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Well to well tracer tests
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. Active tracers interact with the other fluids in the system or with the rock surface. 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.
Practical applications of tracers
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 1 pointed out six areas in which tracers could be used as a tool to improve the reservoir description.
- Volumetric sweep. The volume of fluid injected at an injection well until breakthrough of the traced fluid at an offset producer is a measure of the volumetric sweep efficiency between that pair of wells. Very small volumes injected before breakthrough [relative to the interwell pore volume (PV)] would indicate the existence of an interwell open fracture (or a very thin high-permeability stringer) and would give an idea of the volume of that channel. Knowledge of channels is important to the sizing of remedial treatment.
- Identification of offending injectors. Problem injection wells can be identified by associating the breakthrough of a specific tracer to its point of injection. At this well, a remedial treatment to seal a channel normally would be applied.
- Directional flow trends. When fluids are injected in a regular pattern (five-spot, nine-spot, line drive, etc.) and the fluids injected at each well tagged with a different tracer, directional flow trends will be obvious from the repeated early tracer breakthrough at producers in a preferential direction from the injectors. Where directional flow trends are prevalent, the interwell sweep efficiency often can be improved by altering the injection pattern and/or the injection and withdrawal rates at selected wells.
- Delineation of flow barriers. Faults with large displacement along the fault plane and permeability pinchouts can represent barriers to the flow of fluids perpendicular to their axis. Normally, such barriers are detected by bottomhole pressure buildup surveys run in nearby wells. However, the course of these barriers can be delineated further from the production well’s response (or lack of it) to traced water injection at an array of wells surrounding the producer.
- Relative velocities of injected fluids. When different fluids are injected simultaneously, alternately, or sequentially in the same well with each fluid tagged with a different nonadsorbing tracer, the relative velocities of these fluids can be measured from the individual-tracer arrival time at offset producers. For example, assume that traced solvents and traced water are injected alternately in the same well. The early arrival of one of the traced fluids at the producing well would indicate that the early arriving fluid had contacted less of the reservoir than the slower fluid. This shows a need to alter one of the fluid injection cycles to achieve more uniform sweep of the reservoir. Similarly, in a micellar flood in which water is injected sequentially, the overrunning or fingering of one injected fluid through another points out the need for better fluid-mobility control to achieve more uniform sweep by the various injected fluids.
- Evaluations of sweep-improvement treatments. Remedial treatments to correct sweep problems can be evaluated by comparing the before-and-after-treatment interwell volumetric sweep as determined by tracing.
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. Adequate data presentation and simple hand calculation can give further knowledge about the flow behavior in the reservoir. More quantitative information can be obtained by fitting response curves obtained from numerical simulation to the observed response curves. Additional information also can be obtained by applying analytical procedures on the basis of generic or simplified reservoir models.
Field tracers, whether chemical or radioactive, are currently the only feasible, direct means of tracking the movement of injected fluids in a reservoir. In many fields, this information has been crucial for improving injection and production programs. Investments in new wells and equipment for injection programs are large and decisions should be based on the best possible data. A tracer test is a cost-efficient method to obtain important data that allow the analysis of injection and production options.
Types of tracers available
A passive tracer that labels gas or water in a well-to-well tracer test must fulfill the following criteria. It must have several characteristics:
- Have a very low detection limit
- Be stable under reservoir conditions
- Follow the phase that is being tagged and have a minimal partitioning into other phases
- Have no adsorption to rock material
- Have minimal environmental consequences
The tracers discussed in the linked articles have properties that make them suitable for application in a well-to-well test in which dilution volumes are large. For small fields in which the requirement with respect to dilution is less important, other tracers can be applied.
Health, safety, and environment constraints
The regulations for the use of radioactive and chemical substances vary from country to country. The application of radioactivity is generally restricted, and it is important to plan the tracer test with the actual regulations in mind.
Radioactive tracers are either gamma emitters, beta emitters, or both. Tritium is a low-energy beta emitter. A sheet of paper stops this beta radiation, and it will not cause any harm to humans as long as it is kept outside the body. If spillage occurs, operators can be exposed to radiation because of direct contact with skin or because of evaporation and inhalation. Procedures must be followed carefully to ensure a safe operation.
Radioactive tracers such as 22Na and 131I emit gamma radiation. This radiation penetrates steel walls, which means that an operator can be exposed to this radiation. The tracers are transported in lead containers and special handling procedures are needed to reduce the radiation exposure to a minimum. The activity, measured in becqeurels, is normally 2 to 3 orders of magnitude less than the activity applied for tritium-labeled compounds.
Gas tracers like the perfluorocarbons (PFCs) have a high global-warming potential. Most of the back-produced gas will be burned in the end, and the tracers will decompose; therefore, the amount of PFCs entering the atmosphere is low. The tracers, however, may be banned in the future, and research activities are ongoing to identify new gas tracers that are more environmentally friendly.
Noteworthy papers in OnePetro
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read