This paper discusses the successful trials of a new approach to drill Formation-A Reservoir in Field-A. Field-A is one of the most challenging fields in the Sultanate of Oman due to the presence of a gas cap combined with total losses encountered when penetrating a highly fractured and depleted shallow reservoir. Several unsuccessful attempts had been made in the past to overcome the challenge of total losses using conventional methods (LCM, cement plug, etc). Since then, it was agreed to stop the attempts to cure these losses and control these wells instead with a unique well control situation through dynamic filling, which is a continuous pumping of water (70 - 120 m3/hr depending on the hole size) in the annulus. This method acts as primary barrier by having higher downward velocity of water than upward velocity of gas. However due to large amounts of water being pumped into the reservoir, it caused a commercial impact on oil production due to water flow back in production and large number of emissions (12 T/year/well) from this unproductive operation. The other downside of dynamic fill method is the risk of well process safety due to it relies heavily on human behavior and actions. There are many cases where the filling was interrupted by different factors. The team carried out a study on the existing process of drilling 8 ½" and 6 ⅛" holes with a view of eliminating the need for dynamic fill practice. This led to the selection of Nitrogen Cap Drilling (NCD), which is a variation of the Managed Pressure Drilling technique (MPD) technique. NCD is a closed-loop drilling system with no returns to surface based on the presence of a nitrogen cushion establishing the communication between surface and a reduced liquid column in the annulus. The well is continuously monitored, and the annulus pressure is controlled through the wellhead pressure and a balanced fluid level without the need for filling while drilling with total losses. In 2021, the team delivered 2 (two) trial wells with NCD technique successfully without interruption and saving approximately 8,300 m3 of water compared to conventional drilling. NCD allows the pneumatic pressure communication via a pressurized nitrogen column on top of the Heavy Annulus Mud (HAM) balancing the formation pressure. The positive wellhead pressure allows better control of the well behavior and monitor potential gas migration. A positive injectivity test confirmed the applicability of the NCD technique to contain the drilling fluid and drive the cuttings back into the formation. During the drilling process, well site water is injected down the drill string and HAM is injected intermittently in the annulus form the back side of the RCD based on the annulus surface pressure changes related to the gas migration. Tripping out of hole was done using NCD keeping the well balanced and monitored to avoid gas migration to surface. This technology is a step change in our journey to address drilling high-risk Formation-A gas-cap wells in Oman with the NCD concept. It addresses the Well Process Safety concerns, generates a huge value by accelerating oil production from the drilled wells without affecting the nearby ones and finally improves the environment sustainability goals in PDO’s drilling operations.

This paper discusses the journey of finding alternate solution for having to run the Expandable Liners operations in the Fahud field which is already one of the most operationally challenging fields to drill in Petroleum Development Oman (PDO), due to the presence of a gas cap in highly fractured and depleted limestone formations with total losses and the need for dynamic annulus fill to maintain primary well control. In Fahud field, there is a highly reactive shale formation within reservoir limestone formation. Due to high likelihood of total losses, this shale formation caused bore hole instability challenges while drilling. And with more depletion took place, the challenges became more frequently to occurred. In 2001, expandable tubular liner was introduced to address these bore hole instability challenges while drilling highly reactive shale formation under total losses in the 8-1/2″ section. The use of expandable technology was sustained over the years in delivering all wells drilled to traverse this reactive shale column. Previously before 2001, wells used to have fat well design by installations of extra casing to cover the formations and problematic zones. Also, Fahud field was not depleted as it is now, and the problematic shale zone used to drill by normal conventional way without any issue using inhibition frilling fluid. Petroleum Development Oman (PDO) identified expandable liner as a preferred alternative to ‘Fat’ well design. The ‘Fat’ well design would have a large hole size through potential loss zones, resulting in unmanageable volumes of water being required. Expandable liber was fast-tracked - various technical options were considered by PDO with expandable liner technology being identified as the best solution to address the problem of the shale column. However, the deployment of expandable tubular liner technology supported to drill & deliver wells but also has its associated challenges incurring additional time and cost with reasonable installation and low operations success rate due to number of operational steps required prior and after the expandable liner. Adding to that, all the challenges associated with each step. The installation of the expandable liner required eight operational steps with multiple trips to under-ream, install and expand, cement, caliper log and drill through the liner which increased the probability of something going wrong due to mainly the challenging well profile and multiple operations steps. The expandable liners technology was required when the target formation was below the reactive shale interval. The team carried out a study of previous deployments with the intention of identifying well planning and operational contributors to the installation difficulties and operations failures, with a view of eliminating the need for installing the expandable liner and drilling the well to the desired landing point at designed section total depth. Most of the unsuccessful installation rates were observed to be prevalent in wells with high angle applications. The team also observed that the length of the hole interval below the reactive shale column contributed to the number of unsuccessful installation and operational failure rates recorded. The team evaluated the impact of reducing well inclination on the ability to deliver the hole section without installing the expandable liner. Subsequently the team developed an optimization plan which involved keeping all build activities above and below the problematic interval and holding tangent at less than 45° inclination while drilling across the problematic shale. In conclusion, in 2020 the team delivered six wells (90% of wells crossing reactive shale formation delivered) using the above described approach and traversed the historically highly reactive shale formation without installing expandable liners. This resulted in a 20% reduction in total well construction time and 17% reduction in total well delivery cost per well. In addition to the time and cost saving, with the new approach, described in this paper, less water needed to be pumped for dynamic fill. This allowed bringing the wells quicker to production, thus reducing oil deferment.