Measurements of Clay-Bound Water and Total Porosity by Magnetic Resonance Logging
Authors: Prammer M.G. , Drack E.D. , Bouton J.C. , Gardner J.S. , Coates G.R. , Chandler R.N. , Miller M.N. 
Published: December 2007

Pulsed nuclear magnetic resonance (NMR) logging has until now been limited to measurements of capillary bound water and of free fluids, the sum of which is considered the "effective porosity" of rock. Clay-bound water and fluids trapped in micropores generally exhibit NMR relaxation times too fast to be detected, given the echo sampling rates and sensitivity limitations of current state-of-the-art NMR logging tools.

Core studies performed on representative clay samples confirm a linear relationship between the transverse relaxation time T2 and the water content. At 1 MHz, clays with the largest specific surface areas (smectites) exhibit T2's in the sub-millisecond range; illites have characteristic T2's of one millisecond, and kaolinites, having the smallest specific surface areas, relax with T2's in the range of ten milliseconds.

A new MRIL application was implemented based on the industry-standard MRIL logging tool. By incorporating twice the standard sampling rate and an acquisition scheme designed to boost the signal 40-noise ratio of very fast decay modes, the tool is sensitive to transverse decay components as short as 0.5 ms. During a field test campaign, the tool demonstrated the feasibility of simultaneous acquisition of effective porosity and total porosity. Neither porosity measurement requires prior knowledge of rock matrix properties.

In shaly sands, the difference between MRIL total porosity and effective porosity can be interpreted as the clay-bound water volume, relevant as the clay correction term for resistivity analysis.

Introduction

Nuclear magnetic resonance (NMR) logging is used to characterize free fluid volumes and the "effective," i. e. the clay-free, porosity of reservoir formations. Early hardware constraints limited NMR logging to free-fluid identification; whereas modern tool designs quantify both free fluids and immovable fluids as components of effective porosity.

The resistivity measurement, necessary to determine oil and water saturation levels far from the borehole, is sensitive to total porosity, which includes the clay-bound water fraction (see Fig. 1). A synthesis of NMR and resistivity requires an estimate of the wet clay volume and its contribution to conductivity. Since clays and/or clay-bound water typically affect all logging devices, a variety of clay estimators can be constructed. Being more qualitative than quantitative, they rely on local knowledge and/or the log analyst's good judgment.

Log analysis becomes more straightforward if effective porosity, clay-bound water volume and therefore total porosity are all available from only the NMR log. Significant improvements both in data sampling rate and signal quality led to a "next-generation" MRIL measurement, capable of at least partially bridging the gap between effective and total porosity. Compared to the currently commercial MRIL tool, the range of detectable transversal decay times (T2) has been extended from a minimum of 3-5 ms down to 0.5 ms. This added range includes T2 times characteristic for a variety of clays as well as for very small pore systems with clay- and silt-like pore dimensions, which have been traditionally labeled as "missed NMR porosity."

The positive identification of clay-bound water reduces the possible sources of "missed NMR porosity" to the presence of gas (low hydrogen index, long longitudinal relaxation times T1) and very light oils (also long T1 times). Provided a porosity estimate from another tool (typically density porosity) is available, the MRIL can be used as a straightforward and fast hydrocarbon detection device.

Characterizing the fast end of the T2 relaxation spectrum may improve production estimation because clay morphology can have a dramatic impact on flow properties.

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