The traditional plug-and-abandonment (P&A) method of exploration wells in the North Sea is to set a series of cement plugs to isolate the pressurized zones from each other and from surface. This paper describes a North Sea P&A field case. In this case, an alternative method was used with a Bingham-plastic unconsolidated plugging material with high solids concentrations. This alternative method addresses well-integrity issues such as those caused by shrinking of cement or gas migration during setting, fracturing after setting, or long-term degradation by exposure to heat and chemical substances in the well.The gas-tight well-barrier element described here does not set up after placement and does not shrink. Furthermore, it cannot fracture even when shear forces exceed its strength. When this happens, the material floats and shear forces are reduced below yield strength, causing the plug to reshape. Because this is a purely mechanical process, the transition between solid and fluid phase is repeatedly reversible (in principle, an infinite number of times).The plug is thermodynamically stable because its sealing property is decided by the solids particle-size distribution (PSD) and bound water only. The closely packed particles and absence of free water mean that the entire column is kept homogeneous and no internal redistribution of particles may occur. Hence, the permanent gas-tight barrier will prevent influx through the wellbore.In the field case, a successful implementation of the technology was obtained. The field case shows how the fast and efficient placement of the plug contributes to overall cost reduction. The paper explains how the well-barrier element complies with Norwegian requirements for permanent P&A; these requirements also apply to the UK sector (NORSOK D-010 2004;Oil and Gas UK 2009). Operational procedures are also presented in some detail.

The traditional Plug and Abandonment (P&A) method of exploration wells in the North Sea is to set a series of cement plugs to isolate the pressurised zones from each other and from surface. This paper describes a North Sea P&A field case. In this case an alternative method with a Bingham-plastic unconsolidated plugging material with high solids concentrations was used. This alternative method addresses well integrity issues like those caused by shrinking of cement or gas migration during setting, fracturing after setting, or long term degradation by exposure to temperature and chemical substances in the well.The gas tight Well Barrier Element (WBE) described here does not set up after placement and does not shrink. Furthermore it cannot fracture even when shear forces exceed its strength. When this happens the material floats and shear forces are reduced below yield strength causing the plug to reshape. Since this is a purely mechanical process the transition between solid and fluid phase is repeatedly reversible forever.The plug is thermo-dynamically stable since its sealing property is decided by the solids particle size distribution and bound water only. The closely packed particles and absence of free water means that the entire column is kept homogenous and no internal re-distribution of particles may occur. Hence, the permanent gas-tight barrier will prevent influx through the well bore forever.In the field case a successful implementation of the technology was obtained. The field case shows how the fast and efficient placement of the plug contributes to overall cost reduction. The paper explains how the well barrier element complies with governmental permanent P&A requirements and the operational procedures are also presented in some detail.

Shallow water flow was severely hindering further drilling in the North Sea exploration well 16/1–9, "Draupne". The water depth was 111.5m. The 20" casing was set at 590m; above a shallow gas zone. After mounting the Blow Out Preventer (BOP), drilling the 17 ½" section started and continued for a few 100 meters. The wellhead was routinely inspected using an ROV (Remotely Operated Vessel). After some time, a tiny flow was observed around the wellhead. The flow increased in strength and later a large wash out area was observed, and further drilling had to be terminated. Most likely, the water flow originated from a zone at a depth of 172m MSL. To cure the shallow water flow it was decided to grout cement on the outside of the casings. Regular well cements was not desirable to use since these cements are somewhat retarded in itself by mineralogical composition. It was decided to use standard construction industry cement with a very short curing time. Before the grouting could start the BOP had to be removed. Because of the shallow gas zone underneath the 20" casing, two barriers had to be included in the well for well control. Two packers were used, one drillable and one retrievable. The BOP was removed and the grouting operation was performed successfully. No water flow or gas flow were observed while the cement was setting. After the cement was cured drilling resumed, first by drilling and retrieving the packers, then continuing to drill the 12 ¼" section. The paper describes all aspects of these operations in detail. It also focuses on necessary cleaning operation of bulk systems and logistical tanks to hinder polluting well cements with construction cements, and thereby avoid complicating other cementing operations. Introduction Shallow water flow (SWF) is known to be a major and expensive problem for drilling operations and is especially recognized in deepwater regions around the world. Alberty et al. (1999) identified SWF to be the result of four different mechanisms: Drilling of geopressured sands in surface casing intervals, transmission of geopressure through cement channels, induced storage and induced fractures. Geopressured sands are the most common cause of SWF, and can simplified be explained as overpressure in a sand generated by two different compaction mechanisms: Compaction disequilibrium and differential compaction. Drilling of geopressured sands is the mechanism most operators are addressing when referring to SWF and it is known to be the most damaging mechanism. Drilling of these zones is subject to high risk of large washouts and caves, formation compaction and collapse, and subsequent buckling of casing because of the common practice to drill conductor and surface casing intervals riserless. API (2002) recognized three different scenarios that are related to development of transmission of geopressure through flow channels in and around the cement surrounding the casing: Bad primary cement job, flow occurring during cement operation and flow occurring before the cement has hardened. The power of the flow at the mudline will typically increase because particles in the fluid filling the cement channels, usually drilling fluid, settle with time. Alberty et al. (1999) describes this mechanism to be difficult to diagnose due to time delay influenced by the settling solids, and failure may occur long after the cement has set. Induced fractures are related to lost circulation failures as a result of small difference between drilling fluid and formation strength in deepwater areas, and there is no need for any sands to be present. Induced storage typically takes place during surface casing intervals in permeable and porous silts, sands and even shale due to charging of formations generated by pressure from the drilling fluid column.

The paper describes field experience with a method for efficient anchor handling and rig move during North Sea exploration drilling campaigns with a semi submersible drilling rig requiring anchors for positional fix. A conventional anchor handling operation including a rig move operation is presented, followed by a detailed presentation of an innovative field operation including a method for combining anchor pick up, rig move and pre-setting. Four anchor handling boats were used to pick up anchors at an abandoned well. Thereafter the anchors and ground chains were sent off to next drilling location with three of the boats. These boats pre-set the anchors. The anchor pre-setting sequence was selected such that one boat was released after only a short period at the new location. The last anchor handling vessel, the smallest one, was used in the rig move operation as a tug. When the rig arrived at the new drilling location, the three remaining boats were involved in connecting the anchors. As a result of the pre-set anchors, the mooring operation could be performed in a weather window, being roughly one half of the weather window needed for normal mooring operations. The anchor handling boats were released sequentially as early as possible reducing the cost of the entire operation significantly. As will be described a traditional anchor handling operation would have resulted in two days waiting on weather in addition to the extra time spent on anchor handling because the weather conditions deteriorated. The paper also describes the challenges of retrieving pre-laid anchors tensioned to the anticipated maximum 100 years load.

Plug and Abandonment (P&A) can contribute with 25% of the total drilling costs of exploration wells offshore Norway. Cost efficient P&A technology is therefore necessary. In this paper, qualified technology for cutting and retrieval of wellheads using a separate vessel is described in detail. It is shown how to use this technology to significantly reduce the costs of exploration drilling. The technology has now been used on several abandonment operations on the Norwegian continental shelf.During the P&A phase of the North Sea offshore exploration well, Trolla , the rig was unable to pull the 9-5/8" production casing and was therefore unable to secure and abandon the well according to plan. This incident prevented the wellhead from being retrieved in accordance with Norwegian legislation. The required barriers were reestablished by squeezing cement through cuts in the production casing. Then the well was permanently plugged; leaving the wellhead temporarily abandoned by installing a trawl protection. This is used while waiting for a vessel to arrive for completing the operation. A high pressure abrasive water jet cutter was run from this vessel.The abandonment operation with this vessel, the world's deepest multistring conductor cut using such a system, was successful. It is shown how the casing was cut, and how the complete wellhead including the guide base was retrieved with the vessel active heave compensated crane.On a different Norwegian Sea well, a spud foundation "can" was used at a water depth of 274 m MSL to preinstall a wellhead foundation on the seabed. The paper show how the surface casing was cut approximately 12.5m below the top of the wellhead by using water jet cutting technology and how the wellhead together with the can was retrieved in the P&A operation.