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Proactive Geosteering in Safaniya Sands in KSA (World’s Largest Offshore Oilfield)
May 13, 2016

Proactive Geosteering in Upper Shuaiba Carbonates in Oman

Upper Shuaiba play in North Oman is a prograding shoals and mounds into the Bab Basin interfinger with argillaceous limestones and are truncated by Nahr-Umr shales. Individual reservoirs, in the clinoform apex, are too thin to be resolved on seismic. The clinoforms present imply that top Shuaiba is not a uniform hard-kick pick; phase is depending on reservoir thickness and manual picking is necessary. Well placement and geosteering are key factors in increasing success and maximising the value of discoveries.

Geosteering Objective:

The geosteering objective was to optimize horizontal wellbore position within porous limestone layer keeping the wellbore trajectory smooth, utilizing real-time distance-to-bed-boundary (DTBB) calculations.

Challenges:

  • Clinoform with inconsistent target interval thickness and boundary truncation.
  • Lateral variations in facies.
  • Uncertainty in seismic resolution
  • Unconformity lying above with Nahr Umr shale, causing non-uniform formation top.

Tools Utilized:

  • Baker’s Deep Azimuthal Resistivity (AziTrak) tool.
  • Triple combo.

Geosteering Summary:

The wellbore was landed by the client’s geologist to ensure optimal landing in the zone. The resistivity was very low at the edge of the clinoform or the landing point. The formation porosity was low and the thickness was 3-4 metre. Utilizing AziTrak tool’s APR signals and DTBB, the wellbore position was optimized in the porous interval of target zone. The resistivity data indicated that the reservoir facies quality was improved in the clinoform. Approaching further with lateral extent, the reservoir quality deteriorates at the edged of the clinoform with reduction in porosity and resistivity. The DTBB analysis showed the good quality reservoir was pinched out towards well TD with increase in argillaceous material.

The wellbore was placed 100% in zone with the utilization of DTBB and accurate interpretations that supported geosteering specialists in providing right targets to the directional driller for optimal well placement.

Conclusion:

  • AziTrak distance-to-bed boundary calculation, utilized APR signals, 400 KHz and 2MHz attenuation and phase resistivity, was invaluable in making right decisions in time.
  • Reservoir boundary mapped for new wells planned in the vicinity.
  • The boundary mapping tool added value in precise well placement within zone with no exposure in unwanted zones.
  • Knowledgeable personnel were the key for accurate interpretation and precise wellbore placement.