated sands. Prior to the tests, the resistivity probes were calibrated against the water saturation of unsaturated sand samples. The calibration relationship was compared to Archie's law. The miniature probes proved to be a valuable tool for monitoring water saturation variations during three-fluid phase flow under 1 g conditions, as well as under the accelerated gravity field of 20 g.

Simulation of Gas Dipole Tests in Fractures at the Intermediate Scale Using a New Upscaling Method

A high-level radioactive waste disposal site may lead to gas generation by different physical mechanisms. As these sites are to be located in areas with low water flow, any small amount of gas can lead to relative high gas pressures, so that multiphase flow analysis becomes relevant. The movement of gas and water through the system has two important implications. Firstly, water flow takes place in unsaturated conditions, and thus travel times of the radioactive particles transported are affected; and secondly, gas can also carry radioactive particles. Therefore, one of the key points in such studies is the time when gas would break through the biosphere under a number of different flow conditions. In fractured zones, gas would flow preferentially through the most conductive features. We consider a two-dimensional system representing an isolated fracture. In each point we assign a local porosity and permeability and a local pressure-saturation relationship. A dipole (injector-producer) gas flow system is generated and the variation in water saturation is studied. A simple method is proposed for obtaining upscaled values for several parameters involved in two-phase flow. It is based on numerical simulation on a block scale assuming steady-state conditions and absence of capillary pressure gradients. The proposed method of upscaling is applied to simulate a dipole test using a coarser grid than that of the reference field. The comparison between the results in both scales shows an encouraging agreement.

Effects of water salinity, saturation and clay content on the complex resistivity of sandstone samples

Abstract Complex resistivity measurements were made on sandstone samples in the frequency range from 10 Hz to 2 MHz. The main objective was to investigate the frequency response of complex resistivity and phase angle as a function of salinity, water saturation and clay content. The results showed the classical frequency dependence behaviour where the complex resistivity decreases with increasing frequency. The complex impedance behaviour in the intermediate frequency range (10–100 kHz) was used to relate the effect of frequency dispersion with interface polarization and, hence, pore geometry, specific surface area and permeability. Both water saturation and salinity were found to influence the gradient and the relaxation frequency of the complex resistivity versus frequency relationship. A variation in water saturation from full to partial saturation resulted in a dramatic increase in the gradient and a clear shift of the relaxation frequency. Both the saturation and salinity dependence can be attributed to the polarization of both the rock-fluid and fluid-fluid interfaces within the pore space, which depend on the geometry and physical characteristics of the interfacial layers. The results presented in this paper can have important applications in identifying low resistivity and low contrast pay zones.

Log Interpretation Problem in Low Resistivity Sands

Abstract All recent investigations have attributed low electric log resistivities in producing formations to a conductive ion layer associated with clay material present in the sands. In this study another mechanism is proposed. It is believed that connate water saturation variations (which may or may not be associated with shale contact) also contribute to low resistivities. In addition to the mechanistic studies, evaluation of various recently proposed interpretation methods have been made. Of these, the Poupon, Loy, and Tixier and de Witte methods seem best. Statement of Problem Low resistivity sands are often encountered which turn out to be commercial hydrocarbon zones but appear from electric log data to be water bearing. Adequate methods of recognizing these cases are needed. Industry-wide interest in this problem is evidenced by the number of papers published in recent years. It seemed that several zones in the Mustang Island field (Nueces County, Tex.) were affected by the shaly sand problem. This was evidenced by:low hydrocarbon saturation calculations from logs and measured formation factor data in producing zones,changes in apparent measured formation factors with changes in water resistivity, andcore descriptions which indicated the presence of shaly "streaks" or "spots." A section of a typical log is shown in Fig. 1. All sands on this log are hydrocarbon zones. Scope of Investigation The Mustang Island field, Nueces County, Tex., has been chosen for the study of the shaly sand problem because the oil-water contacts are well established for most of the sands and some core data were available for study. This problem has been approached by both empirical and experimental methods. The empirical portion of this study is a comparison of calculated hydrocarbon saturations from logs by various methods with expected or known production (oil, gas, or water).