--> Resistivity not Required: Fluid Identification From Nuclear Magnetic Resonance Logs
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Resistivity not Required: Fluid Identification From Nuclear Previous HitMagneticNext Hit Resonance Logs

Abstract

The Mississippian Formation, which is productive through much of Kansas and Oklahoma, is a difficult, altered lime reservoir with a significant amount of secondary porosity. Production generally has a very high water cut. The reservoir exhibits much secondary porosity development in the form of vugs and fractures. Water-productive portions of the reservoir may be separated from hydrocarbons by reservoir stratification and segmentation or through direct contact. Both the water and the hydrocarbon portions of the reservoir often have the same resistivity measurements. Archie calculations of water saturation are not always helpful. Closely monitoring cuttings from drilling and performing drill-stem tests at shows is the most common evaluation technique. This quickly becomes very costly. Thus, nuclear Previous HitmagneticNext Hit-resonance (NMR) logs were added to the evaluation program to evaluate fluid type. Previous HitPolarizationTop (T1) measurements from NMR have been the most applied approach to this difficult fluid identification. Because this measurement responds to the type of molecule being manipulated, in simple 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 presence of secondary porosity in the form of fractures or vugs can distort the T1 measurement. These alterations may be primary porosity changes from deposition or compaction or secondary porosity development after deposition. Either condition could distort the T1 signal and would definitely affect the resistivity measurement. This paper presents a 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. A comparison of the production forecast vs. production actually achieved indicate a powerful new capability to establish fluid type, independent of any other logging or coring result.