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.
Drilling a surface hole with total losses through shallow aquifers is normal practice in Oman and most of the Gulf region. If losses cannot be cured while drilling, and the casing cement does not return to surface, required zonal isolation is most likely being compromised. During the life of the well, flow behind the casing from the shallow aquifers results in accelerated casing corrosion that compromises the integrity of the well and results in relatively costly workover operations to restore the well and allow it to be operated. Squeeze cement that repairs each corroded section of casing is a relatively high-risk and expensive operation with a low chance of success. During a workover on one of the water-injection wells in North Oman, severely corroded 9 5/8-in. production casing was confirmed. The initial investigation confirmed multiple leaks in the 9 5/8-in. casing, of which the exact location was difficult to establish; however, it was expected to have resulted from a failed 13 3/8-in. surface casing caused by corrosion from the shallow aquifers. Communication between the 9 5/8-in. production casing and the 13 3/8-in. surface casing was confirmed in multiple zones. Different options were evaluated to restore the integrity of this well. The options evaluated included the use of mechanical casing clads (expandables) to fix each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string and the option to squeeze cement across each corroded section in the 9 5/8-in. production casing before running a 7-in. cemented tieback string. Both of these options were abandoned because of economic concerns and low chances of success. It was decided to approach this integrity repair differently by running a 7-in. tieback casing to surface and using foamed cement to cement the 7-in. tieback casing and existing corroded 9 5/8-in. casing to surface. To maximize the chance of success, the 7-in. tieback casing incorporated a multistage cementing collar to help ensure foamed cement returned to surface. The two-stage, foamed-cement job with controlled returns from two annuli might be considered to be the first of its kind, globally. The foamed-cement job was executed successfully as per program, with good foamed cement returns from both the 7-in. × 9 5/8-in. annulus and the 9 5/8-in. × 13 3/8-in. annulus. The overall job met all the objectives and this technique has been repeated successfully on subsequent wells with similar integrity issues.
In a well integrity, repair for well AB, 9 5/8″ casing perforated to fix gas breakthrough problem behind 13 3/8″ casing, right after the casings perforation, contaminated cement circulated to surface from the annulus between 9 5/8″ and 13 3/8″ casing. Oil extracted from contaminated cement sample using two solvents, hexane and dichloromethane. The extracts analyzed using gas chromatography. Hexane and dichloromethane extracts, showed that the cement has hydrocarbons in the carbon number region from C8 to C30. Hard cementitious material milled and circulated out of hole from 900 meter downward to S formation at 1440 meter. Having contaminated cement in the top part of the cement column and hard cementitious material downward to the oil reservoir at 1440 meter, then outstanding cement downward to the section TD at 2530 meter, indicates that cement above this reservoir did not hydrate at all. Cement contaminated as a result of the losses encountered right after displacing the cement, the entire cement column form S formation upward to the top of cement contaminated with S formation oil, contaminated cement settled and compacted because of the hydrostatic head above it. Compacted solids prevent further influx to take place. This case history along with the detailed demonstration including the novel lab work will present in this paper.
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.
PDO is the biggest Oil and Gas Producer in Oman drilling in several fields with a number of drilling and cementing challenges. One of the key challenges that the operator has started to encounter is sustained casing pressure (SCP) because of poor zonal isolation. Hydrocarbons are observed in B or C annuli and lab analysis confirms it is coming from one of the Limestone reservoirs. More than 4 wells that were drilled in 2012 and 2013 have been identified with SCP and others are being monitored. The drilling campaign in this particular field was halted because of SCP problems and failure to comply with government regulations. Expensive work-over operation had to be done to abandon the wells.Owing to the fact that conventional cement systems and practices have proven ineffective, an innovative self-healing cement system was introduced. This system has the property of self-repair when in contact with hydrocarbons, seals any pathways and restores well integrity without any intervention thus providing long term zonal isolation. The system is placed conventionally in the annulus as part of the cement slurry and the healing material stays dormant till the time it sees the hydrocarbons that may flow through the cracks. Subsequently it expands and heals the cracks.The self-healing cement system has been tried in four wells in the same field with excellent results. After more than one year of completion, no gas pressure has been observed in the annulus. The operator has standardized this system for this field for all future wells and plans to extend the use to other fields with similar issues. This paper will cover details on the system, mechanism of work, validation process of the system, operational aspects and field implementation to elaborate how SCP problems can be overcome.
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