Oman is a hotspot for drilling activity and wells are being drilled in different environments varying from Deep exploration and development for gas and oil and water injection/disposal. One challenge tops all other challenges: Lost Circulation. Due to the fractured/fissured nature of the formation and low existing reservoir pressures, all major operators are suffering from lost circulation challenges. Some of the challenges include: Mud losses while drilling leading to cost overruns and HSE concerns, primary cement job failure due to not getting the cement up to the desired height resulting in subsequent sustained casing pressure and corrosion, not able to perform work over activity on certain wells due to losses. Enormous quantities of water are required to maintain well control, and due to the limitation of water availability all over Oman, this becomes another critical issue. An Engineered fiber-based Loss Circulation pill has proved successful to address these challenges in multiple fields for Petroleum Development Oman. Drilling shallow wells in Oman through the naturally fractured limestone formation of Natih, usually results in significant losses of up to 55 m3/h (346 bbl/h) even with a low density drilling fluid of 1,033 to 1,070kg/m3 (8.6 to 8.9lbm/gal). Packoffs are often observed due to the swelling shale section, which leads to several attempts with kick-off plugs and sidetracking. Engineered fibers pills enabled total returns to surface when no other loss circulation solution had worked before. This also enabled to bring cement all the way to surface using 1,410kg/m3 (11.8lbm/gal). In another field, a work over rig was mobilized to perform a well kill operation and pullout. Due to total losses through perforations into the reservoir, the well kill could not be completed. In addition, every time the water level fell gas started to flow in the well. After 17 attempts and 8 loss circulation material pills, a total of 763m3 (4,800bbl) of well-supply water had been pumped. An engineered fiber pill at 1,474kg/m3 (12.3lbm/gal) was designed and bullheaded into the perforations. The pressures while pumping and squeezing rose to 11,031kPa (1,600psi). The well was shut and observed for 3 hours without any pressure increase indicating losses were cured and gas flow stopped. Engineered fibers have proved their value in all sorts of lost circulation applications in North Oman. These pills have been successfully used to mitigate losses while drilling, while cementing, during mud circulation before cement job when the casing is on bottom and in work over jobs in depleted reservoirs. With the level of success achieved with such treatments, in some fields it has become a standard practice for curing losses.

Petroleum Development Oman, PDO, is planning to improve ultimate recovery of condensate from a retrograde condensate gas field by reducing the rate of reservoir pressure decline. This shall be accomplished by re-injecting into the reservoir some of the produced gas and all of the acid gas extracted from the sweetening process. The composition of the injected gas will vary over time, from 15% CO2 and 3% H2S to 56% CO2 and 10% H2S. These combinations of CO2 and H2S can cause the wells cement to deteriorate. Portland cement tends to strongly degrade once exposed to such acid gases by reacting with calcium hydroxide formed from hydrated calcium silicate phases. As carbonates are dissolved in a low pH environment, the cement-carbonation products will not act as a self-plugging agent / s in the cement sheath. The resulting decrease of compressive strength and increase of permeability could lead to loss of zonal isolation and casing corrosion. These requirements led PDO to investigate and trial CO2-resistant cement to enable zonal isolation and ensure long term containment of the reservoir fluids. The nominated new technology cement system was trailed in a deep gas well which penetrated a reservoir which has high concentrations of CO2 and H2S at a super critical condition. The CBL/VDL log which was run after well completion showed excellent results. The well-cement quality shall be re-logged prior to any zonal shutoff work-over or well decommissioning. This paper will discuss the design, execution, and evaluation of the first acid gas resistant cement in PDO in one of the high profile gas well in South of Sultanate of Oman.

In Oman, certain fields contain heavy oil and recovery of this oil is done through steam injection, which leads to rapid heat-up of the cemented annulus to very high temperatures. Throughout the lifecycle of steam injection wells, stresses in the cement sheath induced by rapid temperature cycling, results in mechanical damage and ultimate failure of the cement sheath. Such failure leads to loss of steam down-hole and an increased amount of steam is required to extract the oil. This translates into higher energy costs for steam production. In extreme cases steam can be seen breaking through to the surface. In Oman heavy oil reserves are found in naturally fractured limestone formations prone to severe losses while drilling. To ensure proper cement placement, systems with densities below 1,400 kg/m3 are required. In the past certain wells were cemented using foam cements with density close to that of water. Data collected from earlier steam injection drilling campaigns by PDO suggests that maintaining cement integrity is a key challenge. The issue is related to initially not being able to place the cement properly due to losses and subsequent degradation of set cement as it does not withstand the stresses created during steam injection process. A recently developed specialized cement system was used to successfully cement one such well. The system was placed successfully using fibers based pill ahead of the slurry to cure the losses. Stresses created on the cement sheath during steam injection were simulated, mechanical and thermal properties of the cement system were optimized to prevent failure, and evaluation was performed for wellbore integrity. Excellent mechanical and thermal properties for a 1,400 kg/m3 slurry system showed no breakthrough of steam when exposed to multiple temperature cycles of up to 300 degC. Multiple wells in Oman have been cemented using this technology. The current paper looks at various aspects of design, execution and evaluation of such cement systems.

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Petroleum Development Oman (PDO) is drilling sour reservoir wells in the south of Oman. The challenge for these wells is that the completions are suffering from an iron sulfide scale formation during the production stage. Iron sulfide scale precipitation occurs from the reaction of hydrogen sulfide (H 2 S) and iron oxide (hematite or other metals oxides). The latter is usually used in the high density conventional cement slurries to cement the liner across the reservoir intervals. The iron sulfide scaling on these wells reduces the internal diameter of the liner and the tubing. The consequence of this is the restriction of access to the wells with surveillance equipment for data acquisition (for example production logging) and other tools. A new solution was found to eliminate the formation of the iron sulfide scaling in future wells. This solution is the implementation of an innovative optimized particle size distribution cement system, with low water content and high mechanical performance, that meets the well design requirement. The cement slurry formulation contains no iron and hence meets the cement system specifications. Extensive laboratory work was undertaken to engineer the slurry to the required specifications and yard trials were also performed. PDO and Schlumberger have worked very closely and have been able to execute the first ever metal-free cement system for such environments. This paper will cover the long term zonal isolation challenges that wells, with the risks of iron sulfide scaling, are facing. New technologies and techniques used to seal and cement these wells will be presented with case studies from well operations. BackgroundThe field N is a high profile field was discovered by exploration well A-1 in 1989. It is situated 40 km west of the NM field and 60 km north of the MM field in the South Sultanate of Oman salt basin in an area where no infrastructure was present at the time of discovery. To date thirty vertical wells, including one 4 hole multi-lateral, have been drilled in field N. The field contains the unique Athel silicilyte formation, some 4.5 km below the surface. The Athel reservoir has a gross thickness of up to 400 m and is fully encased in sealing salt as a result of which the reservoir is geo pressured to 80 MPa. Whilst porosity (23 Pu), net-to-gross (> 90%) and oil saturation (80 su) are favorable in field N, the permeability is extremely low. This is due to the fact that the rock consists mainly of micro-crystalline silica with a uniform size of around 2-3 m, which leads to extremely small pores and pore throats. The oil in field N is very light and volatile. It has a density of 0.622 g/cm3 at reservoir conditions, with a solution GOR of 410 sm 3 /m 3 and a bubble point pressure of 11600 psi, resulting in an API gravity of 48 at stock tank conditions. The oil is also sour, containing 1.5 mol% H 2 S and 3 mol% CO 2 , no formation water and under 1 %, by volume, water of condensation. Through special PVT experiments the oil has been shown to be miscible with hydrocarbon...