Borehole Nuclear Magnetic Resonance: Experience and Reservoir Applications in West Kuwait Carbonate Reservoirs

Abstract

Abstract Recently introduced nuclear magnetic resonance well logging has been tested and evaluated in West Kuwait and is now being used to better define effective reservoir porosity in the most complicated reservoirs and under difficult conditions, including oil-base drilling muds. A number of West Kuwait carbonate oil reservoirs are difficult to evaluate because of the complexities of their pore systems. Borehole imagery has helped to define fractures and faults in wells and, thus remove a major uncertainity in formation evaluation. Standard open hole well logs provide porosity and saturation which are often not enough in evaluating these complex carbonate reservoirs with high-resistivity. In fact, apparent porosity and saturation may be misleading as to what fluids and at what rates will flow from a given zone. Low porosity layers can have the highest permeability within a reservoir and similarly the highest porosity layer may be comprised of micropores and be impermeable. Moreover, zones which correctly calculate high water saturation with standard well logs may flow oil with no water while tar mats may be misleadingly interpreted as an oil zone. Borehole nuclear magnetic resonance helps to define the potential of the most complex carbonate reservoirs in West Kuwait. Borehole NMR, in addition to providing porosity independent of lithology, can be related to pore size once hydrocarbon effects are accounted for. Understanding pore size distribution within a reservoir helps to define effective porosity, identifies the presence of microporosity and quantifies irreducible water saturation when used with standard well log evaluation. The definition of effective porosity and pore size distribution allow better determination of reserves. Initial estimates of permeability without any modeling of specific reservoir rock types have related well to the independent assessment of permeability by acoustic technique and wireline formation testing. Our assessment of borehole nuclear magnetic resonance has been very encouraging. It has been possible to use borehole NMR to better identify reservoir facies, predict well productivity and select completion zones. Many other reservoir and geological applications are being examined at present time.