A novel mechanism of residual oil reduction during low-salinity water flooding by induced fines migration and consequent permeability damage is discovered. Dry corefloods (without oil) with ...piecewise-constant decreasing salinity are conducted to verify the presence of movable fines that yield permeability decrease. Sequential two-phase coreflood tests are performed using the displacement of nonpolar oil by high-salinity water, followed by full resaturation of the core by nonpolar oil and low-salinity waterflooding. The test then continued in tertiary mode, in which brines of decreasing salinity were injected in the presence of residual oil. Four Berea cores with high clay content, one Bentheimer core with low clay content, and two artificial cores with no clay content were used for two-phase waterflooding experiments. Reduction in permeability for water, fines production, and reduction in residual oil saturation accompanied the abrupt salinity decrease in all tests. This effect is attributed to fines mobilization that is due to salinity decrease, followed by fines migration and straining in thin pore throats that resulted in local hydraulic resistance and consequent pore-scale flux diversion, yielding S or reduction. The S or dependencies of the induced formation damage are derived from five series of laboratory tests. All the laboratory tests confirmed the proposed S or reduction mechanism by fines-assisted low-salinity waterflooding.
The data input for reservoir simulation of low-salinity waterflood in sandstones includes adsorption isotherms, or equilibrium constants for all cation-exchange reactions, and cation-exchange ...capacity (CEC). We develop a novel method for determination of those functions from the laboratory data of low-salinity coreflooding; the inverse problem is solved; rather, matching is applied. Changing independent time variable to Lagrangian coordinate transforms the governing system for two-phase multicomponent flow into a single-phase system of mass balance equations for each ion and the volumetric balance equation for water. It allows determining the adsorption isotherms for all ions, or equilibrium constants and CEC, from the breakthrough ion concentrations, using the exact solution of ion-exchange inverse problem. Further, the fractional flow and relative permeability can be determined from the effluent water cut and pressure drop histories, using the Welge’s and JBN methods; here each data point corresponds to the breakthrough values of ion concentrations. We show that for four-ion waterflooding, only two constants from two equilibrium constants and CEC can be determined. Treatment of five coreflood data sets shows a close agreement between the laboratory data and the results of inverse problem solution.
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•Determination of both relative permeability and capillary pressure from corefloods.•Lab validation of steady-state transition test by secondary and tertiary imbibitions.•Method ...validity is proven by close match for steady-state and continuous injections.•Match of pressure-drop data is significantly more accurate than that for saturation.•Concentric-circle distributor yields much less 3D effects than half-moon distributor.
The work aims at the laboratory validation and practical implementation of the steady-state-transient method for simultaneous determination of relative permeability Kr and capillary pressure Pc, which uses steady-state data along with the transient data between the steady states. A new methodology for preliminary choice of Kr and Pc and the detailed test modelling yields planning and design of the steady-state-transient test SSTT – the definition of admissible flow rate, number of steady states and injected water fractions, frequency and volumes of the effluent samples. The test-parameter choice is based on the newly derived theoretical criteria for validity of the model for two-phase flow in porous media, and the operational criteria for the accuracy of the measurements. The preliminary modelling also allows determining the type curves for transient pressure-drop histories, which are used to approximate the measured data and input the inverse solver. Two laboratory SSTTs with inlet half-moon and concentric-circle distributors are carried out. The agreement between the matched-modelling and experimental data for pressure-drop is significantly higher than for average saturation for both tests. The agreement for the test with the concentric-circle distributor is significantly closer than that of the half-moon distributor. This is explained by non-uniform saturation distributions in core cross-sections due to 3D flow, which is more pronounced in the case of half-moon distributor.
•High- and low-salinity brine was injected into a Berea sandstone core plug.•Micro-CT images of the core plug was taken before and after injection.•SEM-EDS analysis showed clay fines ...production.•Laboratory measurements showed an 80% decrease in permeability.•Image analysis show clay straining near the production end of the core plug.
Fines migration is considered one of the mechanisms of permeability alteration associated with water injection in aquifers and oil reservoirs. However, there has been limited direct evidence of fines migration. This paper presents conclusive evidence for the migration and capture of fines, obtained from experiments utilising 3D micro-CT imaging.
A single-phase flooding experiment was performed on a Berea sandstone core plug. First, a micro-CT image of the dry core plug was taken before flooding. Then the core plug was saturated with 4% NaCl brine, further injected with 4% NaCl brine, and finally flushed with fresh water. The pressure difference across the core plug was monitored during these fluid injections, and produced water samples were analysed for fines concentration and type by combining particle counting methods and Scanning Electron Microscope – Energy Dispersive Spectroscopy (SEM-EDS) analysis. After the flooding experiment, the core plug was reimaged, and the second micro-CT image was compared to the first to quantify the fines migration.
Laboratory measurements showed an 80% decrease in permeability during fresh water injection, which was accompanied by a large concentration of fines in the produced-water samples. SEM-EDS analysis of those fines showed them to be clay. Comparison of initial and final tomogram images showed increased micro-CT intensity in the flow direction near the core outlet, due to relocation of clay. Permeability computations made on micro-CT images showed a 50% decrease in permeability during fresh water injection, due to fines straining near the core outlet. Modelling of retention concentration along the core length and of breakthrough fines concentration versus pore volume injected indicated that fines retention was nonlinear.
We discuss the governing system for oil–water flow with varying water composition. The model accounts for wettability alteration, which affects the relative permeability, and for ...salinity-variation-induced fines migration, which reduces the relative permeability of water. The overall ionic strength represents the aqueous phase composition in the model. One-dimensional displacement of oil by high-salinity water followed by low-salinity-slug injection and high-salinity water chase drive allows for exact analytical solution. The solution is derived using the splitting method. The analytical model obtained analyses the effects of wettability alteration and fines migration on oil recovery as two distinct physical mechanisms. For typical reservoir conditions, the significant effects of both mechanisms are observed.
The Welge–JBN method for determining relative permeability from unsteady-state waterflood test is commonly used for two-phase flows in porous media. We discuss the theoretical criteria that limits ...application of the basic Buckley–Leverett model and Welge–JBN method and the operational criteria of the accuracy of measurements during core waterflood tests. The objective is determination of the waterflood test parameters (core length, flow velocity and effluent sampling frequency) that fulfil the theoretical and operational criteria. The overall set of criteria results in five inequalities in three-dimensional Euclidian space of these parameters. For known rock and fluid properties, a formula for minimum core length to fulfil Welge–JBN criteria is derived. For cases where the core length is given, formulae for test’s flow velocity and sampling period are provided to satisfy the test admissibility conditions. The application of the proposed methodology is illustrated by two coreflood tests.
ESPRESSO at VLT Pepe, F; Cristiani, S; Rebolo, R ...
Astronomy and astrophysics (Berlin),
01/2021, Letnik:
645
Journal Article
Recenzirano
Odprti dostop
Context. ESPRESSO is the new high-resolution spectrograph of ESO’s Very Large Telescope (VLT). It was designed for ultra-high radial-velocity (RV) precision and extreme spectral fidelity with the aim ...of performing exoplanet research and fundamental astrophysical experiments with unprecedented precision and accuracy. It is able to observe with any of the four Unit Telescopes (UTs) of the VLT at a spectral resolving power of 140 000 or 190 000 over the 378.2 to 788.7 nm wavelength range; it can also observe with all four UTs together, turning the VLT into a 16 m diameter equivalent telescope in terms of collecting area while still providing a resolving power of 70 000. Aims. We provide a general description of the ESPRESSO instrument, report on its on-sky performance, and present our Guaranteed Time Observation (GTO) program along with its first results. Methods. ESPRESSO was installed on the Paranal Observatory in fall 2017. Commissioning (on-sky testing) was conducted between December 2017 and September 2018. The instrument saw its official start of operations on October 1, 2018, but improvements to the instrument and recommissioning runs were conducted until July 2019. Results. The measured overall optical throughput of ESPRESSO at 550 nm and a seeing of 0.65″ exceeds the 10% mark under nominal astroclimatic conditions. We demonstrate an RV precision of better than 25 cm s−1 during a single night and 50 cm s−1 over several months. These values being limited by photon noise and stellar jitter shows that the performance is compatible with an instrumental precision of 10 cm s−1. No difference has been measured across the UTs, neither in throughput nor RV precision. Conclusions. The combination of the large collecting telescope area with the efficiency and the exquisite spectral fidelity of ESPRESSO opens a new parameter space in RV measurements, the study of planetary atmospheres, fundamental constants, stellar characterization, and many other fields.
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