Neutron radiography: A technique to support reservoir analysis

Dynamic neutron radiography is a method to image fluid flow in porous media, based on the tendency of neutrons to be preferentially attenuated by hydrogen. Fast neutrons with energies of several MeV can be produced by particle accelerators, and are suitable for detecting fluids in rock samples of 0.15 to 0.30 m (15 to 30 cm) thick. Slow (thermal) neutrons with energies of about 0.03 eV are produced by nuclear reactors, and are ideal for imaging fluid flow in rock samples in the order of 0.01 m (1 cm) thick.

Thermal neutrons are attenuated by an exponential law, which relates the incident intensity of a neutron beam (I0) to the transmitted neutron beam (I): I = I0 exp(−ρμh), where ρ is the bulk density of the rock in the neutron path, μ is the bulk neutron attenuation coefficient in the neutron path, and h is the thickness of the rock in the neutron path. The exponent term (ρμh) can be linearly related to the components making up the rock:

ρμh = (ρ1μ1V1 + ρ2μ2V2 + ρmμmVm)h, where V1; is the volume fraction of fluid 1, V2 is the volume fraction of fluid 2, Vm is the volume fraction of the rock matrix, the subscript “m” refers to the rock matrix, and “1” and “2” refer to fluid 1 and fluid 2, respectively.

Knowing that V1 + V2 = ø, which is the rock’s porosity, an estimate of the relative fluid saturations can be made. These are the principal economic quantities required for either a hydrogeological or petroleum reservoir. Examples of water infiltration and petroleum imbibition into the Visingsö Sandstone, Sweden, are provided to illustrate the application of the technique.