The paper presents the results of laboratory nuclear magnetic resonance (NMR) studies of oil-saturated core samples of the Sheshminskian horizon of heavy oil fields in the Republic of Tatarstan. The analysis of the results showed that T2 spectra of oil recovered via centrifugation from the pre-heated core and oil remaining in the core are substantially different from each other. Magnetic relaxation rate of residual oil is much faster, and thus its viscosity is significantly higher than that of the recovered oil. The difference can be up to ten times. The spectrum of relaxation times T2 in the original oil-saturated core integrates information about all the fluid contained in the rock.
The results and findings underscore the ability of NMR logging tools to characterize the entire oil fluid in the rock in situ.
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.
This paper presents the results of laboratory and nuclear magnetic resonances researches of seventy-seven crude oil samples from the Devonian, Carboniferous and Permian formations of Tatarstan oil fields with a wide range of viscosity varying from several cps to 60,000 cps.
The results can be used to estimate the viscosity of the oil, including in situ viscosity. It has been found that the NMR relaxation characteristics of the tested samples presented in the form of T2 time distribution spectra confirm that crude oil is multicomponent. Numerical characteristics resulting from the analysis of the spectrum of spin-spin relaxation time T2 have been determined, which show good correlation with the viscosity of the hydrocarbon fluid. The paper presents the obtained experimental correlations between the rheological properties and NMR characteristics for oil samples studied.
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