The continual increase in exploration drilling in southern Africa has translated into a number of remote deepwater campaigns, the most recent ones being in Namibia. One particular three-well campaign was exceptionally challenging as there was no near offset-well data available. The challenges were especially acute in the riserless tophole section.The well designs called for top of cement (TOC) at seabed for the surface casing. This was of the utmost importance for adequate structural and axial support for the blowout preventers (BOP) and subsequent casing strings. The very low fracture gradient near the seabed was the main challenge as the formations would not support the hydrostatic pressure of the cement column. On the first well, total losses were encountered prior to and during the entire cementing operation. As a result, no cement returns were observed at seabed, contrary to what was expected from hydraulic simulations and volume calculations and required to meet the job objective.To achieve objectives required for the success of the subsequent two wells, all aspects of drilling and cementing operations were reviewed based on the findings of the first well. Mud weight and casing setting depth were critically challenged, with other parameters adjusted. Cement formulation and density were optimized to reduce hydrostatic and hydrodynamic pressures and to increase the chance of success. The cement slurry was changed to a bimodal lightweight system with better fluid-and set-cement properties. Lost circulation fiber technology was also incorporated in the spacer preflush and in the cement slurry to mitigate any losses during placement.Alignment of service company and operator objectives and optimization of drilling and cementing parameters were critical for the successful cementation of these challenging tophole sections. Continuous improvements resulted in the second well being effectively cemented to seabed, even though intermittent losses were observed. After further optimization, the third well was cemented to seabed with full returns. Reaching the target TOC eliminated the need for a top-up job, saving valuable rig-time.

The job objective in a UK North Sea field was to permanently abandon a well that had poor cement bonding behind the casing across the intended isolation intervals. The challenge was to provide lateral isolation across two separate intervals in the most efficient way possible. Two "perf-and-wash" operations were executed successfully during the well abandonment. The deeper barrier envelope was validated by tagging and pressure testing the plug. The shallower section had been logged prior to the operation and, on completion of the perf-and-wash job, the plug was drilled out to allow for relogging, which indicated more than 76% of the perforated interval had circumferential coverage. After the bond log results were confirmed, a further cement plug was set across the shallow interval by conventional methods and verified by tagging and pressure testing. This paper outlines the detailed design preparations and presents both case histories where these steps were implemented successfully.

In line with the rise in remote deepwater exploratory work in East Africa, a major operator drilled a well in one of the two petroleum block licenses that they own offshore Kenya. The well was drilled in a water depth of 723m (2372ft) with the primary objective of establishing the presence of net pay and fluid type at specified stratigraphic levels. Prior to spud, the geological prognosis of the well indicated the presence of carbonate formation characterized with vugs and the imminent risk of severe losses were predicted. The drilling of this well indeed proved to be a challenging process. Lost circulation while drilling and cementing continues to be a consistent problem in most deepwater operations that costs significant time and money. Firstly, the operator is faced with the inability to reach planned total well depth within the projected time interval, and secondly the necessary hydraulic isolation (cementation) covering problematic formations is not achieved. In addressing the loss and weak zones, different approaches are used notably by reducing the hydrostatic pressure column, utilizing lighter weight slurries and reducing the penetration of fluids into fractures with the use of lost circulation material. Losses were encountered in all hole sections below the 20-in casing just as predicted; estimated between 40 to 180 bbl/hr. As per a planned line of action, the mud service company pumped a range of lost circulation materials with different formulations, but none was successful. The executed approach was not effective in curing the losses and this resulted in total down time of 16 days of rig time causing the operator an additional $20 million over the AFE. Out of this additional cost, $0.34 million was due to mud lost into the formation and the rest was for daily operating rig cost. The extent of the losses was so severe that it took a total of three engineered fiber pills and cement plugs placed at different depths before the operator could drill beyond the troubled zones. This paper is intended to cover the use of an engineered fiber and solids technology in a spacer system that was pumped ahead of cement slurry to aid in curing the losses.

SUMMARYTraditional abandonment of a well is accomplished by setting multiple cement plugs to achieve zonal isolation. By setting longer plugs to cover multiple zones and thus having fewer cement jobs, rig time and operating costs can be significantly reduced. However, there are limitations to the length a cement plug can be set conventionally without increasing the operational risks such as stuck pipe. The Hydraulic Tubing Release Tool was introduced on a deepwater exploration and appraisal well campaign in East Africa. It has been used successfully on 5 wells to set long abandonment plugs, ranging from 700m to 1800m.

With the increase in decommissioning/abandonment work in the North Sea, a major operator had the objective to permanently abandon a well that had been temporarily suspended. There were several key challenges associated with this project: high pore pressure and temperature, a completion packer set in a target abandonment interval, and small cement plug volumes. These obstacles had to be overcome to complete the abandonment safely and efficiently. The primary barrier to the deepest potential flow zone, zone 1, was established by running flush slimline tubing through the 5-in. tubing and 9 7/8-in. production packer. The cement plug was then successfully placed below, across, and above the production packer. The cement plug was successfully tagged and inflow tested, creating a primary barrier against zone 1. This approach eliminated the need to remove the production packer prior to the abandonment plug being set, which saved the operator approximately 10 days of rig time. This paper outlines the detailed design preparations and presents the case history where these steps were implemented successfully.