Radioactive Tracer SurveyingA Comprehensive Report
Authors: Kelldorf W.F.N. 
Published: December 2007

Presented here is a discussion of the many factors involved in planning and Presented here is a discussion of the many factors involved in planning and obtaining accurate tracer surveys, as well as brief descriptions of radioisotopes, gamma-ray detectors, tracer ejector tools, interpretation methods, and application in the fieldparticularly the Wasson San Andres field of West Texas.

Introduction

Radioactive tracer surveys are primarily run in production and injection wells to investigate dynamic production and injection wells to investigate dynamic conditions of fluid flow in and around a wellbore,. The greatest number of radioactive tracer surveys are run in water injection wells in secondary recovery or pressure maintenance projects as a part of a periodic pressure maintenance projects as a part of a periodic surveillance program. The surveys are run routinely to determine if the injection profile is reflecting the programmed waterflood. In producing wells, programmed waterflood. In producing wells, radioactive tracer surveys are run to locate water influx, channeling, or communication between perforations.

Properties and Selection of Radioisotopes Properties and Selection of Radioisotopes Radioisotopes are classified in three categories with which the potential user of radioactive tracer surveys must be familiar: half-life value, millicurie strength, and energy level. Half-life is the length of time required for any given number of atoms to lose half their measurable intensity. Half-lives range from fractions of seconds to billions of years. The radioisotopes used in tracer surveying usually have a half-life of 8 days to 5 years. Fig. 1 compares radioisotopes and their characteristics. The millicurie strength or specific activity level of a radioisotope denotes the number of disintegations per second that are available for the gamma-ray per second that are available for the gamma-ray detector to measure (a curie being 3.70 × 10(10) radioactive disintegrations per second). For tracer surveys inside the wellbore, a radioisotope of 5 millicurie strength is sufficient. The kinetic energy level of radioisotopes is expressed in units of millions of electron volts (mev). Nearly all energies fall in the range of 0.1 to 10 mev. The higher the mev level, the greater the intensity of the gamma-ray emission per millicurie. An important property of radioisotopes is their ability to penetrate or be absorbed by most materials. The penetration qualities of gamma rays is in proportion to the specific activity or mev level of the proportion to the specific activity or mev level of the radioisotope. The ability of materials to absorb gamma rays varies and is expressed as the material's half-value thickness (HVT). Fig. 1 relates the HVT of materials common to the oil industry. HVT is the thickness of a material required to reduce the incident gamma-ray beam to half its intensity. The more electrons in the composition of the material being used to reduce an incident gamma ray, the more radiation will be stopped. Two half-value thicknesses will reduce the beam of radiation to one-fourth of its original strength. The primary concerns in the radioisotope selection are well conditions and the characteristics of the radioisotope. Because of the many aspects involved in the selection of a suitable radioisotope, there is no one material that can be used in every situation. The radioisotope selected must be completely soluble in the injection fluid. Interpretation of any surface recording will be extremely difficult or impossible if the radioisotope and the injection fluid are not compatible.

JPT

P. 661