George R. Coates

Measurements of Clay-Bound Water and Total Porosity by Magnetic Resonance Logging

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 T 2 and the water content. At 1 MHz, clays with the largest specific surface areas (smectites) exhibit T 2 s in the sub-millisecond range; illites have characteristic T 2 s of one millisecond, and kaolinites, having the smallest specific surface areas, relax with T 2 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-to-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.

Lithology-Independent Gas Detection by Gradient-NMR Logging

It has been shown that the Magnetic Resonance Imaging Log (MRIL{reg_sign}) can detect hydrocarbon gases present in the near wellbore zone (approximately 10 cm from the borehole wall). By exploiting the T{sub 1}- and diffusion-weighting capabilities of this tool, mineralogy-independent water and hydrocarbon saturations can be determined in this zone. Gas and oil saturations are used to correct the reduction in apparent NMR porosity, an effect due to reductions in hydrogen index and incomplete recovery of magnetization polarization. The proposed method exploits the predictability of density, hydrogen index, T{sub 1} and diffusivity of the non-wetting phase from its bulk properties. Gas is always and oil is often non-wetting. The MRIL{reg_sign}/C`s multi-frequency capabilities are used to integrate changes in recovery time and/or echo spacing into a single logging pass. The resultant data streams are processed in the complex time-domain to extract signal components characteristic for hydrocarbons.

Characterization of light hydrocarbon reservoirs by gradient-NMR well logging

New methods for acquiring and processing gradient NMR well log data enable signals from gas, light oil, and water to be unambiguously separated and, in many cases, quantified. These methods exploit the combined effects of T1-based and diffusion-based contrast on log response. T1 contrast, which separates the water and light hydrocarbon (oil or gas) signals, is measured by subtracting spin-echo decays measured at different, appropriately chosen wait times. Gas and oil signals are then separated based on the large contrast in the diffusion-induced T2 relaxation times for gas vs. liquid. Practical application of these principles is illustrated with new log examples that also highlight the advantages of NMR over traditional logging methods for detecting and typing light hydrocarbons, especially in mineralogically complex rocks.

Petrophysical Implications of Laboratory NMR and Petrographical Investigation on a Shaly Sand Core

Nuclear magnetic resonance (NMR) measurements on rocks directly respond to the hydrogen content of the fluids in the pore space. Thus, besides the routine application of NMR in measuring the producible water- and hydrocarbon-filled porosity in the rocks, NMR technology has the potential to characterize the surficial clay-bound water. The exact nature of NMR transverse-relaxation T 2 spectra obtainable from the clay-bound water has been discussed in recent studies conducted on both pure clays and reservoir rocks. The paper presents the results of a laboratory investigation on systematically sampled shaly sand rocks. We explore the relationship between the T 2 distribution as a function of the rock texture, pore size distribution, clay mineralogy, and clay morphology. A 10-ft conventional core, cut in water-bearing, clay-rich sandstone, was sampled for this study. Mineralogy, texture, pore size distributions, and clay morphology were analyzed by X-ray diffraction (XRD), thin section petrography, scanning-electron microscopy (SEM), and petrographic image analysis (PIA). In addition, fully polarized NMR signals were acquired from brine-saturated and desaturated samples with inter-echo spacings (T e ) of 0.3 and 0.6 millisecond. NMR porosity, computed by summing the signal in the T 2 spectra, match favorably with the helium porosity, indicating that the full spectrum of pore sizes is included in the NMR porosity. The NMR T 2 spectra obtained from brine-saturated samples show three distinct T 2 classes: 1 to 3, 5 to 20, and 30 to 200 milliseconds. These appear from core properties to be controlled by pore-size changes, which are primarily influenced by the changes in clay type and morphology. The <5-millisecond components likely correspond with <5-μm sized pores typically trapped within the flakes of detrital illite. Features in the 5- to 20-millisecond range are also likely associated with the authigenic aggregates of kaolinite and chlorite that enclose 5- to 30-μm sized pores. T 2 spectra obtained from desaturated rocks support the match of the specific features in the T 2 spectra to the clays present in the rocks. Finally, the water in the relatively larger pores (30 to 150 μm) is associated with the signals at about 30 milliseconds and greater.

A real-time well site log analysis application using MRI logs

With any complex logging tool, there exists a need for a qualitative and reasonable analysis, run in real-time at the well site that both provides a preliminary interpretation of the data and also serves as a means for enhancing quality control. Such an application has been developed to provide a quick-look analysis of data from NUMARs MRIL®-Prime tool. The application takes advantage of the Prime tools ability to simultaneously acquire data from multiple nuclear magnetic resonance (NMR) activations for purposes of fluid identification. The application is based on several well-established NMR fluid-characterization methods. These methods compute total porosity, effective porosity, bulk volume irreducible, flushed-zone fluid volume, and fluid type in a variety of reservoirs. The methods used in quick-look analyses include the Differential Spectral Method (DSM ), Time Domain Analysis (TDA ), Shifted Spectrum Method (SSM ), Enhanced Diffusion Method (EDM ), and the Total Porosity Method (TPM ). Accomplishing these objectives, i.e., computation of total porosity, effective porosity, etc., in a real-time environment requires consideration of the wellsite engineers level of experience and responsibilities. In addition, the dynamics of the real-time wellsite analysis presents several competing and important challenges: acquiring quality measurements, working with limited data and information, and providing results in a timely and efficient manner. Thus, a real-time analysis should be robust and consistent with any additional analysis done in the post-acquisition environment. A set of procedures has been developed to guide the process so that reasonable interpretations can be obtained over a broad range of operating conditions. To simplify the engineers tasks, the analysis procedure is automatically driven by the type of NMR data being obtained. Thus, meaningful results can be obtained without increasing the burden on the field engineer. This paper briefly describes the techniques that have been developed and demonstrates their application through field examples.