Abstract

The undeveloped sublayers of the prolific offshore Arab reservoir were recently targeted for appraisal drilling, aligning with the ever-increasing efforts to expand production and book new reserves. The first extended reach horizontal well was drilled in this field to evaluate the economic potential of four different reservoir sublayers. Reservoir characterization and the evaluation of lateral permeability changes were the primary objectives for these low permeability layers. Maximizing reservoir contact while maintaining minimum borehole tortuosity presented substantial geosteering challenges. Another challenge is that the bottomhole assembly (BHA) must be free from radioactive chemical sources.

A well placement workflow was developed that honors the structural geological setting, based on the existing field knowledge and offset petrophysical data. The optimized BHA consists of a point-the-bit rotary steerable system (RSS) and logging-while-drilling (LWD) sensors. These LWD sensors include high-resolution microresistivity imaging, laterolog resistivity, azimuthal multipole acoustic, nuclear magnetic resonance (NMR), ultrasonic calliper, and near-bit azimuthal gamma ray sensors.

High-resolution microresistivity imaging and the near-bit azimuthal gamma ray sensor were used to geosteer in the thin reservoir subunits and to facilitate fracture identification. NMR was used to help remain in sweet zones in real time and to provide pore size distribution, based on T1 measurements for permeability evaluation. Acoustic and high-resolution image data were used to derive empirical permeabilities.

The 8,000-ft horizontal section was successfully geosteered with 100% reservoir contact, tapping into four thin reservoir sublayers. Real-time high-resolution microresistivity images, dip picks, and near-bit azimuthal gamma ray data helped to maintain the wellbore attitude parallel to the stratigraphy within each sublayer; they also facilitated a smooth transition from one sublayer to the next with minimum borehole tortuosity, aided by the point-the-bit RSS and at-bit inclination measurements.

Fracture evaluation from high-resolution images, NMR, acoustic, and image-based permeabilities are integrated with production log results to enable a better understanding of the field, to benchmark flow unit identification in these undeveloped reservoirs, and to optimize future geosteering and petrophysical data acquisition requirements.

The traditional reactive geosteering concept is challenged by placing the 8,000-ft extended reach section in four different sublayers that are as thin as 3 ft true stratigraphic thickness (TST) without penetrating any boundaries.

The multidisciplinary approach helped to assess the economic potential of these undeveloped layers within the local reservoir sector and to formulate plans for a future field development program.

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