Natural Fractures Help Unlocking Tight Carbonates: Re-Developing the Natih B Reservoir in a Giant Field in the Sultanate of Oman
Authors: Bazalgette Loic , Aamri Nawal , Field Chris , Bos Robert , Tooqi Sabah , Wei Lingli , Nofli Kholood , Manji Khoula , Hadhrami Wafa , Shizawi Wafa , Hasni Adil , Shuaili Salim , Shuaili Khalid , Al Bahri Sharif , Al Jabri Yousuf 
Published: October 2022

Within the stratigraphy of one of the largest fields in the northern Sultanate of Oman, is the Natih B reservoir. Although in a brown-field setting, the Natih B reservoir has undergone several challenges during its development journey over the past decades due to the complexity of the reservoir setting and difficulties in locating the natural fractures. Success in developing such tight carbonates depends on the integration of high quality seismic, borehole imagery, petrophysics, and dynamic data. The approach allows the right access to natural fractures by the wellbore and to release the trapped oil at economic rates. The objective of this paper is to develop an integrated technical approach that can be used to unlock one of the largest undeveloped resources in PDO's current portfolio (up to one billion barrels of oil originally in place).

The Natih B reservoir is one of the largest undeveloped resources in PDO's current portfolio. Various attempts at producing this reservoir in the 1980s and 1990s were not successful, essentially because of poor reservoir permeability or due to early fluid breakthrough from the surrounding Natih A or C reservoirs. Such breakthroughs were related to the common presence of high permeability sub-seismic faults or fracture corridors directly intersected by producing wells or subsequently connected by induced fractures caused by well stimulation. The opportunity to re-develop Natih B reservoir arose late 2018, focusing on a new integrated approach consisting of: (1) State-of-the-art understanding of static and dynamic characteristics of fractured carbonate reservoirs; (2) interpretation of up-to-date, high quality seismic volumes, and detailed structural analyses; (3) integration of petrophysical, dynamic and production data.

The integrated study described above clearly demonstrated the negative role of large-scale fault zones and fracture corridors on reservoir producibility. It also highlighted that poor matrix properties could be compensated for thanks to the support of small-scale ("background") natural fractures locally present in the reservoir. Targeting the stratigraphic intervals with high densities of small-scale natural fractures was done with the help of a simple but robust characterization of the reservoir's mechanical stratigraphy. Carefully interpreting the recent high-quality 3D seismic also allowed optimization of well locations and design, and to avoid large-scale faults and fracture corridors responsible for fluid breakthrough. As a result, the first producer well of the re-development phase was drilled in 2019. Data acquisition included fluid production logging (including SNL "noise" logs) and production tests, and consequently confirmed the successful concept. This well is still producing to date without significant decline or fluid breakthrough. Based on this success, new wells were proposed following the development strategy based on the new-simplified concept of natural matrix depletion enhanced by small-scale natural fractures. Furthermore, a two-step development staircase has been developed in to further maximize recovery. The staircase includes Fishbones Enhanced Recovery and Miscible Huff-&-Puff (HnP) Enhanced Recovery.

This project demonstrates how a thorough multidisciplinary study supported by a good understanding of the simple but robust static and dynamic concepts of fractured and carbonate reservoirs can support unlocking a very large but previously untouched volume of oil. Under certain conditions, the workflows described in this paper can certainly be adapted to other challenging resource accumulations locked in low permeability, fractured reservoirs.