CMR-MagniPHI NMR Service Quantifies Movable Water in Oil-Wet Carbonate for Managing Water Production

Published: 08/26/2019

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Source of high water obscured in conventional logs

An operator needed to address the high water cut in producing wells from residual oil zone (ROZ) of the San Andres Formation. High water cuts of 90% or more are common and hard to predict. Conventional NMR logs, which are based on only T2 relaxation, are not able to differentiate bound and free water in oil-wet rock.

NMR is commonly used to quantify irreducible- (bound-) versus free-fluid volumes. In carbonate pores containing water, the T2 cutoff for capillary-bound water is set at 100 ms. Water in pores with T2 values higher than 100 ms is considered free and producible. However, in oil-wet rock, such as this San Andres reservoir, the T2 signature of hydrocarbon can be less than 100 ms, and therefore not distinguishable from the bound-water T2 signature.

Effectively mitigating the high water cut needed to begin with identification of the source of the movable water.

High-definition T1 and T2 measured in the smallest pores

CMR-MagniPHI high-definition NMR service introduces the new dimension of continuous T1 longitudinal relaxation time measurement made simultaneously with that of standard T2 relaxation. Both are made at the industry’s shortest echo spacing of 200 us. This combination captures high-definition NMR data from the smallest pores without any lithologic sensitivity. In addition to its high fluid sensitivity, even in micropores, the CMR-MagniPHI service is tolerant of high formation water salinity.

The continuous acquisition of T1 and T2 data by CMR-MagniPHI service enables an independent assessment of the fluids volumes and distributions, highlighting wettability contrasts and providing a lithology-insensitive alternative to resistivity-based calculations that conventionally inform the petrophysical model.

Porosity-fluid signature partitions in the oil-wet dolomite.
Porosity-fluid signature partitions in the oil-wet dolomite were identified by T₁T₂ cluster volume analysis.

Free and bound water differentiated with separate oil and water T₂ signatures

The conventional T2-only interpretation identified a larger volume of oil than  for the volume from NMR T2 > 100 ms. This lack of agreement indicates that some of the oil must have a short T2 as a result of oil-wet pores.

To obtain a more accurate bound- and free-water volumes, Schlumberger analysts matched the volumes obtained by CMR-MagniPHI service to oil and water volumes from core measurements. The resulting oil signature had a higher T1T2 ratio than the water. Using this distinctive signature in identifying the cluster volume partitions from the T1T2 fluid maps supported differentiating the free- and bound-water signatures.

The free water identified by CMR-MagniPHI service’s interpretation is consistent with the produced water/oil ratio to enable mitigating the high water cut.

The log produced with the sum of the source clusters assigned to the oil and water signatures compares well with core-derived results for volumes of porosity, hydrocarbon, and water.
The log produced with the sum of the source clusters assigned to the oil and water signatures compares well with core-derived results for volumes of porosity, hydrocarbon, and water.
United States, North America

Challenge: Characterize water saturation distribution versus pore size and wettability to assist in mitigating excess water production.

Solution: Deploy CMR-MagniPHI high-definition NMR service to acquire simultaneous, continuous T1 and T2 measurement for analysis differentiating bound and free fluids.

Results: Separated the oil and water T2 distributions to enable accurate estimate of bound and free (movable) water.

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