Applications of a Subsurface Solid-State Isotope Injector To Nuclear-Tracer Survey Methods

Abstract

Abstract The techniques of following the movement of dispensed isotopes in a wellbore with a gamma-ray detector have been described in previous literature. The method of determining fluid-movement profiles and velocity readings in wellbores has proved valuable for defining producing zones and thief zones, for water flooding for secondary recovery programs and for solving lost-circulation problems. The Electric isotope injector utilizes a new principle to eliminate many of the disadvantages of previous down-hole dispensers. This method injects radioisotope from a solid state directly into the surrounding fluid. The radioactivity is "fixed" on a metal "button" carried by the tool; when ready to perform an injection operation, the button is lowered to the appropriate depth in the well. The isotope is dispensed from the button by energizing the proper circuits in the logging system from the control panel in the logging service truck. The isotope remains in stable solution in the well fluid. The injector is run in combination with the Scintillometer gamma-ray logging tool for tracing the movement of the fluid. Using isotopes in solid-state form is a major improvement from the standpoint of radiation safety. The problems of radioactive-material spillage and contamination have been virtually eliminated. This new tool has been field-tested under high-temperature and high-pressure well conditions and has proved very reliable and dependable under various well conditions. The substantial saving in operating time and the more reliable control permitted provides an economical and efficient subsurface tracer-survey system. Introduction Numerous techniques have been developed in the literature regarding the dispensing and tracing of radioisotopes in subsurface operations. The solution of problems relating to the detection of fluid movement in the well and subsequently into subsurface strata can become complex by using radioisotopes as tracer material. Probably the earliest application of radioactive materials in oilfield operations was the addition of carnotite to cement slurry and the subsequent detecting of the height of the cement column through location of the gamma-ray emissions. Possibly the most important and widely developed use of radioisotopes has taken place in injectivity-profile studies. The success of injection and permeability profiles in waterflood operations has been amply demonstrated for a number of years. Oil- or water-injection profiles, made both before and after fracturing operations, are extremely beneficial in demonstrating the efficiency of fracture treatments. Additional applications for tracer surveys have been developed for locating lost-circulation zones in drilling operations, locating casing leaks, locating cement-channeling behind casing and for determining the effects of vertical permeability on fluid movement through a reservoir. Techniques have been developed and utilized for almost any application in which the detection of fluid movement is essential for sound drilling or production practices. Prior to the development of the Electric Isotope Injector, utilization of radioisotopes in either oil- or water-soluble states necessitated injecting the liquid tracer either at the wellhead or from a container lowered on a cable into the wellbore. This type of downhole dispenser depended entirely upon electro-mechanical and/or hydraulic action to evacuate the isotope from the container. Injecting radioisotopes through wellhead equipment demands a high degree of control in handling procedures, in preventing surface radioactive contamination, and in monitoring practices. Operations with older down-hole dispensers have been plagued by a number of serious problems, including radioactive-material leakage at undesired depths and the failure of tools to operate at high temperatures and high pressures. To overcome many of these problems, the McCullough Tool Co. designed and has successfully utilized the Electric Isotope Injector. JPT P. 121^