Abstract
Drilling horizontal wells in the mature giant carbonate fields offshore Abu Dhabi, where high uncertainty regarding the lateral distribution of fluids results in variable water saturation, is very challenging. In order to meet the challenges and reduce uncertainty, the plan was to drill pilot holes to evaluate the resistivity of the target zones and plan horizontal sections based on the information gained. To investigate the possibility of avoiding pilot holes in the future, an ultra-deep electromagnetic (EM) tool was deployed to map the mature reservoirs, identifying formation and fluid boundaries before penetrating them, avoiding the need for pilot holes.
Prewell inversion modeling was conducted to optimize the spacing and firing frequency selection and to facilitate early real-time geosteering and geostopping decisions. The plan was to run the ultra-deep resistivity mapping tool in conjunction with shallow propagation resistivity, density, and neutron porosity while drilling the 8 ½-in. landing section. The real-time ultra-deep EM inversion was run using depth of inversions up to 120 ft., to be able to detect the reservoir early and evaluate the predicted reservoir resistivity. This would allow optimization of any geostopping decision.
The ultra-deep EM tool delivered accurate mapping of thin reservoir layers while drilling the 8 ½ inch section, as well as enhanced mapping of low resistivity zones up to 85 ft. True Vertical Thickness (TVT) in a challenging low resistivity environment. The real-time EM inversion enabled the prediction of resistivity values in target zones prior to entering the reservoir; values were later crosschecked against open-hole logs for validation. The results enabled identification of the optimal geostopping point in the 8 ½-in. section, enabling up to seven rig days to be saved in the future by eliminating pilot holes, in addition to eliminating the risk of setting a whipstock at high inclination with subsequent milling operations. In specific cases, this minimizes drilling risks in unknown/high reservoir pressure zones by improving early detection of a formation tops, thus improving geostopping decisions. Plans were modified for a nearby future well and the pilot-hole phase was eliminated because of the confidence provided by these results.
Deployment of the ultra-deep EM tool in these mature carbonate reservoirs may reduce the uncertainty associated with fluid migration. In addition, use of the tool can facilitate precise geosteering to maintain distance from fluid boundaries in thick reservoirs. Furthermore, due to the depths of investigation possible with these tools, it will help enable the mapping of nearby reservoirs for future development. Further multi-disciplinary studies remain desirable using existing standard log data to validate the effectiveness of this concept for different fields and reservoirs.
