Summary
The enhanced diffusion method (EDM) exploits the diffusion contrast between oil and water separating their respective nuclear magnetic resonance (NMR) signals. Unlike standard NMR logs acquired with short interecho time (TE), measurements, EDM data are acquired using long T E accentuating diffusion. Fundamentally the EDM establishes an absolute upper bound for the T2 of water, thus any T2's greater than this limit unambiguously indicates that oil is present.
The EDM's best application is with intermediate viscosity oils (approximately 1 to 50 cp) complementing other NMR hydrocarbon-typing applications designed for lighter hydrocarbons (i.e., the differential spectrum method). While expanding the viscosity range of NMR hydrocarbon-typing applications, the EDM also provides a method by which to determine residual oil saturation (ROS), which is the main focus of this article.
The potential use of NMR as a direct indicator of hydrocarbon saturation via techniques such as the differential spectrum method (DSM) has generated significant interest in the petrophysical community in recent years. Although originally developed for applications involving natural gas, the DSM has also been used successfully in light hydrocarbon environments. However, success has been limited to the low end of the viscosity spectrum because of the T1 separation requirements between the brine and hydrocarbon phases.
The T1 separation requirement imposed on diffusion applications in higher viscosity oils can be eliminated by using the EDM, where diffusion is turned into the dominant relaxation mode for the wetting brine phase. Given that brine is more diffusive than the hydrocarbons, the longest apparent T2 from the brine phase can be made short enough to cause separation between the two phases in T2 space, thereby eliminating the need for T1 separation. Wait time manipulation can then be used to quantify hydrocarbon volumes when the two phases are separated in the T2 domain.
In this article we focus on determination of the residual oil saturation using the EDM, while also providing guidelines for job screening and acquisition parameter selection. Several case histories that are provided are used to illustrate the basic concepts and different methodologies available.
Introduction
The enhanced diffusion method is a new method developed to distinguish oil and water NMR signals in a gradient magnetic field by exploiting the diffusion contrast between the two fluids. The method is applicable for moderate oil viscosities, approximately in the ~1 to ~50 cp range.
The major objective of this article is to discuss EDM signal processing techniques for residual oil saturation, and the reader is referred to existing literature1 for a detailed discussion regarding the petrophysical concepts and related laboratory measurements of the EDM. A secondary objective is to provide guidelines that can be used to screen potential EDM applications and to determine optimal acquisition parameters.
Within the context used in this article, residual oil saturation is defined as the oil saturation in the flushed zone after drilling fluid invasion, and the terms residual and flushed zone oil saturation are used interchangeably.
Theory
The basic concept of the EDM is to turn diffusion into an effective transverse relaxation mechanism while minimizing the dominance of surface relaxation by acquiring NMR logs at long interecho times. Three different mechanisms, which operate in parallel, contribute to the overall apparent relaxation rate of water in porous media: $$1/T {2AW}=1/T {2BW}+1/T {2SW}+1/T {2DW},\eqno ({\rm 1})$$ where the subscript W stands for water, and A, B, S, D denote apparent, bulk, surface-induced, and diffusion-induced mechanisms, respectively. The surface and diffusion induced relaxation rates are given by $$1/T {2SW}=\rho {2}S/V,\eqno ({\rm 2})$$$$1/T {2DW}=((\gamma GT {E})^{2}D {0W})/12,\eqno ({\rm 3})$$ where ?2 is surface relaxivity, S/V is the surface-to-volume ratio, ? is the gyromagnetic ratio, G is the magnetic field gradient, TE is the interecho time, and D0W is the self-diffusion coefficient of water.
In standard logging modes using short TE surface relaxation dominates since (1) T2BW is very long, especially at elevated temperatures, and (2) T2BW is also very long because of the short TE values used, despite large magnetic field gradients of the logging tools.
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