Innovative Cased Hole Logging Identifies Opportunities | SLB
Case Study
Location
Yemen, Asia
Details

Challenge: Log a cased interval in an exploratory well where openhole logging was not possible.

Solution: Used a combination of ECS elemental capture spectroscopy sonde, Sonic Scanner acoustic scanning platform, and three-detector lithology density (TLD) tool run as the CHFD cased hole formation density service to log the cased interval.

Results: Identified new opportunities using the same methodology for other wells, saving rig time and additional logging costs.

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Innovative Cased Hole Logging Identifies Opportunities

SEPOC Yemen achieves accurate calculation of formation density behind casing

Difficult logging conditions

Safer Exploration & Production Operations Company (SEPOC) Yemen needed to log a cased interval in a recently drilled exploratory well. Openhole logging had not been possible owing to drilling and operational reasons. Cased hole logging was attempted, but a large borehole and thick cement made calculation of the formation density especially difficult.

Successful use of cased hole logging tools and analysis workflows

Schlumberger proposed a solution combining the ECS elemental capture spectroscopy sonde, Sonic Scanner acoustic scanning platform, TLD tool, and CHFD cased hole formation density service over the cased hole interval. With the technology in place, multidisciplinary workflows were used to achieve reliable density. Three different approaches were adopted to calculate the formation density behind the casing:

  • cased-hole TLD data using the GeoFrame* integrated reservoir characterization system to derive the CHFD density
  • density computation using ECS lithology and Sonic Scanner compressional slowness
  • density derived with Gardner’s equation (used for qualitative purposes only).
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ECS lithology resolved for sandstone, carbonates, and clay.

Because of the very hard formation and thick cement behind the casing, it was difficult to distinguish compressional slowness from casing arrival. However, the Sonic Scanner platform's data quality enabled the computation of compressional slowness, and the density was computed successfully using back calculation based on the ECS sonde's lithology analysis and Sonic Scanner platform's rock porosity measurement.

The evaluated average cement thickness behind the casing along most of the interval varied from 1.5 to 2 in—too high for reliable density from CHFD service. The vertical borehole and well-centered casing also imposed an additional challenge on the density calculation based on the CHFD service's logging results.

Innovative Cased Hole Logging Identifies Opportunities
Z-plot of CHFD density vs. ECS and DTCO density, showing cement thickness behind casing is more than 1.5 in at sizeable data interval.

After applying casing corrections, the Schlumberger team resolved the ECS sonde data for sandstone, carbonates, and clays, the three main lithologies recorded in the mud log. Sonic Scanner platform data was carefully processed using different filters and parameters. After elimination of the effects of casing, data quality was excellent and different slowness arrivals in the casing could easily be discerned on the quality control (QC) plot. Hence, the Sonic Scanner platform distinguished the minimum level of change in rock properties and enabled computation of reliable compressional slowness—a feat that would be difficult to achieve with conventional sonic tools.

Innovative Cased Hole Logging Identifies Opportunities
Sonic Scanner data QC plot giving good compressional and shear arrivals in thick cement and casing.

Validation of density

ELANPlus advanced multimineral log analysis was used to build a model for the computation of density using compressional slowness and ECS sonde lithology, which produced satisfactory results owing to the accurate inputs. The same parameters and inputs were next used in the ELANPlus analysis model to generate the density over a 50-ft section below the casing shoe where the density had beenlogged in the open hole with the TLD tool. Results from this interval were then crosschecked with the logged openhole density and found to be accurate.

Generation of rig-time and logging savings

Besides the initial exploratory well, use of the methodology led to new opportunities for the company’s other wells, saving rig time and additional logging costs. SEPOC Yemen now plans to use Sonic Scanner and ECS tools in all future exploratory wells.

Contact your local Schlumberger representative to learn more.

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