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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. Patent rights were issued in 1971. Since then, numerous oil companies have used the SWCT method.
The single-well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs. The most common use is the assessment of residual oil saturation (Sor) prior to improved oil recovery (IOR) operations (post-waterflooding). The SWCT test for Sor uses only one well and involves the injection and back production of water carrying chemical tracers. A typical target interval for SWCT testing is shown in Figure 1. The candidate well should be completed only to the watered-out zone of interest (zone at Sor).
Simple analytical interpretation of single well chemical tracer (SWCT) is possible if one assumes uniform oil saturation, negligible hydrolysis during injection and production and assuming similar dispersion for all reservoir layers. In complex reservoir settings, including multilayer test zones, drift, cross-flow etc., reservoir simulation tools, capable of handling the hydrolysis reaction are commonly applied (Jerauld et al., 2010; Skrettingland et al., 2011). In practice, coupled flow and chemical reaction simulators (see e.g. CMG, 2010; and UTCHEM, 2000) are used. Such coupled simulations are CPU-demanding enough that execution time may be an issue, especially when small grid-size are applied to avoid numerical smearing.
Paskvan, F.. (State of Alaska Department of Natural Resources Division of Oil and Gas) | Blas, P. San (State of Alaska Department of Natural Resources Division of Oil and Gas) | Young, J.. (State of Alaska Department of Natural Resources Division of Oil and Gas) | Bakun, F.. (State of Alaska Department of Natural Resources Division of Oil and Gas) | Carlisle, C.. (State of Alaska Department of Natural Resources Division of Oil and Gas) | Pope, G.. (State of Alaska Department of Natural Resources Division of Oil and Gas)
Abstract Single well chemical tracer tests were used to measure saturations and oil-water fractional flow accurately, quickly, and cost-effectively. The Aurora oil field in the Prudhoe Bay Unit, Alaska, was identified for fast-paced development leveraging existing facilities. A series of single well chemical tracer tests (SWTT) determined key volumetric and reservoir performance properties including: Initial oil saturation, Waterflood residual oil saturation, Miscible gas EOR residual oil saturation, and Waterflood water-oil fractional flow. The SWTT measurements of initial oil saturation, residual oil saturation to water, residual oil saturation to miscible gas, and oil-water fractional flow made during the initial field development in 2001 closely match current model parameters determined based upon 18 years of field history. Methods, Procedures, Process Given a fast development pace and relatively small field size, it was deemed impractical to collect low invasion core and perform expensive, complex, and time consuming special core analysis. Instead, a series of SWTT were performed on a single production well to determine key reservoir parameters within six months. This compares favorably to core acquisition and analysis which can take six times longer and cost ten times as much. Also, SWTT can be performed after a well is drilled and on production, so key tests can be performed without early, time-intensive special core analysis acquisition planning and rig time. Aurora production well S-104 was chosen as the key data collection well to describe Aurora field. The well was conventionally cored and had a full suite of open hole logs including nuclear magnetic resonance and focused micro resistivity. This well was an ideal candidate for a SWTT as it had a good quality cement job, no water or gas injection, a detailed near wellbore reservoir description, and the well would produce to surface with gas lift. The SWTTs were performed over a 30-foot perforated interval. A typical SWTT involves tracer injection, shut-in time, then production flowback. No downhole interventions were needed since SWTT are performed using readily transportable surface equipment like chemical injection pumps and well tracer sampling and measurement equipment including a gas chromatograph. Results, Observations, Conclusions Connate water saturation was lower than expected (13% ±2), increasing the estimated oil in place and calibrating the well log water saturation log model and reservoir model saturation-height function. Due to increased initial oil saturation proved by the SWTT, additional wells were justified in the southern portion of the field which added an estimated 2.5 million barrels of recovery. The waterflood residual oil saturation was higher than expected (30% ±2) indicating a more oil-wet system than previously assumed. The oil-water fractional flow data also indicated a more oil wet relative permeability curve than estimates from available analog curves. Finally, the miscible gas EOR test demonstrated miscibility and enhanced oil recovery in-situ by measuring very low residual oil saturation (4.5% ±2) to miscible injection gas. Novel/Additive Information This is the world's first successful SWTT water-oil fractional flow measurement. If data collection had included downhole pressure gauges, the test would have also measured relative permeability endpoints. The Aurora SWTT program provides an innovative solution to a classic challenge: Accurately determining key reservoir properties in a timely and cost-effective way. Reservoir simulation using SWTT results match 18 years of field performance, demonstrating the accuracy of SWTT measurements.
Abstract The residual oil saturation obtained from SWCTT is critical for designing enhanced oil recovery (EOR). However, a key assumption in conventional SWCTT is that only single phase (water) is mobile. In reality, this is often not the case, and significant error can occur if the conventional SWCTT analysis method is used when multiple phases flow at the same time. The objective of this study is to improve the accuracy and precision of SWCTT interpretation in multi-phase flow condition. In this paper, we propose an innovative procedure of modified SWCTT and the method of moment (MoM), aiming at the two-mobile-phase condition. In the development of the algorithm, a ratio parameter is introduced to adjust the calculated swept volume difference between the conservative tracer and partitioning tracer. In addition, a mixture injection of oil and water is required, instead of pure water injection in SWCTT. The proposed approach is verified through numerical simulation on synthetic cases with known input parameters. The simulated models consist of a radial flow regime with a single vertical well in the center. The input oil saturation varies from 0.1 (immobile oil saturation) to 0.9. Our results show that the saturation estimated from modified MoM matched the simulation input data, which indicates that our approach is able to capture the saturations under two-mobile-phase condition. Moreover, the modified MoM can also be applied in single-mobile-phase condition and improved accuracy of conventional MoM.