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.

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.