The single well chemical tracer (SWCT) test can be used to evaluate an Improved oil recovery (IOR) process quickly and inexpensively. The one-spot procedure takes advantage of the nondestructive nature of the SWCT method. The single-well (one-spot) pilot is carried out in three steps. First, Sor for the target interval is measured (see Residual oil evaluation using single well chemical tracer test. Then an appropriate volume of the IOR fluid is injected into the test interval and pushed away from the well with water.

Using the single well chemical tracer (SWCT) test avoids the problems of too-wide well spacing and excessive tracer dispersion caused by layering that can occur with well to well tests. In the SWCT test, the tracer-bearing fluid is injected into the formation through the test well and then produced back to the surface through the same well. The time required to produce the tracers back can be controlled by controlling the injected volume on the basis of available production flow rate from the test well. In a single-well test, tracers injected into a higher-permeability layer will be pushed farther away from the well than those in a lower-permeability layer, as indicated in Figure 1a; however, the tracers in the higher-permeability layer will have a longer distance to travel when flow is reversed. As the tracer profiles in Figure 1b show, the tracers from different layers will return to the test well at the same time, assuming that the flow is reversible in the various layers.

The single-well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs. Most often, residual oil saturation 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.

Even with a properly designed single well chemical tracer (SWCT) test, interpreting the data requires judgment calls, and typically, simulation, to arrive at a final estimation of residual oil. Tomich et al.[1] report one of the earliest SWCT tests, which was performed on a Frio Sandstone reservoir on the Texas Gulf Coast. The results of this test are used here to demonstrate the details of SWCT test interpretation for an ideal situation. The test well in the Tomich et al.[1] report was in a fault block that had been depleted for several years. Because of the natural water drive and high permeability of the sand, the formation was believed to be near true Sor.

The single-well chemical tracer test (SWCTT) is widely used in the determination of residual oil saturation (SOR) in the near-well region. SWCTTs typically require large amounts of chemicals and some days of well shut-down. In the present paper, we propose using thermo-sensitive nanogel carriers for targeted release of tracers for SWCTTs. This approach has the potential to significantly reduce the time and amount of chemicals required by a SWCTT.

The targeted tracer release method was inspired by previously developed drug delivery applications using stimuli-sensitive nano-capsules. Nanoparticles loaded with medical cargo were synthesised to target specific sites. The release of the active principles would then be triggered by an in-situ (temperature, pH) or ex-situ (magnetic field, light) stimuli. For the application to tracer studies (with focus on SWCTT), a poly-N-isopropylacrylamide (pNIPAm) based nanogel was synthesized and tested due to its thermo-sensitive nature. PNIPAm molecule has a unique memory effect with respect to temperature. This effect is explored as a mechanism to both load and release the tracers for a SWCTT.

PNIPAm nanogels or hydrogels is a highly hydrophilic, cross-linked polymeric network. When the temperature of the solution is increased above the lower critical solution temperature (LCST) of PNIPAm molecule, the capsules exhibit a reversible collapse effect, which is capitalised in the release of the tracer molecules. The hydrodynamic diameter of capsules were measured using Dynamic Light Scattering (DLS) and were found to be 195 ± 1.4 nm at 25 °C and 73 ± 1.45 nm at 45 °C respectively. The nanogels exhibit a reduction in volume to 8 times when the temperature is increased from 25 °C to 45 °C. This change in volume acts as a lock-in mechanism once the tracer is loaded and open-up to release loaded tracers. In-order to study the encapsulation and release of tracer compounds, passive and partitioning tracers were loaded into the structures. The capsules showed a significant tracer loading efficiency. For studying the release rate and mechanism, increase in temperature was used to trigger the release of tracers.

The developments in the SWCTT have been slow since it was originally introduced in 1973, even though this test is one of the most used tracer tests in the oil industry. In the present work, we show how nanotechnology can be used to reduce the large amounts of chemicals and time required by SWCTTs. Concepts and results about the synthesis of the nano-carriers, loading and releasing of the tracers are presented and discussed.

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).

The paper presents the results of the single well chemical tracer tests (SWCTT) conducted in the Kharyaginskoye carbonate reservoir to estimate residual oil saturation before and after of low-salinity water flooding. Previously core-studies were conducted to estimate the effectiveness of low-salinity water flooding. The SWCTT is based on chemical transformations of substances into the bottomhole zone of the well. Reactive chemical tracer is injected into the reservoir through the production well. After that the volume of tracers is pushed into the reservoir with an additional volume of water. The well is then 'shut in' while some of the reactive tracer is allowed to undergo a hydrolysis reaction within the reservoir. Following 'shut in' period, the well is produced to bring back the fluid, with sampling and analysis for the presence of indicators. The reaction product, alcohol tracer and unreacted ester tracer undergoes a chromatographic separation. The magnitude of this separation depends on the oil/water ratio in the reservoir and the ester partition coefficient, thereby allowing to calculate residual oil saturation. The depth of investigation area was roughly 3.5 m from the wellbore. Effectiveness of flooding is estimated, based on the values of the residual oil saturation before and after the treatment. The article also presents the results of laboratory tests on core samples to determine the effectiveness of low-saline water injection, the values of the separation and hydrolysis coefficients required when interpreting the SWCTT results, and describes the practical part on the field of the experiment. As a result of the work, a numerical assessment of the effectiveness of low-saline water injection in the Kharyaginskoye oil field was obtained.

Abstract Tracer technology has evolved significantly over the years and is now being increasingly used as one of the effective monitoring and surveillance (M&S) tools in the oil and gas industry. Tracer surveys, deployed as either interwell tests or single-well tests, are one of the enabling M&S technologies that can be used to investigate reservoir connectivity and flow performance, measure residual oil saturation, and determine reservoir properties that control displacement processes, particularly in improved oil recovery (IOR) or enhanced oil recovery (EOR) operations. As part of a comprehensive monitoring and surveillance program for a GAS-EOR pilot project, an interwell gas tracer test (IWGTT) was designed and implemented to provide a better understanding of gas flow-paths and gas-phase connectivity between gas injector and producer pairs, gas-phase breakthrough times ("time of flight"), and provide pertinent data for optimizing water-alternating-gas (WAG) field operations. Additional objectives include the detection and tracking of any inadvertent out-of-zone injection, and acquisition of relevant data for gas reactive transport modelling. Four unique tracers were injected into four individual injectors, respectively, and their elution were monitored in four "paired" updip producers. In addition to the reservoir connectivity and breakthrough times between the injector and producer pairs, the results showed different trends for different areas of the reservoir. The gas-phase breakthrough times are slightly different from the water tracer breakthrough times from a previous inter-well chemical tracer test (IWCTT). Residence times for the tracers indicate different trends for three of the injector-producer pairs compared to the last pair. These trends reflect and support conclusions regarding reservoir heterogeneities also seen from the previous IWCTT, which were not anticipated at the beginning of the GAS-EOR pilot. This paper reviews the design and implementation of the tracer test, field operational issues, analyses, and interpretation of the tracer results. The tracer data has been very useful in understanding well interconnectivity and dynamic fluid flow in this part of the reservoir. This has led to better reservoir description, improved dynamic simulation model, and optimized WAG sequence.

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.

Abstract The success of any improved oil recovery (IOR) project is largely dependent on how much oil is remaining to be mobilized within the targeted area of the partially depleted or mature reservoir. Partitioning tracers are generally used to measure residual oil saturation (Sor) or remaining oil saturation (ROS) in the near wellbore region via a single well chemical tracer test (SWCTT) or in an inter-well region via a partitioning inter-well tracer test (PITT). There is a limited repertoire of nonradioactive and environmentally friendly inter-well partitioning tracers for measuring ROS. A new class of environmentally friendly partitioning tracers was field tested, in a giant carbonate reservoir undergoing peripheral waterflood, for measuring ROS in inter-well regions in a depleted area. The new partitioning tracers were qualified via laboratory experiments and are deemed to be very stable at reservoir conditions (213°F and a salinity range of 60-200 kppm). The field pilot was conducted concurrently with a set of non-partitioning inter-well chemical tracer test (IWCTT) to determine reservoir connectivity, water breakthrough times, and injector-to-producer pair communication in an area selected for an IOR/EOR field pilot. An elaborate sampling and analysis program was carried out over a period of 30 months. This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the passive and partitioning tracers are reported, and inter-well connectivity between the paired and cross-paired injectors and producers are analyzed. The ROS measured by a majority of the novel tracers is comparable to the saturations obtained via SWCTT, core and log derived saturations. The combination of conventional IWCTT and the novel partitioning tracers via PITT has been very useful in analyzing well interconnectivity, understanding the reservoir dynamics and quantifying remaining oil saturation distribution in the reservoir. This has led to better reservoir description and an improved dynamic simulation model.