Carbon dioxide is often injected into oil-producing formations as a miscible flood drive to enhance removal of oil. Although such a carbon dioxide flood can greatly increase the amount of oil which can be recovered from a formation, it is a relatively expensive process. Carbon dioxide is usually recovered from other formations, or removed from natural gases. The carbon dioxide must be compressed and transported to the field into which it is to be injected. The carbon dioxide must therefore be utilized efficiently to result in an economical flooding operation. The efficiency with which the carbon dioxide is being utilized is indicated by the profile in which the carbon dioxide enters the formation. Fingering or channeling of the carbon dioxide, which indicate inefficiency, may be inferred when carbon dioxide enters a formation from a wellbore through some casing perforations disproportionally. These problems can often be remedied when they are detected. These problems are typically detected by determination of the velocity profile within the injection well borehole. Changes in wellbore velocities, along with the wellbore pressure profile and diameter can be utilized to determine the amount of carbon dioxide entering the formation from perforations at different locations.
A common well logging technique to determine the velocity profile within a carbon dioxide injection well involves placing a plurality of radiation detectors within the wellbore, injecting a radioactive isotope, and measuring the time required for the radioactive isotope to travel from the first radiation detector to subsequent radiation detectors. Iodine-131 has been used almost universally as the radioactive source in this well logging. Iodine-131 is readily available as a byproduct from reprocessing of nuclear fuels. But iodine concentrates in thyroid glands of humans, so acceptable human exposure levels are extremely low. The half-life of iodine-131 is also relatively long, about eight days. An isotope which has a shorter half-life would considerably lessen the exposure of personnel to radiation from equipment and from any flow of injected fluids back to the surface.
U.S. Pat. No. 4,421,982 discloses the use of bromine-82 as a preferred isotope for radioactive well logging, along with a downhole injector apparatus. Bromine-82 has a half-life of about one and a half days. This patent discloses using bromide in a granular form. The granular radioactive tracers are utilized to determine the location and extent of fractures between wellbores. For velocity profiling in a carbon dioxide injection well, a tracer component which is miscible in the carbon dioxide is preferred. Being miscible in the carbon dioxide would ensure that the tracer travels at the same velocity as the carbon dioxide. Removal of residues of the radioactive tracer from the wellhead and downhole equipment is also enhanced by the tracer being miscible with the injected carbon dioxide.
It is also desirable to have a tracer which is miscible with the injected carbon dioxide to ensure that the tracer does not plate out onto wellhead and tracer injection equipment. Applicant has found that ionic bromine-82, such as ammonium bromide, will precipitate from supercritical carbon dioxide and plate out on the well logging and wellhead equipment. Although useful velocity profile information can be obtained, injection and logging equipment are contaminated with precipitated bromine-82 and storage in isolation until an acceptable level of radiation is obtained is required.
It is therefore an object of the present invention to provide a method to determine velocities within a carbon dioxide injection well utilizing a radioactive tracer wherein the tracer is miscible in the carbon dioxide, wherein the radioactive isotope has a relatively short half-life, and wherein the tracer does not tend to plate out onto the wellhead and logging equipment.