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Abstract Tracer technology is an efficient and effective monitoring and surveillance tool with many useful applications in the oil and gas industry. Some of these applications include improving reservoir characterization, waterflood optimization, remaining oil saturation (Sor) determination, fluid pathways, and connectivity between wells. Tracer surveys can be deployed inter-well between an injector and offset producer(s) or as push-and-pull studies in a single well. Tracers can be classified several ways. (a) Based on their functionality: partitioning and passive tracers. Partitioning tracers interact with the reservoir and thus propagate slower than passive tracers do. The time lag between the two types can be used to estimate Sor, to ultimately assess and optimize EOR operations. (b) Based on their carrying fluid: water and gas tracers. These can be used in IOR or EOR operations. All gas tracers are partitioning tracers and the most common are perfluorocarbons; they are thermally stable, environmentally friendly, have high detectability and low natural occurrence in the reservoir. On the other hand, water tracers are passive tracers and the most commonly used ones are fluorinated acids. (c) Based on radioactivity: radioactive and non-radioactive tracers. Selecting a tracer to deploy in the field depends on a number of factors including their solubility, fluid compatibility, background concentration, stability, detectability, cost, and environmental impact. This paper provides an overview of various tracer applications in the oil and gas industry. These will include the single-well tracer test (SWCT), inter-well tracer test (IWTT), nano tracers, gas tracers and radioactive tracers. Their use will be highlighted in different scenarios. Field case studies will be reviewed for all types of tracers. Lessons learnt for all the applications, including what works and what does not work, will be shared. Specific cases and examples will include the optimization of waterflood operations, remaining oil saturation determination, flow paths and connectivity between wells, and IOR/EOR applications. The current state-of-the-art will be presented and novel emerging methods will be highlighted. This paper will showcase how the tracer technology has evolved over the years and how it shows great potential as a reservoir monitoring and surveillance tool.
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.
This paper documents the consecutive and concurrent use of both ester andCO2 tracer chemistries in the same single-well test application. The simplicityof the CO2 system, which uses water-soluble-only injectants, and a no-soak/soaktest sequence allowed determination of definitive residual oil saturation (ROS)values.
The tests were applied in a California turbidite reservoir and in a U.S.gulf coast sandstone reservoir. In the turbidite, the CO2 test was run betweenthe mini- and main ester tests. In the sandstone, a separate CO2 test wasfollowed by injection of both reactive chemicals in the sameinjection/production sequence. Including both reactive chemistries in the sametest sequence appears to be a powerful test method to determine and to verifythe tracer responses and oil saturation. Ref. 1 describes the ester chemistryresults; documents the large ester hydrolysis pH-rate dependence; and concludesthat transit-generated tracer can dominate the response, making the testinsensitive to oil saturation. We redesigned the ester test to include ano-soak/soak test sequence to measure ester hydrolyzed in the reservoir duringtransit and soak periods, which indicates whether the tracer was generatedduring the soak period and therefore whether a unique oil saturation can becalculated in principle from the response.
Ref. 2 introduced the use of in-situ-generated CO2 as an oil tracer. Thefield procedures are essentially the same for ester and CO2 chemistries, butreactive and nonreactive tracers in the CO2 system travel together at thevelocity of the brine during injection because they are both whollywater-soluble. A wide range of robust and predictable CO2 generators withdifferent reaction rates is available. This allows optimal generator selectionfor a particular field application. These characteristics greatly simplify datainterpretation.
California Turbidite Tritiated-Water and CO2 Results
The ROS in a California turbidite reservoir was measured with the solubleCO2 tracer method. An ROS of 41 2 PV% was calculated with one layer andreversible flow. Fig. 1 compares the measured and calculated cumulative waterand oil tracer responses. The well was quite competent, and the water tracerarrived within 5 bbl of a perfectly reversible injection and production cyclefree of fluid drift. This can be seen readily by the match between the idealcalculation and the smooth "S"-shaped tritiated-water response.
More than 52,000 water and oil tracer data points were recorded.Approximately 35 points (at every 1,500th water and oil tracer data point) areplotted. The tritium and CO2 tracers were determined by use of acomputer-controlled on-line analysis system that continuously sampled aslip-stream of produced fluids and recorded the response on magnetic media.Tritium was determined by scintillation counting, and CO2 was determined bytriethanol amine titration.
The oil tracer response was good over most of the produced volume andallowed accurate calculation of the volume-averaged ROS. However, high drawdownproduction conditions, especially evident during the last 15% of production,caused gas and oil expulsion and therefore nonchromatographic tracerproduction. Oil production changes the transport mechanism by bringing the oiltracer to the wellbore directly instead of through the water phase only afterpartitioning. The nonchromatographic (fast) tracer transport is obvious in Fig.1 near the end of the test.
Although oil expulsion marred the appearance of the results, it fortunatelyoccurred late in the production cycle and did not interfere with oil saturationdetermination. Bottomhole pressure was 2,000 psi during shut-in and as low as80 psi during production. The bubblepoint pressure of the reservoir oil is1,000 psi. Oil desaturation by gas-bubble generation is a well-knownphenomenon. Separate simulations indicated that pressure drawdown below thebubblepoint would occur in the near-wellbore region ( 2 to 3 ft) under theseconditions. Chromatographic transport simulations placed the remaining tracerin the near-wellbore region and corroborated the observed fast tracertransport. The Appendix provides additional comments on the high drawdownconditions and the small amount of corrosion interference during the first twowellbore volumes of production.
The 41% CO2-tracer-test-determined ROS value agrees well with the 40%calculated from the ester results when the pH-dependent rate effects wereincluded in the analysis. Unfortunately, the ester value cannot be used forcorroboration because of excessive tracer generation during transit (see Ref.1).
ROS Calculation. The same governing chromatographic equations apply to bothtracer chemistries. Therefore, most hydrolysis must occur during the soakperiod. In comparison with the pH-dependent ester calculations that requiredifferent hydrolysis rate constants for the injection, soak, and productioncycles of the test, the same hydrolysis rate constant is used throughout forthe CO2 single-well tracer test (SWTT) calculations. This is appropriatebecause temperature was the only parameter found to affect the reaction ratefor most CO2 generators.
Temperature gradients are not a problem because water-soluble CO2 generatorstravel with the brine front and therefore are placed beyond theinjection-fluid-cooled region. The CO2 generator may be completely hydrolyzedduring the soak period without harming the test because it is independent fromthe water tracer, which is desirable. Complete hydrolysis during the soakperiod precludes reaction during production and simplifies interpretation.Essentially, this condition is achievable under most reservoir conditionsbecause many generators with different reaction rates are available.
Tracer Response. The water tracer response from the turbidite test is nearlyideal; therefore, the calculated oil tracer response is also nearly ideal. Theconstitutive relationship between the associated tracer-determined arrivalvolumes, Voi and Vwi, for calculating the oil saturation is
When the Voi/Vwi ratio is constant throughout tracer production, therequired quasistatic equilibrium condition for application of thechromatographic equation is verified and oil saturation is determined uniquelyfor a specified partition coefficient, Ko. Dispersion is the same for bothtracers because they return in the brine phase from the same position in thereservoir.
The calculated oil tracer response in Fig. 1 is actually a"predicted" response (i.e., predicted from the tritiated-water responsefor any assumed oil saturation by Eq. 1). For example, Fig. 2 shows measuredwater and oil tracer data and the predicted oil tracer responses from 0% to 60%ROS in 10% increments. Fig. 3 overplots the measured and predicted 39%, 41%,and 43% ROS values and defines the sensitivity of the results.
Radioactive tracers, such as tritiated water, iodides, cobaltic compounds, etc., are frequently used in subterranean reservoir studies. Their great advantages over non-radioactive (chemical) tracers are often outweighed by their large losses within the reservoir matrix due to undesired adsorption. The chemical tracers can also be adsorbed at a high rate. However, their high adsorption losses do not necessarily lead to large variations of the tracer retention times in the reservoir (chromatography effects) because of their larger number of molecules or ions in the liquid (mobile) phase if the (a) adsorption is governed by Langmuir type isotherms, and (b) the concentration in the liquid phase is within the flat portion of the isotherm (asymptote).
The adsorption effects experienced with both types of tracers make very precise interpretations of tracer data obtained in field studies almost impossible. For example, material balances have shown a larger acceptable degree of tracer recovery only if very severe and extreme reservoir heterogeneities are encountered. In real matrix flow, i.e., uniform or homogenous zones, the adsorption contributes significantly to the total dispersion of the tracer, thus creating a high degree of uncertainty in the data about various reservoir characteristics obtained from the tracer test in larger reservoirs. It becomes impossible to distinguish between the three main fractions contributing to the total mass dispersion: (a) fluid-dynamics, (b) diffusion, and (c) adsorption/desorption.
A new method is suggested whereby not a single tracer test but a radioactive tracer cocktail is applied. Each individual tracer contains a different radioactive element incorporated into a chemical compound. These compounds have pre-determined adsorption isotherms. These isotherms plus the material balances and the variations in the retention times for the various chemical compounds (chromatography) will allow the determination of reservoir parameters not possible by any other reservoir tracer study.
Basically, this new tracer method employs the same ideas and techniques as those used in the generally accepted analytical laboratory method of high pressure adsorption liquid chromatography (HPLC). Only the objectives are different. In HPLC, a known volume of a mixture of unknown chemical compounds flows through a known, "calibrated" porous media. Determining the "pulses" of the separated chemicals as a function of time, pulse height and pulse shape allows the analytical chemist to determine the previously unknown mixture of chemicals.
In the described reservoir tracer method, a known volume of a mixture containing known chemicals at known concentrations flows through an unknown porous media. But now, the "pulses" of the separated chemicals (function of time, pulse height and pulse shape) will allow the precise description of the porous media itself. The basic principles of both methods, HPLC and "Tracer Adsorption Chromatography Method", are the same, although the methodology (applications) and the final evaluation methods are quite different.
The described new tracer method allows the precise determination of various reservoir heterogeneities and matrix properties. In addition, it allows an experimentally determined properties. In addition, it allows an experimentally determined differentiation between the three main factors contributing to the total fluid mass dispersion (fluid-dynamics, diffusion and adsorption/desorption). This tracer chromatography method, applied in a large reservoir, cannot be duplicated with non-radioactive tracers.
Tracer tests to evaluate the flow patterns between injection and producing wells are common practice in oil and gas field operations. Gulati described the use of tritiated water in a geothermal steam reservoir. In some recent papers we described some general aspects of the use of single tracers for reservoir verification and monitoring in geothermal steam and liquid dominated fields.