Development of Combined Electron Paramagnetic Resonance and Nuclear Magnetic Resonance Technologies for Reservoir Characterization, 15-9083

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Principal Investigators
Qingwen Ni
J. Derwin King
Jorge O. Parra

Inclusive Dates: 07/01/98 - 12/31/99

Background - Accurate oil reservoir description and simulation are dependent on measures of properties of the formation rock such as porosity, permeability, wettability, and fluid saturation. Until recently, core samples were the only source of permeability data measured on a sufficiently small scale to provide critical reservoir heterogeneity data. Permeability is usually derived from core-based permeability-porosity cross-plots, but such an approach is flawed, as the porosity-permeability correlations are often poor, and core data may be sparse and expensive to obtain. Nuclear magnetic resonance (NMR) well logging tools are currently used by the oil industry to determine, in situ, the porosity and permeability of fluid-rocks. Particularly for permeability determination, NMR is better than other well logging methods because the NMR signal relaxation time (T1 or T2) can be used to provide information about pore size distributions. NMR also provides a measure of the total hydrogen  (oil, water, and gas) based in the rocks. SwRI studies show that another magnetic resonance technique - electron paramagnetic resonance (EPR) - produces a detectable signal from organic-free radicals in crude oils but not from water nor from gas. The amplitudes of these EPR signals are proportional to the amount of oil inside the rock and should, therefore, directly measure the oil fraction in fluid-rocks. When used with NMR, this method allows the components of water and oil in the rocks (and possibly the gas) to be separately determined. Thus, EPR should provide a direct measure of the oil fraction in the formation and, in combination with NMR relaxation data, should provide more accurate assessment of the rock porosity, permeabilities and water fraction.

Approach - Accomplishment of the project included the following tasks: 1) core studies to establish the capability for quantitative determination of the porosity and permeability of porous rocks by NMR method at SwRI; 2) laboratory-improved analysis of magnetic resonance (MR) bore hole logging data provided by the oil company after adjusting different parameters to obtain the pore size distribution and the permeability; 3) MR measurements of core samples from the same bore hole as the data in item 2; 4) comparison on the index of porosity and permeability obtained from the borehole logging data with the core index obtained from SwRI laboratory tests and optimization of processing variables for the bore hole data; 5) establish low-field EPR apparatus for core measurements and conceptually design an MR sensor for field application.

Accomplishments - Carbonate core samples and mudstone samples have been received from different oil fields. The NMR measurements were performed using an SwRI-built NMR system, and a proton frequency of approximately 2.3 megahertz was set up for this program. These samples have been measured to determine the NMR properties, and the differences between the carbonate and the mudstone have been characterized. The porosity was determined by NMR spin density and spin-echo measurements and compared with traditional weighing and the Boyle's Law methods. Additionally, the NMR technique yielded valuable petrophysical information from the well logs of porous media, such as the porosity, total fluid content, producible fluid, permeability, and pore size distribution of the reservoir rock. Most of these properties are determined from the NMR relaxation time (T1 or T2) distributions. For a water-saturated rock, the relaxation time distribution of the NMR signal reflects the pore-size distribution with the longer relaxation times corresponding to the larger pores. A computational method for extraction of the T2 distribution has been developed as part of the project effort. The obtained NMR core data from the different oil fields and the reprocessed results by SwRI have also been used in the study. After calibrations, the valuable petrophysical information, such as permeability, and pore size distribution have been obtained. Particularly, the correlations of the premeability of the rocks as determined by traditional methods and by NMR techniques have been studied and compared. In addition, EPR measurements on cleared and partial oil-saturated core samples were performed by using a standard laboratory EPR instrument, and the basic calibrations for these samples have been made. An experimental EPR laboratory system was set up for a field EPR frequency of about 1,270 megahertz measurements up to 1.0-inch (2.54 cm) diameter for core samples. The theoretical models for porous size and permeability distribution have been validated using NMR and EPR data as well as NMR well logs. The new relationships based on MR measurements have provided processing algorithm to better extract permeability from existing NMR well logs data.

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