In-situ modelling of NMR response

The problem . .

It is well known that interpretation of NMR logs on the basis of core-derived parameters often fails because the down-hole situation is much different from that in the laboratory. The largest difference is usually caused by the presence of hydrocarbons (at some remaining saturation). In addition to this, logging tools always measure in an inhomogeneous magnetic field, which may introduce diffusion effects. Core measurements at full in-situ conditions are in principle possible, but very expensive and therefore bound to span a limited range of properties.

. . and its solution

Using our comprehensive software, the NMR response can be modeled at any reservoir condition. Starting point is a representative set of water-saturated core samples measured at ambient conditions. Forward modeling then introduces changes in the NMR response corresponding to full in-situ conditions, including effects due to the presence of native hydrocarbons, mud filtrate invasion and wettability. Specific hardware features of logging tools, such as their gradient and acquisition parameters, are also included. Interpretation parameters, such as (variable) T2-cutoff, and permeability exponents can now be calibrated on NMR data as they appear on the log. Once the workflow has been set-up, any change in conditions is automatically translated in an update of the interpretation parameters; this would not be possible with laboratory experiments.

My publications can be found HERE.

The modeling is done starting from the measured signal at full water saturation. Gradual desaturation causes the peak not only to shrink, but also to move to shorter T2. Ultimately, it fully matches the actually measured response at humidity-controled saturation.