NMR T2 logs respond to both pore size distribution and fluid properties. The presence of more than one fluid can complicate interpretation due to the overprint of the fluids on the pore size information. For many interpretation techniques, including core-based rock typing applications, the response must be transformed to that of a water-filled formation. Further, NMR logs acquired while drilling can look very different to those acquired later, either while wiping with the same logging-while-drilling (LWD) tool used during drilling or on a later wireline-conveyed pass. The difference in the T2 logs is generally due to mud filtrate invasion. Water-based mud (WBM) filtrate invasion into a hydrocarbon zone or oil-based mud (OBM) filtrate invasion into a water zone can both cause additional fluid-related complications.
In these circumstances, fluid substitution is performed on the NMR logs in which the hydrocarbon response is replaced by water response. This paper details the workflow first proposed by Shell (Volokitin et al., 1999, 2001), enhanced by NMR factor analysis (Jain et al., 2013) to determine the fluid-related bin porosities and T2 cutoffs. A step-by-step workflow is provided allowing petrophysicists to use this technique in their daily work. The fluid substitution process is illustrated through several examples.
In the first example, laboratory T2 measurements were acquired on a core first saturated with water, then with hydrocarbon. The fluid substitution applied to the hydrocarbon-saturated T2 log successfully restores the T2 log as measured on the water-saturated core.
The second example is a case of drill & wipe pass LWD NMR logs acquired across an oil-water contact in a formation drilled with OBM. The wipe pass logs look vastly different to the drill pass logs due to invasion. Once the two logs are transformed to water-filled response by substituting the hydrocarbon and OBM filtrate with water, the drill & wipe pass NMR logs look the same and yield the same answers.
The third example demonstrates the impact of gas substitution. Before substituting the gas response, it must be corrected for hydrogen index and incomplete polarization effects with the help of other logs. This example shows a formation with both a gas-oil contact and oil-water contact drilled with OBM. The T2 distribution in the water zone serves as a reference to quality control the fluid substitution in the oil and gas zones.
The fourth example displays heavy oil substitution. If all the oil components are measurable by NMR, the heavy oil response can be substituted even though its T2 spectrum resides in the bound fluid part of the total spectrum due its fast relaxation time. The permeability and irreducible water saturation must first be estimated using other information before the substitution.
These results suggest that fluid substitution is essential to understand and compare NMR logs across the same reservoir at different time in the drilling / fluid invasion process. Fluid substitution to obtain a water-filled response provides corrected pore size distribution and enhanced reservoir rock quality evaluation.