The single-well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs. Most often, residual oil saturation (Sor) is measured; less frequently, connate water saturation (Swc) is the objective. Either saturation is measured where one phase effectively is stationary in the pore space (i.e., is at residual saturation) and the other phase can flow to the wellbore. Recently, the SWCT method has been extended to measure oil/water fractional flow at measured fluid saturations in situations in which both oil and water phases are mobile. The SWCT test is used primarily to quantify the target oil saturation before initiating improved oil recovery (IOR) operations, to measure the effectiveness of IOR agents in a single well pilot and to assess a field for bypassed oil targets. Secondarily, it is used to measure Swc accurately for better evaluation of original oil in place (OOIP). Fractional flow measurement provides realistic input for simulator models used to calculate expected waterflood performance. This chapter familiarizes the reader with the SWCT method, and offers guidelines for selecting suitable test wells and for planning and executing the field operations on the target well. Test interpretation is also discussed and illustrated with typical examples. The first SWCT test for Sor was run in the East Texas Field in 1968.[1] Patent rights were issued in 1971.[2] Since then, numerous oil companies have used the SWCT method.[3][4][5][6][7]

This paper describes the design and implementation of a one-spot enhanced oil recovery (EOR) pilot using high-salinity water (∼166,000 ppm TDS) in a sour, sandstone, heavy-oil reservoir (∼5 mol% hydrogen sulfide) based on an extensive laboratory study involving different polymers and operating conditions. In view of the results of this one-spot EOR pilot, a multi-well, high-salinity polymer-injection pilot is expected to start in 2020 targeting the Umm Niqqa Lower Fars (UNLF) reservoir in Kuwait.

Polymer flooding is normally carried out using low- to moderate-salinity water to maintain favorable polymer solution viscosities in pursuit of maximum oil recovery. Nevertheless, low- to moderate-salinity water sources such as seawater tend to be associated with a variety of logistical, operational, and commercial challenges. For this study, laboratory experiments were conducted in conjunction with reservoir simulation to confirm the technical viability of polymer flooding using high-salinity water. Thereafter, a one-spot EOR pilot was executed in the field using a well near the location of the planned multi-well pilot to confirm the performance of the selected polymer vis-à-vis injectivity and oil desaturation.

The one-spot EOR pilot described in this paper was successfully executed by performing two Single-Well Chemical Tracer (SWCT) tests. For the first stage of the pilot, 200 bbl of produced water (up to 166,000 ppm TDS) were injected into the test well in an attempt to displace mobile oil out of the investigated pore space. Following this produced water injection, an SWCT test (Test #1) was carried out and measured the remaining oil saturation to be 0.41 ± 0.03. This saturation measurement represents the fraction of oil remaining in the pore space of a cylindrical portion of the Lower Fars reservoir, measured from the wellbore out to a radius of 3.02 feet, after produced water injection. After the completion of Test #1 and subsequent recovery of the injected produced water, the same zone was treated with a 200-bbl injection of polymer solution. Following this 200-bbl polymer injection, a second SWCT test (Test #2) was performed and measured the remaining oil saturation to be 0.19 ± 0.03 out to a radius of 3.38 feet. These results indicate that polymer injection may offer considerable improvement to oil recovery over conventional waterflooding alone.

Performing polymer flooding in a sour, heavy-oil reservoir using high-salinity water is a challenging and unprecedented undertaking worldwide. In addition to the improved incremental oil recovery demonstrated by this pilot, enabling the use high-salinity produced water for polymer flooding is expected to result in significant benefits for cost-efficiency and operational ease by reducing or eliminating problems commonly associated with the sourcing, treatment, and handling of less saline water in the field.

Summary Single-well-partitioning-tracer tests (SWTTs) are used to measure the saturation of oil or water near a wellbore. If used before and after injection of enhanced-oil-recovery (EOR) fluids, they can evaluate EOR flood performance in a so-called one-spot pilot. Four alkaline/surfactant/polymer (ASP) one-spot pilots were recently completed in Kuwait's Sabriyah-Mauddud (SAMA) reservoir, a thick, heterogeneous carbonate operated by Kuwait Oil Company (KOC). UTCHEM (Delshad et al. 2013), the University of Texas chemical-flooding reservoir simulator, was used to interpret results of two of these one-spot pilots performed in an unconfined zone within the thick SAMA formation. These simulations were used to design a new method for injecting partitioning tracers for one-spot pilots. The recommended practice is to inject the tracers into a relatively uniform confined zone, but, as seen in this work, that is not always possible, so an alternative design was needed to improve the accuracy of the test. The simulations showed that there was a flow-conformance problem when the partitioning tracers were injected into a perforated zone without confinement after the viscous ASP and polymer-drive solutions. The water-conveyed-tracer solutions were being partially diverted outside of the ASP-swept zone where they contacted unswept oil. Because of this problem, the initial interpretation of the performance of the chemicals was pessimistic, overestimating the chemical residual oil saturation (ROS) by up to 12 saturation units. Additional simulations indicated that the oil saturation in the ASP-swept zone could be properly estimated by avoiding the post-ASP waterflood and injecting the post-ASP tracers in a viscous polymer solution rather than in water. An ASP one-spot pilot using the new SWTT design resulted in an estimated ROS of only 0.06 after injection of chemicals (Carlisle et al. 2014). These saturation values were obtained by history matching tracer-production data by use of both traditional continuously-stirred-tank (CSTR) models and compositional, reactive-transport reservoir models. The ability of the simulator to model every phase of the one-spot pilot operation was crucial to the insight of modified SWTT design. The waterflood, first SWTT, ASP flood, and the final SWTT were simulated using a heterogeneous permeability field representative of the Mauddud formation. Laboratory data, field-ASP quality-control information, and injection strategy were all accounted for in these simulations. We describe the models, how they were used, and how the results were used to modify the SWTT design. We further discuss the implications for other SWTTs. The advantage of mechanistic simulation of multiple aspects of a one-spot pilot is an important theme of this study. Because the pore space investigated by the SWTTs can be affected by the previously injected EOR fluids (and vice versa), these interactions should be accounted for. This simulation approach can be used to identify and mitigate design problems during each phase of a challenging one-spot pilot.

Abstract Remaining oil saturation (ROS) and waterflood residual oil saturation (Sorw) are key parameters for reservoir modeling and waterflood management in a group of heterogeneous deepwater turbidite reservoirs. A large amount of laboratory special core analysis (SCAL) data indicated high Sorw values and a large target potential target for chemical EOR (enhanced oil recovery). Available SCAL data was not considered reliable. Acquiring additional core was considered to be too costly and too risky due to the highly deviated well paths required for new wells. Single Well Chemical Tracer Tests (SWCTT's) in producing wells were the only viable alternative. This paper describes – to our knowledge – the first applications of SWCTT in a deepwater setting. An early 2010 SWCTT showed ROS / Sorw to be much lower than expected but test interpretation was uncertain. The 1 SWCTT provided a valuable learning experience to improve test design and execution and to improve on significant logistical challenges in the deepwater setting. Using lessons learned we performed two additional SWCTT's in late 2010. The later SWCTT's included well integrity pre-tests and smaller completion intervals. Typical SWCTT volumes were ~5,000 bbl of seawater containing tracers with a depth of investigation of ~4 to 5 meters. All three SWCTT's indicated low Sorw values, ranging from 0.05 to 0.20 with a nominal average of 0.15. Similar results from all three SWCTT's indicate that microscopic displacement efficiency is very good; eliminating the option of chemical EOR. The current field development plan is focused on improving volumetric sweep efficiency. Properly designed and executed SWCTT's can be considered as large-scale "laboratory waterflood tests" at true reservoir conditions (e.g., live oil, wettability and stress history). Compared to conventional SCAL tests using small plug samples, SWCTT's see a much larger rock volume and avoid wettability alteration issues that plague modern cores taken with OBM (oil based mud). Though logistically challenging in deepwater, SWCTT's can be more cost- and time-effective than taking a new core.

Abstract A one-spot EOR pilot was successfully completed to demonstrate the efficacy of a lab-optimized ASP formulation to mobilize remaining oil from a giant sandstone reservoir in Kuwait. This one-spot EOR pilot, which also referred to as a Single Well Chemical Tracer (SWCT) test, was a significant milestone in de-risking ASP flooding for multi-well pilot implementation. The vertical zone of investigation for the Raudhatain Zubair (RAZU) SWCT was chosen to be a confined channel sand with relatively homogeneous and representative properties in a producer near the proposed pilot area. Two SWCT tests were performed and the difference in residual oil saturation from post water flood and post ASP injection tracer tests quantitatively determines the displacement efficiency of the ASP slug. The tracer chemicals for the tests included a hydrolyzing, partitioning tracer (ethyl acetate) and two alcohols (n-propyl alcohol and isopropyl alcohol) that serve as cover tracer and material balance tracer, respectively, to ensure robustness of test interpretation. The water flood SWCT test showed ideal behavior with well-defined profiles. Interpretation of this test was accomplished using a single layer model and showed that at the end of the water flood, the residual oil saturation to water was 0.24 ± 0.02% in the 23 -ft interval for the SWCT test. The ASP tracer test was complicated due to poor injectivity, well mechanical issues, and dilution from a zone which did not accept any SWCT test injection fluids but contributed substantially to production. Due to the dilution from another zone, the ASP tracer test profiles were more dispersed than the water flood tracer test but were adequately modeled using a two-layer model with irreversible flow. Analysis of the ASP SWCT test showed that the average oil saturation was reduced to 0.06 ± 0.05%, which represents a ~67% reduction in residual oil saturation. Despite poor injectivity leading to a reduced polymer drive and taper injection and dilution from another zone resulting in a non-idealized tracer response, careful interpretation of the SWCT test measurements resulted in a reliable estimate of the post-ASP oil saturation. The SWCT test results demonstrate the feasibility of applying ASP flooding to increase oil recovery from a giant high-temperature sandstone reservoir in North Kuwait.