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A New Method of Fluid Identification from Nuclear Magnetic Resonance Logs

Abstract

The Mississippian Formation that is productive through much of Kansas and Oklahoma is a difficult, altered lime reservoir with a significant amount of secondary porosity. Production generally is from horizontal well bores and has a very high water cut. Decisions regarding the best interval of the reservoir for completion in vertical wells and for horizontal drilling in horizontal wells is very problematic due to the rapid alteration and compartmentalization of the reservoir. Like many carbonate reservoirs worldwide, water segments may be above, below, or in some cases, in between hydrocarbon traps. In many cases, the water portions of the reservoir and the hydrocarbon portion have the same resistivity measurements. Any technique to accurately characterize the reservoir for fluid type provides tremendous value in drilling and completing these types of wells. Thus, Nuclear Magnetic Resonance (NMR) logs were added to the evaluation program to obtain this type of evaluation. Fluid type, independent of resistivity, can sometimes be accurately determined using the polarization (T1) measurement. This measurement responds to the type of molecule being manipulated. In many reservoirs, the fluid type can be accurately forecast using only this measurement. Reservoirs like the Mississippian present a situation that cannot be simply solved. The T1 measurement can be distorted by the rapid alteration of the reservoir rock. These alterations may be primary porosity changes from deposition or compaction. They could also be related to secondary porosity development after deposition. This paper presents an evolving technique that incorporates both the T1 measurement and the relaxation (T2) measurement to accurately determine the type of molecules in the reservoir pore space. Numerous examples of characterization will be presented. When possible, production results will also be presented. Comparison of the results forecast vs. those actually achieved, indicate a positive future for this new approach to fluid type, independent of any other logging or coring result.