Nuclear Magnetism Logging Field Results
Authors: Gamson B.W. , Stephenson T.B. , Edwards Lyman , Shane Lee 
Published: December 2007

Nuclear magnetism logging has been field tested under a wide variety of conditions using commercial equipment and in most types of formations encountered in California, Texas and Oklahoma. These field results are generally excellent. The free fluid index shows the minimum producible fluid and the thermal relaxation curves show the minimum amount of hydrocarbon available. Concrete examples demonstrate how this now logging technique can assist in the location of fluid-bearing zones and the evaluation of hydrocarbon-bearing strata. The principles involved are briefly reviewed and the significance of the dual polarization curves given. This presentation emphasizes forward results, and therefore represents money-saving factors to the oil operator.

Introduction

Recent papers describe the application of the Nuclear Magnetism Log (NML) in reservoir rocks containing high viscosity crudes and fresh connate water. Such formations exist in many places in Texas, California and elsewhere, and are being exploited commercially. This unique use of NML is predicated upon observing the fresh connate water and not recognizing the signals with very rapid decay times of the order of less than 20 millisec associated with high viscosity fluids (600 cp and greater). Thus the free fluid index (FFI) indicates that portion of the total volume occupied by formation water. The NML in conjunction with a resistivity log delineates the primary hydrocarbon beds from the water zones. This paper summarizes recent developments in NML based upon its broad use in resolving difficult logging problems other than the previously described heavy crude situation. Wells which have been logged with NML and a variety of other logs are reviewed in conjunction with completion and production histories. Examples are given from a cross-section of geological formations from Oklahoma, Southwestern Kansas, West Texas and South Texas. These cover hard and soft rock formations.

Free Fluid Index

The data obtained from a Nuclear Magnetism Log are presented as a free fluid index and the thermal relaxation time. When the FFI is recorded at a time corresponding to full polarization, that curve measures the minimum effective porosity. If full polarization is not obtained, the (FFI) can be corrected to the total (FFI), by means of a knowledge of the thermal relaxation times of the short and long components and the percentage of each component. The (FFI) is a direct positive indication of the presence of fluid within a formation based on its hydrogen density. It is defined as the percentage of the formation containing movable fluid. No physical and chemical properties of the reservoir rocks or fluids are required for the calibration of the (FFI). The only qualifications are that if excessive amounts of magnetite are present in the rocks, the FFI is zero; and the high viscosity crudes greater than 600 cp are not detected. In clean sands and limestones, the (FFI) approaches full porosity values. In dirty or shaly sands, (FFI) yields values less than the core porosity and the reduction reflects shale or silt content. Shales yield zero FFI.

Fluid Identification

At the present time, it appears that the most satisfactory thermal relaxation data for oil-water distinction according to the previously described criteria, require:

sufficient (FFI), to obtain a valid T curve: full invasion of the formation by a water base filtrate; knowledge of the mud filtrate T at depth and pressure; full polarization of the filtrate: and displacement of most of the hydrocarbon by the filtrate.

The thermal relaxation time is not measured directly, but is obtained from a series of measurements. The thermal relaxation curve is not a simple one for a multi-component system. It has been observed and reported by Gamson and associates that in oil sands the thermal relaxation curve can usually be approximated by a two-component system. The short T is the component wetting the sand grains and the long T is the bulk-mud filtrate at the formation temperature and pressure.

JPT

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