Azimuthal Sonic Data Enables Safe Landing in Very Thin Carbonates Reservoir of Lower Cretaceous
Authors: Ahmad Nazlan Mohd Firdaus , Raza Taufique Ahmad , Al Abd Salem Salem , Al-Amiri Abdelhameed A , Hafez Hafez H. , Sallam Yassin Farouk , Al Arfi Saif Ali , Mostafa Hassan , Market Jennifer , Farouk Omar El 
Published: November 2015

Landing a high-angle well in the very thin carbonate reservoir of the lower Cretaceous can be challenging due to heterogeneity present in thin layers above the reservoir and lack of correlatable non-porosity measurements. The use of radioactive source porosity is effective, but due to the extended trajectory and high differential pressure in over burden depleted reservoirs, the chance of becoming stuck is a considerable risk and a big concern, and thus the use of classic nuclear porosity tools are untenable. This paper is a case study illustrating the use of azimuthal sonic porosity as a highly effective method of landing the well in the reservoir.

Azimuthal sonic data were acquired with a focused unipole tool which recorded the measured waveforms and computed compressional and shear velocities in 16 azimuthal bins. Real-time compressional and shear slownesses were also computed by stacking all 16 bins of data, which gave very high signal-to-noise ratios and excellent data quality - often a challenge in hard formations. These azimuthally averaged slownesses were used by the geologist to identify formation tops while drilling, and to detect the approach of the target reservoir, resulting in a safe landing in the target zone.

While the azimuthally averaged real-time sonic porosities were effectively used to land in the target reservoir, the azimuthal results were examined shortly after the well was drilled to understand the additional advantages of using the azimuthal sonic porosities real-time to detect approaching beds from further away. It is clear from the results that, if the porosities from each quadrant (or even just up and down) were transmitted real-time, the reservoir could be detected an additional 1.5 ft. away (TVD). The hardware has since been modified to apply this approach, allowing for 4 quadrants of sonic compressional and shear slownesses to be transmitted in real-time to enhance the detection range. The technology has been applied in real time in the second well with success. It was proven this extended depth of detection can add an additional safety margin to enable landing the well in the reservoir at the optimal angle. Due to the higher depth of investigation the landing can further be optimized using the bottom quadrant slowness, allowing for the early detection of the soft porous layer well before penetrating it.

These quadrant sonic values can be used in existing geosteering workflows to enhance the ability to detect formations, particularly when other measurements such as resistivity or gamma ray yield very shallow depths of detection due to poor formation contrast or depth of detection of the measurements.

This case study builds upon previous sonic experience by investigating the range of azimuthally binned measurements versus azimuthally averaged results and shows a most convincing example illustrating the effectiveness of the method in an Abu Dhabi formation.