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Well-to-well tracer tests contribute significantly to the reservoir description, which is essential in determining the best choice of production strategy. Direct dynamic information from a reservoir may be obtained, in principle, from three sources: production history, pressure testing, and tracer testing. The value and importance of tracer tests are broadly recognized. Tracer testing has become a mature technology, and improved knowledge about tracer behavior in the reservoir, improved tracer analysis, and reduction of pitfalls have made tracer tests reliable. Many tracer compounds exist; however, the number of suitable compounds for well-to-well tracers is reduced considerably because of the harsh environment that exists in many reservoirs and the long testing period. Radioactive tracers with a half-life of less than one year are mentioned only briefly in this chapter because of their limited applicability in long-term tests. Tracers may be roughly classified as passive or active. In principle, a passive tracer blindly follows the fluid phase in which it is injected. Interpretation of tracer-production curves must account for this. The results from the application of active tracers may give information about fluid saturation and rock surface properties. This information is especially important when enhanced-oil-recovery techniques that use expensive fluids such as surfactants, micellar fluids, or polymers are considered. In the last 50 years, many tracer studies have been reported and even more have been carried out without being published in the open literature. Wagner pointed out six areas in which tracers could be used as a tool to improve the reservoir description. Many companies apply tracer on a routine basis. The reservoir engineer's problem generally is a lack of adequate information about fluid flow in the reservoir. The information obtained from tracer tests is unique, and tracer tests are a relatively cheap method to obtain this information. The information is an addendum to the general field production history and is used to reduce uncertainties in the reservoir model. Tracer tests provide tracer-response curves that may be evaluated further to obtain relevant additional information. Primarily, the information gained from tracer testing is obtained simply by observing breakthrough and interwell communication.
This paper reviews various applications of partitioning tracers in the petroleum industry. While non-partitioning tracers are routinely used for flow characterization and source identification, partitioning tracers have been under-utilized largely because of a lack of publicity and credibility. In theory, partitioning tracing is potentially applicable whenever a phase boundary, for instance, gas-oil, oil-water, and water-rock, exists. Partitioning between phases will slow down the partitioning tracers in a phenomenon known as chromatographic retardation, from which fluid saturations and surface properties can be deduced. properties can be deduced. Single-well tracer testing to determine residual oil saturation to waterflood S orw constitutes the most common application of partitioning tracers. More than 200 tests have been run since its first invention In 1971. In the meantime, to overcome model inadequacies, new features are continually incorporated into the simulators, making the simulators extremely difficult to run. To get around the simulation problems, a mass balance method was proposed for direct calculation of S orw from the hydrolysis rate. Based on the same principle, an internally calibrated method involving a new class of compounds which can undergo hydrogen exchange with water is being investigated. With recent advances in instrumentation and the introduction of the chromatographic transformation technique. several successful interwell tests, mostly by Esso Resources Canada Limited (ERCL), to determine residual oil saturations in watered-out and gas-saturated reservoirs have been reported. The technique involves direct comparison of the partitioning and non-partitioning tracer profiles with no need for simulation. Under ideal conditions, S or can be determined by layers. In a different approach, full-field simulation of a multi-well test has also been employed to estimate oil saturation distribution. These tests clearly demonstrate that the valuable information on S or justifies the incremental cost of including a partitioning tracer with the non-partitioning tracer in any routine tracing projects. projects. In-situ miscibility measurement by ERCL is another interesting application. Separation of partitioning tracers injected with solvent is a measure of deviation from the first contact miscibility condition FCM. Other unproven techniques including determining trapped gas saturation during a foam flood, relative permeability ratio measurement and direct logging of gamma emitting partitioning tracers through casing to determine S or partitioning tracers through casing to determine S or vertical distribution are also reviewed in the paper.
Inert non-partitioning tracers have been widely used in the petroleum industry to tag the injected fluid as well as to characterize the flow path. The tracer results provide a guideline to pattern balancing and reconfiguring provide a guideline to pattern balancing and reconfiguring for better sweep and more effective utilization of the injected fluid. Tracer response can also be indicative of in-fill drilling potential. In addition to their use in reservoir management, non-partitioning (in particular, radioactive) tracers are also employed routinely in drilling/completion and injection profile logging.
An interwell test for residual oil saturation to waterflood (Sorw) was successfully conducted at the Leduc miscible pilot during August of 1987. Although the principle of the test was first disclosed by Cooke in 1971, lack of suitable tracers and interpretative techniques would have hindered previous attempts to apply the test in the field. previous attempts to apply the test in the field. Recent advances in radioisotope tracing technology make it possible to use the test as a cost effective alternative to conventional techniques. The Leduc test appears to be the first successful interwell Sorw test to be reported in the literature. To verify the interwell method, two other independent measurements of Sorw were made at the pilot by well-established conventional methods, namely, single well test and sponge coring. The excellent agreement between the measurements strengthens the credibility of the interwell test for determining residual oil saturation. The paper describes and compares the three techniques, focussing on the design and interpretation of the interwell test.
The Leduc Woodbend D-2A Pool was discovered in 1947. It is situated on the eastern shelf of the Western Canada Basin southwest of Edmonton, Alberta (Figure 1). The Nisku (D-2) formation is a dolomitized carbonate platform which overlies the Leduc-Woodbend (D-3) reef. The reservoir has an areal extent of 9200 hectares with a gross thickness of 45 meters and an average porosity of 4%. The Pool is characterized by numerous impermeable zones resulting from the deposition of thin peritidal laminate beds composed primarily of argillaceous mudstones. These beds limit vertical permeability to an average of less than 0.1 md. Horizontal permeability varies from 1 to 1000 md, averaging permeability varies from 1 to 1000 md, averaging 10 md throughout the pool.
The D-2A Pool recognized original oil in place (OOIP) is 32.7 (106) STM3. The pool was produced by waterflood and according to a depletion study, recovery was only expected to reach 14.2 (106) M3 of oil or 43% of OOIP. Because of its large remaining oil in place and its well developed facilities, the pool has the potential to be an ideal miscible flood candidate. Consequently, a pilot program was undertaken to assess tertiary response to program was undertaken to assess tertiary response to solvent injection. The pilot, which consisted of an unconfined injector-producer pair on 64-metre spacing, is shown in relationship to the pool boundaries in Figure 1. An accurate knowledge of residual oil saturation to waterflood, Sorw was essential to the evaluation of the pilot performance. Accordingly three independent methods, namely, interwell and single well tracer tests and sponge core analysis were used to determine Sorw.
These three methods to determine Sorw are described and compared with other measurement techniques in a comprehensive report published by Chang and Maerefat in 1986.
Abstract The application of interwell tracer tests is becoming increasingly important to the petroleum industry. Interwell tracer tests, as a proven and efficient tool, have been used to investigate reservoir flow performance and reservoir properties that control gas and water displacement processes. Tracer data have been used to reduce uncertainties attributed to well-to-well communications, vertical and horizontal flow, and residual oil saturation. This paper describes the development of interwell tracer tests in the petroleum industry, from the first qualitative tracer test in the 1950s to the latest quantitative tracer test in the 2000s. The results of our study indicate that poor sampling is the most frequently encountered problem that leads to a failure tracer test and only a small number of interwell tracer tests have employed the advanced numerical modeling methods to analyze the test data. In addition, the interwell tracer tests in the petroleum industry are not well studied as hydrology industry. Therefore, the interwell tracer tests interpretation methods deserve to be paid more attention, so that petroleum engineers can take better advantage of the costly interwell tracer tests. Introduction Although tracer tests were developed for tracking the movement of groundwater in the early 1900s, they were neglected by the petroleum industry until mid 1950s.At this time, petroleum engineers[2,3] started to conduct tracer tests for determination of flow of fluids in waterflooded reservoirs. In the petroleum industry, solvent is sometimes injected into oil or gas bearing formations for the purpose of producing more hydrocarbons. Tracers can be added to the injected solvent to determine where the injected solvents go. The subsurface flow in the reservoir is anisotropic, and the reservoirs are usually layered with significant heterogeneity. As a result, solvent movement in the reservoir is difficult to predict, especially in reservoirs containing multiple injectors and producers. However, the flow paths can be identified by tagging solvent at each injection well with a different tracer and monitoring the tracers appearing at each producing well. Therefore, multiple tracers are often used for interwell tracer tests in the petroleum industry. Interwell tracers can provide information on flood patterns within the reservoir. This information is reliable, definitive and unambiguous, thus it helps reduce uncertainties about flow paths, reservoir continuity and directional features in the reservoir. Therefore, petroleum engineers can obtain information on reservoir continuity from the amount of each tracer produced from each well.Reservoir barriers can be identified by non-recovery or delayed recovery of specific tracers. At the same time, tracer test data can help determine residual oil saturation. Tracer test results also provide information on fracture characteristics in a naturally fractured reservoir. However, interwell tracer tests are considerably time-consuming and they must last long enough (from several days to several weeks or even months) for injected tracers to flow from injectors to producers. The interwell tracer tests have been applied in many fields across the world. The majority of the fields are located in the North America and Europe. This study gives a review of the development of inter well tracer tests as it is found in the open literature search in the petroleum industry. Unfortunately, not all field tests are adequately described and this review is limited to the publicly accessed papers. The scope of this review is interwell field tracer tests and studies in the petroleum industry. Consequently, experimental works and theoretical studies on interwell tracer flow are not included in this study. Although single well tracer tests are useful for the determination of residual oil saturation, they are also excluded from this review.
An extensive reservoir management program in a hydrocarbon miscible flood has allowed the reservoir team to stay abreast of reservoir performance and to influence ultimate recovery of oil.
A key component of this program, operated by Canadian Hunter Exploration Ltd. at the Brassey miscible flood in British Columbia, included the downhole manufacture of tritiated methane as part of an innovative tracer injection program. Other technical elements in the reservoir management program include frequent pressure surveys, volume monitoring, and compositional analysis.
Teamwork between geologists, reservoir engineers, production/facilities engineers, and field operations staff was absolutely essential to the success of the program.
The reservoir management program at Brassey has enabled the business unit reservoir team to evaluate performance and influence ultimate recovery of oil from the miscible flood. The program was implemented by field operator Canadian Hunter Exploration Ltd., with the assistance of its partner, BP Exploration Inc. and affiliate BP Research. The program included depletion design using full-field black oil and later compositional modelling; a regular, frequent data collection and monitoring program; and feedback of results to field operations staff and to the reservoir simulation team.
Key objectives in managing the field arc to maintain pressure above the minimum miscibility pressure, and to balance flood fronts in each pattern to minimize breatkthrough gas production. Technical elements in the reservoir management program include an innovative tracer program which involved the first downhole manufacture of tritiated methane, frequent pressure surveys, voidage monitoring, and compositional analysis.
The need for a tracer program was identified early in the design of the five-spot pattern gas miscible flood in order to determine the source of gas breakthrough. Tritiated methane (CH3T), krypton-95 (Kr-85), and sulphur hexafluoride (SF6) were selected as tracers of choice in the five-pattern flooded area. Other tracers considered were halocarbons Freon-11, Freon-12, Freon-13B1 and Freon-114. A potential problem with radiolytic decomposition of tritiated methane led to development of an on-site, downhole manufacturing method.
Tracer results to date have made it possible to identify the origin of breakthrough gas at several producing locations. This gas breakthrough has been counteracted by making adjustments to offset well production rates and by injection reallocation. Tritium and krypton have been detected at several wells, verifying the efficacy of the injection scheme. Sulphur hexafluoride has not yet been detected. Using the results of the surveillance program, an effective history match is being constructed for a full-field compositional model, and the information gleaned from the tracer results in particular has added considerable confidence to the accuracy of the match.
The Brassey field, located in northeast British Columbia, Canada (Figure 1), produces oil from the Artex member of the Triassic Charlie Lake formation. The Artex lies at a depth of 10,000 feea, forming a stratigraphic trap with average net pay thickness of 10 feet, with porosities averaging 16 percent, water saturation of less than 2 percent and permeability of 152 md.
The reservoir sand is interpreted to be an aeolian-deposited sand encased in an evaporitic platform sequence'. lateral sand pinchout forms an effective reservoir seal and renders the Brassey field a closed system. The sand is predominately quartz with minor amounts of chert, feldspar, sulfate and dolomite grains.