Schlumberger

Technical Paper: Core and log NMR measurements indicate reservoir rock is altered by OBM filtrate

Society: SPWLA
Paper Number: 2004cc
Presentation Date: 2004
 

Abstract

A core-to-log NMR calibration program for a Gulf of Mexico deep-water reservoir indicates the near wellbore rock wettability is intermediate-wet to oil-wet with enhanced relaxation that results in significant internal gradients (150 to 200 Gauss/cm). This affects the validity of certain aspects of NMR well log interpretation because the response is usually based on the assumption that the formation is water-wet and the magnetic field gradient is equal to that designed for the logging tool.

NMR logs were obtained on nine wells including the cored well and logging-while-drilling (LWD) NMR on an offset well followed by about a week later by wireline NMR log. LWD NMR wiper pass logs run 2.5 days after drilling indicate a longer relaxing T2 peak than the wireline log run 7 days after drilling. These observations are consistent with the OBMF filtrate invasion of the formation causing enhanced relaxation of the OBMF by wettability alteration and paramagnetic particle invasion.

Fresh state and OBMF at Swi (connate water) core plug saturation states have NMR T2 distributions at reservoir confining stress and temperature similar to the wireline log NMR over the same depth intervals. Cores at Swi saturated with OBMF have oil relaxation rates much faster than the bulk OBMF relaxation rate and the OBMF T2 mode in the cores does not vary significantly with temperature. Both of these observations indicate that the main mechanisms for oil relaxation are surface relaxation and internal gradients and indicating oil is wetting some portion of the rock surface. The T1 and T2 distributions of the OBMF depend on whether the whole mud is pressed or a supernatant is filtered. The filtered OBMF was found to contain 0.08 micron paramagnetic particles.

Diffusion editing and CPMG T2 distributions with multiple echo spacings indicate high internal gradients, in the range of 50 to 100 G/cm for extracted plugs and over 150 G/cm for fresh-state plugs. Thin sections and SEM photomicrographs and XRD show that the rocks often contain "trains" of heavy minerals (iron minerals) and shale laminations with little evidence of dispersed clays. The drilling mud solids had a high magnetic susceptibility and the magnetic fraction was identified to contain both iron and magnetite.

There is ample evidence the fast relaxation of OBMF is a result of diffusion relaxation with large internal gradients and surface relaxation caused by OBMF alteration. Laboratory investigation is ongoing to determine whether the alteration is caused by the OBM surfactant additives or submicon paramagnetic particulate material in the OBMF.

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