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.

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Severe hole stability problems were encountered in a recent exploration well in the Norwegian North Sea. The problems occurred when drilling through Tertiary shale sections interbedded with permeable sand layers. Drilling was initially performed with water based drilling fluid. However, being unable to reach the section target after more than two weeks of operation, the section was plugged back and a sidetrack was drilled using an oil based drilling fluid without encountering major operational problems. On the basis of the post-drill analysis of drilling data, well logs, drill cuttings and borehole cavings sampled from the well, this paper describes how the complex combination of drilling fluid salt concentration and geological constraints may be utilized to ensure successful future drilling operations in this part of the North Sea. Cuttings and preserved cavings collected during the drilling operation were selected from several depth intervals identified as potentially troublesome from drilling experience and log data. Determination of cuttings mineralogy enabled better prediction of how the time dependency of the stable drilling fluid density window is influenced by interaction between the shale and the drilling fluid. Mechanical strength is a key input parameter when predicting borehole stability. Dedicated rock mechanical punch measurements on cavings were used to confirm the prediction of strength from log data alone. Examination of caving surfaces revealed the possible presence of in-situ fractured rock. Such fractures would require special measures while drilling to maintain stability. Subsequently a borehole stability sensitivity analysis was performed focusing on time dependent stability in the shale formations. The analysis used cuttings and cavings properties and logs as input. In particular, the modelling showed how the optimum KCl concentration in the drilling fluid changes with depth. The modelling further identified a relatively large sensitivity towards borehole inclination – even at fairly small inclinations. This paper thus illustrates the significance of properly accounting for rock mechanical aspects when planning new wells.

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