Internal gradient Gas Internal gradient Water
Unrestricted diffusion
Restricted diffusion High GOR
Bound water
OBMF with gas
High GOR
Native oil
Mixed wettability
Restricted diffusion
Mixed wettability
> Interpreting the maps. After inversion, two-dimensional crossplots of the data identify the presence of oil, water and gas. When the plot of the response from formation fluids (shown as color contours) conforms to the interpretation model, the response will fall on or near the expected gas, oil or water lines. This affords a straightforward interpretation. Water falls on the water line as shown (middle). However, signals frequently fall off the lines as a result of competing petrophysical effects, which include internal gradients, restricted diffusion, wettability and high gas/oil ratios (GOR). Because internal gradients shorten the relaxation times, the plots tend to shift upward (top left). Restricted diffusion causes the measured diffusion rate to increase, and, as a consequence, the plots will trend down and away
Fluids with a high GOR tend to plot above and to the left of the oil line. This can be seen in native fluids and gas-bearing zones invaded by oil-base mud filtrate (OBMF). OBMF should plot as a moderate to light hydrocarbon. The response of OBMF mixed with native gas from the reservoir plots between the oil and water lines. The D-T maps are powerful tools for inter - preting fluid types in the reservoir. In many cases the interpretation is straightforward, but
from the expected fluid line (bottom left). Plots from oil-wet reservoirs tend to shift to the right of the oil line, as do reservoirs with mixed wettability (bottom right). The maps from oil-wet and mixed-wet reservoirs tend to have a spectrum of responses, which produce a broader image. Because the model is built with dead-oil responses, maps from high-GOR oil reservoirs may not respond as expected. These plots are shifted away from the oil line toward the gas line (top right). In a reservoir that contains only gas, OBM filtrate may mix with the native gas and produce a response similar to that of high-GOR oil. Deeper measurements often help experts refine their interpretation of these maps because filtrate generally diminishes away from the wellbore and gas response increases.
consideration must be given to external factors that lead to nonideal behavior and mislead a novice interpreter. For this reason, it is important to rely on experts adequately trained in processing and interpreting NMR data. A surgeon relies on a trained radiologist to interpret MRI images. A clean break of a bone is easy to spot, even by a novice user, but experience helps a radiologist differentiate between bone fragments and calcification. The differences may be indistinguishable to
the untrained eye. In a similar manner, the log analyst may easily determine the presence of water or gas in a D-T map, but there are occasions when a skilled NMR expert should be called in to help analyze the results. With the aid of fluid maps and an understanding of the measurement physics, the petrophysicist can diagnose conditions that are hidden from the inexperienced observer.
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Oilfield Review
Oil
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