Method of measuring horizontal fluid flow behind casing in subsurface formations with sequential logging for interfering isotope compensation and increased measurement accuracy

摘要

Fluid in permeable earth formations adjacent well casing is irradiated with neutrons to form radioactive tracer isotopes in the chemical elements comprising the fluid, typically sodium 24 in saline subsurface formation water, and in other elements in the casing and formation each of which decays by emission of gamma rays. By measuring the rate of decay of the radioactive tracer isotope, a measure of horizontal fluid flow in the formation is obtained. The elements in the casing and formation have been found to also respond to the neutron irradiation by forming radioactive isotopes, such as calcium 49 in the formation and manganese 56 in the steel casing, which emit gamma rays which interfere with the gamma radiation measurements of the trace element indicative of water flow. A method of measuring horizontal fluid flow while compensating for the presence of gamma rays from elements in the casing and formation and also increasing the accuracy of the measured linear fluid flow velocity is disclosed.