SPE-169155-MS Monitoring Chemical EOR Processes

A laboratory study on core plugs from a carbonate field confirmed the efficacy of an alkaline surfactant (AS) formulation at reservoir conditions. Continuous monitoring of remaining oil saturation (ROS) in short core plugs using spatially resolved nuclear magnetic resonance (NMR) can provide insights into the processes by which surfactants release post-water-flood remaining oil. In single short plugs, volumetric averages do not correctly quantify remaining oil due to capillary end effects (high remaining oil saturation post water flood observed at the outlet face of the plug), oil banks, and other flow heterogeneities. These effects are all quantifiable by NMR, allowing accurate monitoring of ROS. Here, consistent ROS values are obtained in the laboratory, in single-well pilots, and in single-well chemical-tracer (SWCT) tests. Laboratory core floods with nuclear magnetic resonance (NMR) monitoring at low magnetic fields therefore provide a valuable calibration of the NMR logs, in addition to the direct assessment of ROS. Corroboration across multiple length scales, spatial resolution, and correspondence with preferred logging measurements, all contribute to the value of NMR-monitored core-floods as a complement to field pilot studies. Introduction Monitoring the efficacy of enhanced oil recovery (EOR) processes is an important step in the screening of new chemical agents. This screening process begins at the laboratory scale (Stoll et al. 2011). It is important to understand both the liquidliquid and liquid-rock interactions. A possible EOR agent is identified and then adjusted to provide the required fluid-phase behavior for a given set of reservoir conditions (temperature, salinity, crude composition, restore reservoir wettability). The role of surfactants is to reduce the interfacial tension (IFT) between oil and water; this is a critical requirement of the chosen formulation. The alkaline convert some of the oil into surfactant, further reducing the oil/water interfacial tension. Laboratoryscale core floods constitute the next stage of screening to explore liquid-rock interactions before reservoir pilots. Here, the alkaline alters the charge of the rock surface to reduce surfactant adsorption. Nuclear magnetic resonance (NMR) is a powerful tool for assessing remaining oil saturation (ROS) during EOR both in the reservoir—via NMR logging—and in the laboratory with a low-field bench-top instrument. Quantitative EOR evaluation has been demonstrated on short plugs at the laboratory scale using spatially resolved low-field NMR. The spatial dimension is important at all EOR piloting length scales, but is critical when studying short plugs where bulk volumetrics may be dominated by capillary or geometry end effects and yield inaccurate ROS values. At the laboratory scale, it is usual to construct long composite cores (length > 30 cm [12 in.]) from plugs to reduce the significance of end effects and other flow heterogeneities on bulk assays of remaining oil (e.g., effluent gravimetrics). Commercial NMR systems are not suited to bulk measurements of such long samples, thereby requiring some form of mechanical scanning to explore the entire core. Such methodology prevents continuous monitoring of ROS during a core flood. In the laboratory, NMR offers the capability to spatially resolve the distribution of ROS within a short core plug (length = 5 cm [2 in.]). Previously, we demonstrated the quantitative measurements of ROS, consistent with well logs, in such short plugs by monitoring a central portion of the plug expected to be free from capillary or geometry end effects. Magnetic resonance imaging (MRI) of core plugs has been unsuccessful as a probe of ROS; implemented at high field, the solid/fluid magnetic susceptibility contrast prevents quantitative assay of in-situ liquid volumes, and limits the practical spatial resolution, preventing pore-scale imaging. The history of MRI in special core analysis (SCAL) is given by Mitchell et al. (2013b). Here, we use a low-field permanent magnet (B0 = 50 mT [500 G]) to obtain a spatial distribution of oil saturation in a profile (single image axis). The signal-to-noise ratio