Knowledge of rock texture and wettability are vital for the static and dynamic description of carbonate reservoirs. Conventional log measurements are limited in their applicability for the quantitative assessment of these attributes. Rapid variations of texture in carbonates diminishes the usefulness of core measurements on samples of limited size. Obtaining representative relative permeability by restoring core to native reservoir conditions, especially the original wettability state, is also very challenging.The dielectric response of clean carbonate rocks exhibit characteristic frequency dispersion patterns which depend on their texture and wettability. Dielectric measurements with multiple frequencies have become available enabling us to extract this information by inverting the tool data using a dispersive petrophysical model.The dielectric response is primarily sensitive to water, and provides a measure of the water phase tortuosity, which is a combination of texture and wettability, captured in the Archie's exponents m and n. Previous work has demonstrated the estimation of m from pore size distribution obtained from NMR data using an effective medium model. Formation Resistivity Factor data from core is presented to validate the model-derived m. In this work we propose a way to combine the cementation exponent so derived with the textural answers from the dielectric measurement to make a wettability estimate. We also assess the wettability from 3D NMR stations, using the increase in relaxation of oil manifested as shortening of the oil T 2 signal due to partial wetting of the oil phase. The methods are illustrated on a dataset from a Cretaceous carbonate, onshore Abu Dhabi. The interval surveyed in the subject well straddles an oil/water contact (OWC). The n exponent value derived by the method shows that the rock above the OWC is oil-wet in varying degrees. The inference on wettability state from the NMR data further supports the conclusions.

Using borehole measurements to monitor carbon dioxide (CO2) floods in the reservoir is essential to properly understand the behavior of the injected fluids and the flow patterns. Pulsed Neutron logs provide essential measurements for the evaluation of saturation in the injectors, producers and observers. However, the CO2 environment is as yet uncharacterized in the industry. Hence, reliable inferences require that either the measurement is immune to the borehole environment or that the perturbation is minimal and can be easily corrected. Where corrections are required, suitable benchmarks should be planned in advance to verify the accuracy of the corrections. These corrections should be based on the physics of the measurement to the maximum extent possible. On the first CO2 EOR pilot project in the Middle East—unique in the world because the CO2 flood was implemented with the reservoir at original oil saturation—several pulsed neutron surveys were recorded in the injector, observer, and producer wells. The surveys included capture and inelastic mode acquisition. Several novel techniques of data acquisition and interpretation were successfully tried. This paper presents the steps in planning and executing the jobs and the results of the surveys. Limitations of existing characterization and those imposed by the measurement environments in the subject wells are discussed and we show, through comparison with benchmarks, that correction for the unusual borehole environment is possible. The paper illustrates how the different modes of pulsed neutron data acquisition complement each other in the individual wells in assessing the borehole environment, providing adequate input data to enable a multiphase reservoir fluid analysis, and yielding independent fluid saturations for effective comparison. The results of the analysis are compared with an openhole evaluation to help create a coherent picture of the reservoir.

Summary Logging measurements in the borehole are vital for monitoring carbon dioxide (CO2) floods—for assessing the fluid changes in the reservoir rock as well as in the wellbore. The saturation profile at each well location provides the efficiency of the flood process for fluid displacement within the pore and the vertical sweep across and within the reservoir zones. A snapshot from multiple well locations in the reservoir enables the creation of a picture of the flood flow pattern, and the time-lapse surveys track the progress of the flood with time. Pulsed-neutron logs provide essential measurements for the evaluation of saturation in the injectors, producers, and observers. However, the CO2 environment, with the fluid in the borehole, remains uncharacterized in the industry. Hence, reliable inferences require either that the measurement is immune to the borehole environment or that the perturbation is minimal and can be easily corrected. Where corrections are required, suitable benchmarks should be planned in advance to verify the accuracy of the corrections. These corrections should be modeled after the physics of the measurement to the maximum extent possible. On the first CO2 enhanced-oil-recovery (EOR) pilot project in the Middle East—unique in the world because the CO2 flood was implemented with the reservoir at original oil saturation—several pulsed-neutron surveys were recorded in the injector, observer, and producer wells. The surveys included capture and inelastic mode acquisition. Several novel techniques of data acquisition and interpretation were successfully tried. This paper presents the steps in planning and executing the jobs and the results of the surveys. Limitations of existing characterization and those imposed by the measurement environments in the subject wells are discussed, and we show, through comparison with benchmarks, that correction for the unusual borehole environment is possible. The paper illustrates how the different modes of pulsed-neutron data acquisition complement each other in the individual wells in assessing the borehole environment, providing adequate input data to enable a multiphase reservoir-fluid analysis, and yielding independent fluid saturations for effective comparison. The results of the analysis are compared with openhole evaluation to help create a coherent picture of the reservoir. The fluid analysis from the pilot wells confirms the high displacement efficiency of CO2 as an EOR fluid. The saturation profiles from individual wells portray the vertical sweep of the flood, and the snapshot from the multiple wells gives the areal sweep. Combined with the data from production-log sensors and permeability from the magnetic resonance, the flood-breakthrough layers are identified.

As more and more fields mature in their production life cycle, EOR/IOR methods attain increasing significance. To plan these recovery methods optimally an accurate measure of the remaining hydrocarbon saturation is required to establish if there are sufficient hydrocarbons left in the reservoir to make a recovery process worthwhile. Integrating the state of the art Liquid Trapper coring technique and laboratory analysis on whole core and plugs reduce the uncertainty in this crucial reservoir parameter. New developments in dielectric logging techniques provide accurate remaining saturation answers with depth resolution of the scale of core plugs, for effective comparison and enhanced confidence on the answers. The current study aims to evaluate the remaining oil saturation (ROS) after a water flood in 4 different wells located in a depleted zone of a Cretaceous carbonate reservoir in a giant field, onshore Abu Dhabi. This is the first time a multi-well study involving liquid trapper technique is conducted in the subject field with an objective of better capturing the reservoir heterogeneities and their possible impact on the ROS. Laboratory measurements were conducted on reservoir core samples ranging from full diameter whole cores down to small plugs. The multi-scale analysis confirmed the variations in rock properties and the influence of heterogeneity on the reservoir properties, and ROS distribution. Petrophysical Groups (PG) were also defined and the final ROS values were distributed per PGs. ROS values from cores were compared with the continuous log measurement from the new multi frequency dielectric tool, deployed for the first time in the country. The array sensors of the tool make the borehole correction robust while the dispersion analysis of the multi-frequency data provides saturation answers unaffected by pore fluid salinity and carbonate texture. The uncertainty in ROS measurements and best practices for future is also discussed.

Water saturation (S w ) estimates are of prime importance for reserves estimation, reservoir development, and reservoir management. Traditionally, S w has been derived from the Archie equation with formation resistivity or thermal neutron capture cross-section (Sigma, !) in stand-alone mode. Either the resistivity or ! approach requires good knowledge of formation water salinity which can be difficult, particularly in instances of unknown and/or mixed salinities. When resistivity and ! logs are unaffected by fluid invasion, one can simultaneously compute S w and salinity from the two measurements as has been done recently using wireline logs acquired in flowing wells. This paper addresses 2 drawbacks of wireline resistivity-sigma technique: first, it is not always practical to flow the well, and second is the concern about the ! shallow depth of investigation (DOI). The first drawback might be answered by using LWD to mitigate fluid invasion issues instead of logging a live well. The second drawback is addressed by taking advantage of the newlydeveloped LWD multiple depths of investigation (MDOI) ! measurements (shallow !, medium !, and deep !) to determine the presence of invasion. If invasion has occurred, via a new MDOI inversion process analogous to the resistivity step-profile inversion, we estimate both invaded and virgin zone ! for use in petrophysical applications.We analyzed LWD MDOI resistivity and sigma logs acquired in an oil-producing reservoir drilled with a low-invasion waterbased mud for a remaining Oil Saturation (ROS) study. The coring operation has allowed some invasion to take place, which can clearly be seen from the separation of MDOI sigma curves. The inverted (true) Sigma allows meaningful S w comparisons between LWD, wireline and core analysis. The errors on estimating water saturation and salinity using shallow !, medium !, deep !, and true ! are illustrated to highlight the need to correct for invasion before doing quantitative formation analysis with !. The results indicate the viability of using LWD measurements to determine ROS in flooded reservoirs with mixed/unknown salinities.