111.1. PREPARATION FOR RADIATION SAFETY OF RADIOTRACER TESTS

111.1.1. Introduction and general comments

Tracer techniques have wide application in petroleum exploration and production. For several decades, well-to-well tests have been used to study the movement of injected fluids through reservoirs. When properly carried out, such investigations can render information on the reservoir structure and fluid flow behaviour that could not have been obtained by other means. HTO is regarded as the standard reference tracer for water. With regard to safety, HTO is the most favourable radioactive water tracer generally available. However, there are other applicable radiolabelled water tracers, some of which are labelled with pure beta emitters such as 14C and 35S and some with both beta and gamma emitters such as 58Co, 60Co and 131I, while others emit only gamma radiation (electron capture decay) such as 57Co and 125I.

This appendix attempts to cover both the more general parts of the safety consideration as well as aspects that are nuclide specific for the individual radionuclide. In order to avoid an overly complicated treatment, tritium is considered to represent the beta radiolabels and 60Co to represent the gamma emitting radiolabels.

Radioactive tracer injection can be carried out in a safe manner with little impact on the environment. A typical safety report has to be prepared for the approval of the radiation protection authority and for the acceptance of end user management. The report contains:

• The design of the tracer experiment (calculation of tracer quantities, description of practical procedures for tracer preparation, transport and injection, sampling procedures, etc.);

• The description of safety precautions during handling and treatment of HTO during the whole operation.

To perform tracer injection in a safe manner, the injection equipment should have a sound design and meet pressure and temperature requirements. It should be properly tested before injection and operated by well-trained personnel. If these requirements are satisfied, an injection of radioactive and non-radioactive tracers can be carried out with very little risk.

Experience has shown that typically 370-3700 GBq (10-100 Ci) of HTO is injected into an injection well for interwell investigation in oilfields. A typical quantity for a 14C or 35S labelled tracer is 3.7-37 GBq (0.1-1 Ci) and the same for a gamma emitting tracer. A common design for an injection device for beta emitting tracers is presented in Fig. 7. This equipment can be operated in a bypass mode to the main injection line or by application of a high pressure pump. The pneumatically operated Maximator pump, which can be seen in Fig. 12, can deliver up to 10 L/min at a working pressure of 200 bar; lower at higher pressures. The pump is normally run at a rate of 5 L/min. The available pneumatic pressure for operation of the pump should be at 7-10 bar.