Use of radioactive tracers to determine flow profiles near injection wells.

Abstract

A tracer flow logging technique was developed in this thesis, which can be used to determine the flow paths and in-situ velocities in the near well bore region of a petroleum reservoir. The large mean free paths of gamma radiation along with its scattering properties have been exploited in achieving the goal. Two basic transport phenomena in porous media, namely, the transport of a miscible tracer and the transport of gamma radiation were studied. Three methods were used to determine the time-dependent spatial distribution of a noninteracting radioactive tracer in a porous medium. The first method uses a dispersion model to account for the mixing and spreading of the tracer. The second method uses a capacitance model to quantify dispersion and tailing of tracer in porous media. Previous controversy regarding the velocity dependence of the three parameters used in the capacitance model has been resolved. A new technique for imaging flow through porous media was developed, and was used as the third method to determine in-situ tracer concentration profiles. Neutron imaging is ideally suited for visualizing hydrogen containing fluids in porous media. The porous matrix is virtually transparent to a beam of thermal neutrons whereas hydrogen has large cross section for neutrons. A Monte Carlo method was developed to simulate photon transport in porous media. Spatial distribution of the source, determined from the three methods was provided as an input to the model. The photon energy distribution crossing the detector surface was calculated by simulating several photon histories. The photon energy distribution shifts to lower energies as the distance between the source and the detector is increased. This shift in the energy distribution was quantified using the ratio of scattered photon intensity to the full energy photon intensity. The ratio increases monotonically with the distance between the source and the detector. Excellent agreement between the model predictions and experimental data was realized for the case of a linear bench scale system. It is suggested that the ratio of scatter intensity to full peak intensity can be used to determine the location and the velocity of the tracer in the near well bore region.