Abstract
Heavy oil reservoirs are expected to have viscous anisotropy, both vertical and lateral, with viscosity value varying in the range of several orders of magnitude. Today, there is no any reliable technique to define viscosity anisotropy and to estimate oil viscosity in-situ. The existing methods cannot evaluate the viscosity of crude oil without its lifting to the surface. The solution is the Nuclear Magnetic Resonance (NMR) process which is capable of both reservoir characterization and formation fluid properties analysis.
Oil samples in such a case should be collected from reservoir zones that have not been exposed to heat or chemicals. Such samples can be derived, particularly, from oil-saturated core material recovered while drilling wells in newly discovered fields or in untreated reservoir areas. Centrifuge method (HSC) turned out to be the most successful and less laborious. The obtained by HSC samples most accurately reflect the intrinsic properties of potentially recoverable fluid.
Methods whisch can determine mobile oil viscosity from relaxation time is existing. However, all the earlier empirical equations fail to determine oil viscosity higher than 1000 MPa•s. It has been shown that the logarithmic mean spin-spin relaxation time demonstrates the best correlation with oil viscosity, which in combination with a calculated time cutoff, yields over 80% accuracy in viscosity estimation. The developed technique determines oil viscosity distribution from high-field NMR-logging data after calibrating correlation factors.
Considering oil viscosity anisotropy yields a significant economic benefit including the reduced costs of steam injection and increased cumulative oil production.
