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Development of environmentally friendly tracer technology for improved reservoir description (ENVITRACER)


Cyclopropane has been studied and validated as a gas tracer in the laboratory and in the field. The compound survived at reservoir like conditions and has flow behaviour similar to the present perfluorocarbon gas tracers. It is environmentally friendly. It adds to the list of environmentally friendly tracers for usage in oil exploration and recovery. It should be pointed out that none of the existing until now non-radioactive gas tracers was considered acceptable since these have been implicated as important anthropogenic contributors to global warming and long-term climate change. Hence, the benefits to the environment are indisputable. The other gas tracer candidates are not presently available in the market and have to be synthesized before they can be tested.
Recommendations for guidelines to the oil industry regarding the use of the newly developed water and gas tracer technology. While during the development of the previous generation of tracers their environmental impact was not given sufficient attention, the environmental inertness becomes now on a prerequisite for the selection of the new generation of water and gas tracers. There exist today a limited number of non-radioactive substances that are qualified as gas or water tracers for well-to-well tracer studies in oil reservoirs. The current use of several of these compounds however, is restricted due to their properties, which can pose a potential threat to environment. The environmental control authorities in several countries, following the Kyoto Protocol and OSPAR, exert pressure on the oil industry to reduce or even ban the use of some of these chemicals. The objective of this recommendations is to introduce the enlarged, as resulted from ENVITRACER, portfolio of environmentally acceptable gas and water tracers for reservoir applications. In this manner, the satisfaction is attained of both the requirements concerning the chemical compoundsof being stable in the reservoir over a long time period and at the same time being degradable to avoid bio-accumulation or irreversible accumulation in the atmosphere. Furthermore, the costs of using these compounds in the field should be low in order to make their use economically feasible and attractive.
Development of improved detection and analytical methods both for the water tracer candidates and for gas tracer candidates. Analytical methods based on GC/MS have been developed and can at present determine the water and gas tracer candidates in reservoir matrices down to the low ppb concentration range. These methods have been used to analyse the tracer candidates during the stability tests and flow experiments. These methods allow for a cost effective way towards the attainment of commercially attractive compounds by reducing the manufacturing costs and the detection limits. Investigations on the synthesis of the gas and water tracer candidates were thoroughly performed. Suitable synthesis pathways have been compared and the necessary preparations for the small-scale synthesis were accomplished. Although the deuterated organic acids could be found in the market, their high price makes them uneconomical to be used as reservoir gas tracers. Their price is high due to their very high purity (all the H-atoms have been replaced by D-atoms) something that is not required for their use in oil reservoirs. On the contrary, the partial deuteration can be turned into advantage by creating several tracers from the same chemical compound but with different degree of deuteration, resulting in a considerable cost reduction. In addition, the gas tracers (D-alkanes) can be derived easily by decarboxylation from the deuterated organic acids reducing further the cost per tracer. An additional measure to reduce the cost effectiveness of a tracer program is to minimize the amount of tracer that has to be injected into the reservoirs. This can be achieved by reducing the detection limit of the compounds via the development of new extraction and pre-concentration techniques and the upgrade of HPLC, MS and GC techniques. Besides the further development of the analytical method for the tracer detection of the deuterated fatty acids in aqueous reservoir fluids and testing of their ecotoxicological properties, the main focus was placed on the production of rather large amounts of the selected water tracer candidate (in kg). On the other hand, cyclopropane was purchased and not produced in the laboratory.
An electronic tool for studying the tracer flow behaviour has been constructed for water tracers. The development steps of such tool were the following: - Stochastic Reconstruction of the Porous Domains; - Calculation of Permeability, Formation Factor, Dispersivity including Partitioning Aspects and Tracer Interfacial Transport; and - Interpretation of Tracer-tests. The ability to simulate the tracer transport in the core and predicting the experimentally measured tracer concentration profile at the outlet, allows for new insights in interpreting the measured data and offers valuable information on the capability and adequacy of a tracing substance to qualify as a field tracer for the oil industry. The features of the tool enable the tracer developers to perform comparisons with experimental measurements and support essentially the efforts for the interpretation of the observed laboratory tracer concentration values. Therefore, the tool can contribute substantially to the cost-effective development and testing of tracers by reducing the number of required experiments and enhancing the understanding of tracer performance in the field.
New environemtally friendly water tracers have been developed by IFE. The new chemical compounds are non-radiactive and have been tested and validated in the laboratory as well as in the field. In total, five new tracers passed the stability tests and survived under reservoir conditions at 200°C. Seven tracer candidates passed the tests at 150 °C. All investigated fatty acids passed the dynamic flow experiments. In consequence, a new suite of possible tracers for water flow has been qualified. Most importantly, the compounds have been selected so that they do not pose any threat to the environment. It is well known, how useful the use of tracer technology is in order to reducing time and costs, extending production and optimizing oil recovery. Until now, only three water non-radioactive tracers were considered as environmentally safe for use in the North Sea oilfields. The development of the new water tracers is a solution to the tracer shortage problem that the oil companies have been facing.
Successful planning and performance of field experiments were carried out offshore (Statoil fields). All safety aspects had to be taken care of. The qualified gas and water tracers were injected in appropriate amounts and gas samples were regularly taken and analysed. One representative compound for each of the two groups of gas and water tracers was selected for field tests in Statoil offshore wells. Cyclopropane was chosen as the test candidate for the cycloalkane series of gas tracer compounds, whereas deuterated valeric acid (pentanoic acid) was selected for water injection field testing. The injected amounts of each tracer were estimated according to a standard Statoil procedure based on calculation of the volume available for dilution of the tracer between the injector and the producer. The gas injection was performed on September 7th 2006 at Statfjord field while the water tracer injection was carried out on June 1st, 2006 at Gullfaks. At the time of the preparation of this report, breakthrough of the tracers had not been observed in either field (Statfjord or Gullfaks). Since the methods for the analysis of the water and gas tracers are ready for implementation, the analyses of the tracer samples can start as soon as the samples arrive at the laboratory. The major tracer responses for both tracers are expected to pass the selected production wells within half a year (end of 2006). The partners will continue the work following the breakthrough of the tracers after the official end date of the project at their own expences and they intend to issue a complimentary report on the final conclusions.