Single-well Tracer Test Research Articles

Algorithm for estimating boundary conditions of a distributed tracer for application in a single-well tracer test

An important tool in determining residual oil saturation today is the single-well tracer test, as the preferred method for assessing the potential for using enhanced oil recovery methods (EOR) and developing pilot projects. The success of the test performed directly depends on the optimal choice of the tracer composition, which will contribute to the qualitative determination of the parameters required in the calculation of the residual oil saturation of the formation. To assess the boundary conditions for the applicability of the tracer in the field, the kinetic and thermodynamic properties of tracers are considered under various reservoir conditions of the field. Based on the results of this work, an algorithm for assessing the applicability of the tracer for reservoirs in a wide range of salinity and temperatures is presented.

Analysis and Numerical Simulation of Single-Well Tracer Test in Homogeneous, Layered and Slightly Tilted Formation.(Dept.C)

Analysis and Numerical Simulation of Single-Well Tracer Test in Homogeneous, Layered and Slightly Tilted Formation.(Dept.C)

A constant rate salt tracer injection method to quantify pumped flows in long-screened or open borehole wells

Quantifying vertical flows in long-screened or open wells is essential for their reliable use in all types of groundwater investigations. In ambient conditions, a flow profile shows the producing/receiving zones of head-driven flow, the relative vertical head gradient in the aquifer system and potential bias in chemistry samples. A flow profile while pumping can be used to quantify aquifer heterogeneity and the sampled water mixtures. This paper describes a novel approach to a single-well tracer test to quantify the flow regime in a pumped well, which is unique in its utility over a wide range of discharge rates. During constant pumping, a tracer is injected at the opposite end of the well and, as it is drawn towards the pump, the tracer is diluted in proportion to each inflow. A dilution model using the advection-dispersion equation is used to visually fit a flow profile that explains all tracer profiles (pre-injection, transient phase and steady-state). Results compare favourably to borehole EM flowmeter data, particularly if tracer density issues are correctly interpreted and head-loss in the flowmeter is avoided. A dimensionless Froude number is provided to assist both with understanding and minimising the role of free convection when planning all types of in-well tracer tests involving a density contrast. Like the flowmeter, this method is particularly suited to screened wells, where packers are ineffective. Used together or separately in existing wells, these in-well methods can provide considerable information on aquifer-well hydraulics without the cost of additional drilling.

Application of Inert Gas Tracers to Identify the Physical Processes Governing the Mass Balance Problem of Leaking CO2 in Shallow Groundwater System

The CO2, leaked from a deep storage reservoir, can arrive at the shallow groundwater system. In shallow aquifer, the leaked CO2 forms a multi-phase plume and its mass balance will be controlled by various physical, chemical and biological processes. Inert gases such as sulfur hexafluoride (SF6) and noble gases are biochemically stable in groundwater system which can separate and explain the physical process governing the mass balance of the multi-phase plume. In this study, two pilot tests were conducted using inter gas tracers to evaluate the influence of physical processes on multi-phase plume migration. First injection test was made in Wonju, Korea. Three different tracers including chlorine, helium and argon tracer were jointly injected in 7.5 – 10.5 m below water table and recollected at same point after 1 day drift time in groundwater system. The mass recovery of the Single-Well Tracer Test (SWTT) was 49.3% for SF6, 58.1% for helium, 78.2% for argon, and 73.1% for chlorine. The recovery of inert gas tracers was relative to their solubility. Second injection test was made in Eumseong, Korea. Four tracers such as chlorine, helium, argon, and krypton were released along an induced pressure gradient where 0.0135% of helium, 0.0137% of argon, 0.0602% of krypton, and 75.0% of chlorine were retrieved. In this Inter-Well Tracer Test (IWTT), the recovery of inert gas tracer was also relative to tracer’s solubility. This study suggested the applicability of inert gas tracers to evaluate the mass balance of leaking CO2 plume in shallow aquifer system.

The Case for Flux‐Based Remedial Performance Monitoring Programs

The Case for Flux‐Based Remedial Performance Monitoring Programs

Evaluation of multiple tracer methods to estimate low groundwater flow velocities

Four different tracer methods were used to estimate groundwater flow velocity at a multiple-well site in the saturated alluvium south of Yucca Mountain, Nevada: (1) two single-well tracer tests with different rest or “shut-in” periods, (2) a cross-hole tracer test with an extended flow interruption, (3) a comparison of two tracer decay curves in an injection borehole with and without pumping of a downgradient well, and (4) a natural-gradient tracer test. Such tracer methods are potentially very useful for estimating groundwater velocities when hydraulic gradients are flat (and hence uncertain) and also when water level and hydraulic conductivity data are sparse, both of which were the case at this test location. The purpose of the study was to evaluate the first three methods for their ability to provide reasonable estimates of relatively low groundwater flow velocities in such low-hydraulic-gradient environments. The natural-gradient method is generally considered to be the most robust and direct method, so it was used to provide a “ground truth” velocity estimate. However, this method usually requires several wells, so it is often not practical in systems with large depths to groundwater and correspondingly high well installation costs. The fact that a successful natural gradient test was conducted at the test location offered a unique opportunity to compare the flow velocity estimates obtained by the more easily deployed and lower risk methods with the ground-truth natural-gradient method. The groundwater flow velocity estimates from the four methods agreed very well with each other, suggesting that the first three methods all provided reasonably good estimates of groundwater flow velocity at the site. The advantages and disadvantages of the different methods, as well as some of the uncertainties associated with them are discussed.

Use of Partitioning Tracers To Estimate Oil-Saturation Distribution

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 179655, “Use of Partitioning Tracers To Estimate Oil-Saturation Distribution in Heterogeneous Reservoirs,” by R.M. Dean, D.L. Walker, V. Dwarakanath, T. Malik, and K. Spilker, Chevron, prepared for the 2016 SPE Improved Oil Recovery Conference, Tulsa, 11–13 April. The paper has not been peer reviewed. Partitioning interwell tracer tests (PITTs) have been used to estimate remaining oil saturations (ROSs) during waterflooding. Compared with core tests, well logs, and single-well tracer tests, PITTs sample a much larger representative elemental volume (REV) and provide interwell estimates of remaining oil saturation. In this paper, the authors present the information gained from conducting a polymer PITT and the saturation estimated during the PITT. The polymer PITT allows characterization of polymer-flood efficiency and is a useful tool in polymer-flood evaluations in heterogeneous reservoirs. Introduction The difference in arrival time between conservative tracers has long been used to estimate the volume of oil remaining in a reservoir after tracer tests. The method-of-moments solution to calculate ROS from mean arrival times of conservative and partitioning tracers is based upon single-phase flow. When large changes of saturations occur in between the initial conservative and partitioning tracers, the volume calculated is hard to interpret. Use of equations with single-phase-flow assumptions makes it a challenge to understand the correlation between the volume calculated and its corresponding time. The method of moments has been used to calculate oil volumes and saturations. With the method of moments, the mean residence time of the tracer for a given slug duration is calculated from the concentration of tracer in the produced effluent. Oil volume is normally related to the difference in mean residence volume between a partitioning tracer and a conservative tracer, although two partitioning tracers can be related. The remaining oil volume can be calculated from the mean residence volumes of the conservative and partitioning tracers. When oil cut and saturation change are large, the volume becomes difficult to correct. Often, correcting for production on the basis of the mean residence time of the first tracer gives the right value if oil production is small, but this does not give the correct answer if oil cut is high. Conducting a PITT with a favorable mobility ratio helps alleviate the problem of high oil cut during tracer production because it allows the partitioning tracer to contact oil at residual saturation. In order to calculate the volume of oil and saturation more accurately during an active waterflood with mobile oil, a technique is required that estimates oil remaining behind the waterflood. To achieve that objective, modifications to the method of moments, known as the residence-time-distribution- analysis (RTDA) method, were made in order to account for two-phase flow. In this new derivation, oil and water can be produced simultaneously and are accounted for by the fractional flow of each phase.

Simulation of single well tracer tests for surfactant–polymer flooding

A single well tracer test (SWTT) is a method to investigate the residual oil saturation near the wellbore. It presents an important tool to evaluate enhanced oil recovery (EOR) processes. For EOR evaluation, two SWTTs (one before and another after EOR application) can be used to estimate the reduction in S or due to the application of an EOR process. The change in S or is a measure of the incremental oil recovery of the applied EOR technology. In this work, we use University of Texas Chemical Flooding Simulator to guide the design of SWTTs that will be later run to evaluate chemical flooding potential. First, we perform thorough sensitivity simulations using an idealistic homogeneous model. Second, we perform simulations using a realistic model, which was generated based on the selected evaluation well (Well-X). In the sensitivity runs, we investigate the effects of various parameters such as partitioning coefficients, reaction rates, injection rates, injection volumes, and shut-in times. Based on the results, we provide recommendations for designing the SWTTs. Furthermore, simulations using the Well-X model suggest an incremental oil recovery factor of 14.7 % OOIP due to surfactant-polymer flooding. This is consistent with lab data and provides assurance to multi-well field applications. More importantly, those simulation results support the utility of SWTTs in evaluating chemical flooding potential. Based on the results, we expect to observe distinct back-production peaks, clear separation between the reactive and product tracers, and measurable variation in separation due to chemical EOR application that can be categorically analyzed.

Field Performance of Point Velocity Probes at a Tidally Influenced Site

AbstractConventional methods to estimate groundwater velocity that rely on Darcy's Law and average hydrogeologic parameter values are insensitive to local‐scale heterogeneities and anisotropy that control advective flow velocity and direction. Furthermore, at sites that are tidally influenced or have extraction wells with variable pumping schedules, infrequent water‐level measurements may not adequately characterize the range and significance of transient hydraulic conditions. The point velocity probe (PVP) is a recently developed instrument capable of directly measuring local‐scale groundwater flow velocity and direction. In particular, PVPs may offer distinct advantages for sites with complex groundwater–surface water interactions and/or with spatially and temporally variable groundwater flow conditions. The PVP utilizes a small volume of saline tracer and inexpensive sensors to directly measure groundwater flow direction and velocity in situ at the centimeter‐scale and discrete times. The probes are installed in conventional direct‐push borings, rather than in wells, thus minimizing the changes and biases in the local flow field caused by well installation and construction. Six PVPs were installed at a tidally influenced site in North Carolina to evaluate their implementability, performance, and potential value as a new site characterization tool.For this study, a new PVP prototype was developed using a rapid prototyping machine (i.e., a “three‐dimensional printer'') and included both horizontally and vertically oriented tracer detectors. A site‐specific testing protocol was developed to account for the spatially and temporally variable hydraulic conditions and groundwater salinity. The PVPs were tested multiple times, and the results were compared to the results of several different groundwater flux and velocity estimation tools and methods, including a heat‐pulse flowmeter, passive flux meters, single‐well tracer tests, and high‐resolution hydraulic gradient analysis. Overall, the results confirmed that the PVP concept is valid and demonstrated that reliable estimates of groundwater velocity and direction can be obtained in simple settings. Also, PVPs can be successfully installed by conventional methods at sites where the formation consists primarily of noncohesive soils and the water table is relatively shallow.Although some PVP tests yielded consistent and reliable results, several tests did not. This is likely due to the highly transient flow conditions and limitations associated with the PVP design and testing procedures. PVPs offer particular advantages over, and can effectively complement, other groundwater flow characterization techniques for certain conditions, and objectives may be useful for characterizing complex flow patterns under steady conditions; however, this study suggests that PVPs are best suited for conditions where the flow hydraulics are not highly transient. For sites where the hydraulic conditions are highly transient, the most reliable approach for understanding groundwater flow behavior and groundwater–surface water interactions would generally involve both a high‐resolution hydraulic gradient analysis and another local‐scale method, such as tracer testing. This study also highlighted some aspects of the current PVP design and testing protocol that can be improved upon, including a more robust connection between the PVP and injection line and further assessment of tracer solution density effects on vertical flow. © 2013 Wiley Periodicals, Inc.

The use of chemical tracers to water injection processes applied on Romanian reservoirs

The hydrocarbon reservoirs are extremely complex, each reservoir having its own identity. Reservoirs heterogeneity (mainly regarding the layered ones) frequently results in low recovery efficiencies, both under the primary regime and when different agents are injected from the surface. EOR processes efficiency depends on how detailed the reservoir is known and on the information related to fluids flow through reservoir. There are certain analyzes, investigations and tests providing good knowledge about the reservoir. The tracer tests are among them, being frequently used to water injection processes. Depending on the method used, IWTT (Interwell tracer test), SWTT (Single-Well Tracer Test), TWTT (Two-Well Tracer Test), information are obtained as related to: the setting of the preferential flow path of the injected fluid, the identification of water channels, evidencing the geological barriers, determining the residual oil saturation, around the well bore or along the tracer's path between two wells. This paper is focused on ICPT Cefforts related to the use of the chemical tracers to the water injection processes applied to the oil reservoirs of Romania. It describes the usual tracers and the methods used to detect them in the reaction wells. Up to now, more than 50 tests with IWTT tracers have been performed on-site and this work presents some of their results.

A Re-Circulating Tracer Well Test method for measuring reaction rates in fast-flowing aquifers: Conceptual and mathematical model

Summary Reactive groundwater tracer tests provide a practical means by which rates of reaction processes that constitute natural attenuation can be measured in aquifer systems. We present a two-well Re-Circulating Tracer Well Test (RCTWT) method that we conceive applicable to determining in situ reaction rates in fast-flowing alluvial aquifers where conventional small-scale single-well tracer tests, such as the push–pull test, might be impractical. The RCTWT concept was analysed mathematically and breakthrough curve datasets were generated using a numerical model to simulate hypothetical aquifer systems from which the sensitivity of the RCTWT method to governing mathematical variables was analysed. A simplified data interpretation method for determining reaction rates from observed RCTWT breakthrough data was developed. It was demonstrated that the efficiency of the RCTWT is strongly affected by the degree of aquifer heterogeneity and the performance can be optimised by applying a high pumping rate along with a short doublet well-spacing. The simplified method provided reasonably accurate estimates of first-order reaction rate coefficients when evaluated using the hypothetical datasets, from which it was concluded that the RCTWT is a valid concept for determining potential in situ reaction rates. The findings are to be incorporated in the design of practical RCTWTs aimed at measuring denitrification rates in fast-flowing alluvial aquifers, in New Zealand.

Diffusive partitioning tracer test for the quantification of nonaqueous phase liquid (NAPL) in the vadose zone: Performance evaluation for heterogeneous NAPL distribution

A partitioning tracer test based on gas-phase diffusion in the vadose zone yields estimates of the residual nonaqueous phase liquid (NAPL) saturation. The present paper investigates this technique further by studying diffusive tracer breakthrough curves in the vadose zone for a heterogeneous NAPL distribution. Tracer experiments were performed in a lysimeter with a horizontal layer of artificial kerosene embedded in unsaturated sand. Tracer disappearance curves at the injection point and tracer breakthrough curves at some distance from the injection point were measured inside and outside of the NAPL layer. A numerical code was used to generate independent model predictions based on the physicochemical sand, NAPL, and tracer properties. The measured and modeled tracer breakthrough curves were in good agreement confirming the validity of important modeling assumptions such as negligible sorption of chlorofluorocarbon (CFC) tracers to the uncontaminated sand and their fast reversible partitioning between the soil air and the NAPL phase. Subsequently, the model was used to investigate different configurations of NAPL contamination. The experimental and model results show that the tracer disappearance curves of a single-well diffusive partitioning tracer test (DPTT) are dominated by the near-field presence of NAPL around the tip of the soil gas probe. In contrast, breakthrough curves of inter-well tracer tests reflect the NAPL saturation in between the probes, although there is no unique interpretation of the tracer signals if the NAPL distribution is heterogeneous. Numerical modeling is useful for the planning of a DPTT application. Simulations suggest that several cubic meters of soil can be investigated with a single inter-well partitioning tracer test of 24-hour duration by placing the injection point in the center of the investigated soil volume and probes at up to 1 m distance for the monitoring of gaseous tracers.

Pore-Scale Simulation of Dispersion in Porous Media

Summary Mixing of miscible gas with oil in a reservoir decreases the effective strength of the gas, which can adversely affect miscibility and recovery efficiency. The level of true mixing that occurs in a reservoir, however, is widely debated and often ignored in reservoir simulation in which very large grid blocks are used. Large grid blocks create artificially large mixing that can cause errors in predicted oil recovery. This paper examines mixing that occurs in porous media by solving for single-phase flow in a connected network of pores. We differentiate between true mixing that can reduce the effective strength of a miscible gas or surfactant from apparent mixing caused by convective spreading. This work differs from network models in that we directly solve the Navier-Stokes equation and the convection-diffusion equation to determine the velocities and concentrations at any location within the pores. Flow in series and layered heterogeneous porous media are modeled through use of many grains in different arrangements. We consider slug, continuous, and partial injection as well as echo tests (single-well tracer tests) and transmission tests (interwell tracer tests). We match the concentrations from the pore-scale simulations to the analytical convection-dispersion solution that includes both transverse- and longitudinal-dispersion coefficients. The results show that for flow in series and in layers, echo- and transmission-longitudinal dispersivities become equal and reach an asymptotic value if complete mixing over a cross section perpendicular to flow has occurred. In practice, the asymptotic value of dispersivity may never be reached, depending on pattern-scale heterogeneity and well spacing. Transverse-dispersion coefficients also are scale dependent, but they decrease with traveled distance. We further demonstrate that the classical Perkins-Johnston relationship between longitudinal-dispersion coefficient and fluid velocity is obtained. We conclude that echo dispersivities are reliable indicators of true mixing in porous media.

Low-Salinity Waterflooding Improves Oil Recovery - Historical Field Evidence

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 109965, "Low- Salinity Waterflooding To Improve Oil Recovery - Historical Field Evidence," by Eric P. Robertson, SPE, Idaho National Laboratory, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed. Crude-oil/water/rock interactions can lead to large variations in the displacement efficiency of waterfloods. Laboratory waterflood tests and single-well tracer tests in the field have shown that injection of low-salinity water can increase oil recovery, but testing on a multiwell-field scale has not yet been undertaken. Three Minnelusa-formation waterfloods in the Powder River basin of Wyoming were compared on the basis of salinity of the formation and injection water and of reservoir characteristics. Introduction Different wetting states of crude-oil, water, and rock ensembles can yield different oil recoveries from laboratory-waterflood tests. The wetting state, or wettability, of a rock/fluid system can be altered. In the laboratory, wettability can be altered by changing the crude-oil composition, changing the temperature while aging the rock and crude oil, or by changing the temperature during water displacement. Also, water composition can have a significant effect on wettability and on oil recovery. Therefore, cases may exist in which attention to injection-water composition could increase oil recovery, and, likely, increase the economic profitability of a waterflood. An optimal composition of dissolved solids in the injection water may exist that would yield the highest oil recovery. The composition could involve many variables with respect to ionic composition and concentration, but current knowledge of how and when water composition can be manipulated to increase oil recovery is limited. Several examples of improved recovery by injecting low-ionic-strength brine have been reported for both outcrop- and field-core samples. Fig. 1 shows the potential for increased oil recovery from low-salinity waterflooding. These corefloods were performed on two cores from the CS reservoir under identical conditions except for the composition of the injected water. The conditions necessary for improved recovery, such as the type of crude oil and rock, composition of the formation and injected waters, and initial water saturation, still are not understood fully. The crude-oil/water/rock interactions that determine displacement efficiency are highly complex. Nevertheless, laboratory observations were sufficiently encouraging to justify more studies aimed at field application. The Idaho National Laboratory (INL) through funding from the US Department of Energy searched historical waterflood records for anecdotal evidence of increased oil recovery resulting from the injection of lower-salinity water to displace oil in reservoirs with higher-salinity initial formation water. The objective was to research and compare historical field data and to compare waterflood responses from low-and high-salinity injection waters.

Estimation of True Dispersivity in Field-Scale Permeable Media

Summary Simulation studies (Solano et al.1; Stalkup2) show that oil recovery for enriched gas drives depends on the amount of dispersion in reservoir media, but the value of dispersion, expressed as dispersivity, is unknown at the field scale. This work focuses on three types of dispersion in permeable media to obtain realistic estimates of dispersive mixing at the field scale. The dispersivity from single-well tracer tests (SWTTs), also known as echo dispersivity, is dispersivity that is unaffected by fluid flow direction. Layering in permeable media tends to increase the dispersivity observed in well-to-well tracer tests, also known as transmission dispersivity, but should not affect the echo dispersivity. We analyzed a collection of SWTT data to estimate echo dis-persivity at the SWTT scale. The estimated echo dispersivities closely match a published trend with length in dispersivities obtained from groundwater tracer tests. This unexpected result—it was thought that transmission dispersivity should be greater than echo dispersivity—is analyzed with numerical simulation. Another type of dispersive mixing is local dispersivity, or the mixing observed at a point as tracer flows past. Numerical simulation results show that the local dispersivity is always less than the transmission dispersivity and greater than the echo dispersivity. It is closer to one limit or the other depending on the amount and type of heterogeneity, the autocorrelation structure of the medium's permeability, and the lateral (vertical) permeability. The agreement between the SWTT echo dispersivities and the field trend suggests that the field data are measuring local dispersivities. All dispersivities appear to grow with length.

Analysis and numerical simulation of a single-well tracer test in homogeneous, layered and slightly tilted formations

Simulation of a field tracer experiment from an injection well in an axially symmetrical flow field in homogenous, stratified and slightly tilted aquifers is presented. The simulation has been verified by analytical solutions of the evolution of the first and second radial spatial moments of the tracer displacements derived in the current study under pure advective transport in case of layered formation. The study focused also on the discrepancies in the transport mechanisms between uniform (linear) and axially symmetrical radial flow fields in homogenous, layered and slightly tilted formations. Excellent agreement exists between analytical solution and the numerical simulation for the case of pure advection in both first and second moments supporting the validity of the numerical simulations. A subdiffusive dispersion regime in case of transport by advection and dispersion in homogeneous aquifer is observed due to the decline of the velocity field. N K;) is the representative effective medium of the layered aquifer in case of pure advection under axially symmetrical flow field.

Interpretation of single-well tracer tests using fractional-flow dimensions. Part 2: A case study

Generalized equations using fractional-flow dimensions were derived to estimate the Darcy and groundwater-flow velocities obtained from the point-dilution and the single-well injection-withdrawal field tests. Flow velocities can only be estimated from single-well tests if the kinematic porosity or the hydraulic conductivity and hydraulic gradient are known a priori. A pumping test performed on the boreholes will yield an estimate of the fractional-flow dimension and the extent of the flow region by applying the generalized radial flow (GRF) model of Barker [Barker JA (1988) A generalized radial flow model for hydraulic tests in fractured rock. Water Resour Res 24(10):1796–1804]. These parameters are used in the generalised equations for the single-well tracer tests to estimate the flow-through area and, therewith, the Darcy or flow velocity.

Interpretation of single-well tracer tests using fractional-flow dimensions. Part 1: Theory and mathematical models

Generalized equations using fractional-flow dimensions were derived to estimate the Darcy and seepage velocities obtained from the point-dilution and the single-well injection-withdrawal field tests conducted in fractured-rock aquifers. Seepage velocities can only be estimated from single-well tests if the hydraulic conductivity and the hydraulic gradient are known a priori. However, if a radial-convergent test is also performed between two boreholes, the kinematic porosity can be estimated and be used to estimate the seepage velocity from the single-well test results.

Analysis of a Single-Well Chemical Tracer Test To Measure the Residual Oil Saturation to a Hydrocarbon Miscible Gas Flood at Prudhoe Bay

Summary In 1990, a single-well chemical tracer (SWCT) test was performed in Prudhoe Bay to measure the effective waterflood and miscible gasflood residuals over a 12 ft reservoir interval. This is believed to be the first such use of this technology for a hydrocarbon miscible gas. This paper describes how the usual SWCT design was modified to accommodate the miscible gas, the results of the SWCT, which indicate significantly higher residual oil saturation for miscible gasflood than expected from coreflood experiments, and the subsequent simulation of the test which has provided good agreement with the observed results. The paper shows, with compositional simulation support, that the high apparent residual oil saturation was a consequence of incomplete volumetric sweep by the miscible gas and draws on the experiences of this test to make recommendations for the design of future SWCT tests measuring residuals to gasflooding.

Performance Analysis and Optimization of the Prudhoe Bay Miscible-Gas Project

Summary Because EOR oil response at Prudhoe Bay has been difficult to measure directly, a number of different types of field measurements have been made to evaluate the miscible-flood displacement efficiency. These measurements include water- and solvent-injection profiles, logging data from an observation well, and single-well tracer test (SWTT) data. Despite ambiguity in these data, the measurements support the simulation and laboratory data and generally indicate that the Prudhoe Bay Miscible Gas Project (PBMGP) is performing well. The most useful EOR surveillance data have been the separator-gas-sample database, with ≈4,000 compositional analyses. Separator flash analysis and allocation programs use this database to infer EOR performance on the basis of produced solvent. Reservoir mechanisms that adversely affect the EOR process efficiency have been identified. The project has exceeded initial expectations in terms of solvent retained within the reservoir, which has favorable implications for solvent sweep efficiency. Procedures have been developed to use the field and simulation data to determine how the solvent should be allocated to the existing patterns and when the project should be expanded into new areas. These procedures are designed to maximize the value of the PBMGP.