Permanent Abandonment of a North Sea Well Using Unconsolidated Well-Plugging Material

Abstract

Summary 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.