Nuclear Magnetic Resonance Solves Challenges of Identifying Hydrocarbons in Low-Resistivity Pay Zones
Many reservoirs in the world that produce hydrocarbons do not exhibit traditional log responses that indicate the presence of these valuable fluids. In the past, these productive intervals have been identified by blow outs, painstakingly accurate mud logging, drill stem testing, wireline testing, and local knowledge. These methods happen later in the formation evaluation process, are time consuming and more expensive, or are random, thereby increasing uncertainty and delaying development decisions. In these cases, the identification of the fluid type and productivity have not been identified by direct Archie calculations. Sometimes, after lab studies, alternate cementation factors allow a relatively accurate reserve and water saturation calculation. However, even this approach does not resolve issues in carbonates with segmented and non-connected reservoir conditions.
Nuclear magnetic resonance (NMR) logs offer a direct and immediate solution to this low-resistivity characterization problem. Polarization (T1) data from the NMR tool supplies a direct evaluation of the fluid type, regardless of the apparent resistivity or alterations in cementation of the particle sizes in the reservoir. Relaxation (T2) can be recorded in the same logging pass as the polarization and is used to establish permeability of a low-resistivity but potentially productive reservoir.
This paper compares induction and porosity information to the fluid type, porosity, and permeability information from the NMR. The selection of productive horizons within very difficult reservoirs is demonstrated. Production results are included from low-resistivity carbonate and clastic reservoirs. Low-resistivity and layered reservoir formation evaluation issues are resolved directly.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013