Certain abandonment operations require plugs to be set off-bottom in a well. The challenge in setting plugs in highly deviated wellbores lies in obtaining a competent hydraulic seal that can qualify as a barrier to any potential flow zones. Failure to achieve a good cement plug leads to increased nonproductive time (NPT) from cleanout, drillout, and setting additional plugs. Small slurry volumes, unstable fluid interfaces with respect to gravitational forces, and fluid contamination add to the complications. In the Caspian Sea abandonment operations, plug-placement simulation aided in designing cement plugs and setting procedures in inhibited seawater, water-base, or oil-base drilling fluid environments. The advanced cement plug placement simulator was used to design fluid properties and optimize volumes for plug-placement operations in highly deviated Caspian Sea wells.
For successful placement, physical fluid properties and fluid mechanics were used in the advanced placement simulator; in addition to the fluid behavior, the simulation took into account well geometries, tubular dimensions, and mechanical separation devices. Various parameters affecting successful plug placement, including but not limited to, length of tailpipe, gel strengths, underdisplacement volumes, and placement procedures were input into the simulations. Balanced cement plug placement is widely practiced by many operators. In the literature, it has been shown that when smaller diameter tubing or tailpipe is run on the bottom of drillpipe, the assumption that all fluids both inside and outside the drillpipe will remain in hydrostatic equilibrium is wrong. In this study, advanced placement modeling illustrated that an initially balanced cement plug may become unbalanced while pulling a cement string with tailpipe out of the hole. Multiple abandonment operations performed in Caspian Sea wells provided key lessons for wellbore preparation, fluid design, and wellsite execution. Design decisions made from the simulation results and improvements made with implementation of industry best practices increased the ability to get plug placement right every time from the first attempt.
Jain, Bipin (Schlumberger) | Khattak, Mohammad Arif (Schlumberger) | Mesa, Alvaro Martin (Schlumberger) | Al Kalbani, Sultan (Schlumberger) | Ahmed, Junaid (Schlumberger) | Al Aghbari, Salim (Petroleum Development Oman) | Qassabi, Nabhan (Petroleum Development 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.
Jain, Bipin (Schlumberger) | Mesa, Alvaro Martin (Schlumberger) | Kalbani, Sultan Al (Schlumberger) | Meyer, Arnoud Willem (Schlumberger) | Aghbari, Salim (Petroleum Development Oman) | Al-Salti, Anwar (Petroleum Development Oman) | Hennette, Benjamin (SHELL) | Khattak, Mohammad Arif (Schlumberger) | Khaldi, Mohammed (Petroleum Development Oman) | Al-Yaqoubi, Ali (Petroleum Development Oman) | Al-Sharji, Hamed Hamoud (Petroleum Development Oman)
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.