De-Risking a Plugged and Abandoned Offshore Well Using Coupled Reservoir Geomechanics Fracturing Modelling
Authors: Castillo Edgar , McCurdy Phil , Walters Dale , Nassir Mohammad , Littlejohn Derek 
Published: June 2020

This paper will describe a workflow undertaken to address potential risks highlighted by a review of a well abandonment plan. The specific issue is described by the potential for failure of a secondary abandonment plug, should the primary plug set relatively deeper had failed. In the case of these events, pressures were sufficient to generate a risk of uncontrolled fracture propagation at the second plug.

The initial high fracturing risk was evaluated based on a simple comparison of fracture pressure vs. reservoir pressure column analysis; e.g. the reservoir would deliver a pressure greater than the original fracture pressure at the second and shallower plug depth. To better evaluate the real fracture behaviour and the risk associated with failure of the first plug, a more comprehensive approach was deemed necessary.

The main objective of the more detailed study was to investigate the likelihood of fracture initiation, the associated height growth and containment within immediate plug depth formation and overburden. This would be based on the charging pressure and flow volume from the reservoir zone into the plug depth formation.

A comprehensive coupled reservoir-geomechanics-fracturing numerical approach was adopted. This included a dynamic fracturing calculation using a non-linear Barton-Bandis material model embedded in a coupled finite-element geomechanical and a multiphase thermal finite-difference flow simulator. In the coupled solution, the fracture propagation is controlled by the nonlinear fracture stiffness and the normal effective stress. The fracture permeability is a function of the fracture width resulting from the poroelastic stress-strain coupled calculations triggered by the reservoir inflow pressure.

The modelling results have shown that when a simple low shale permeability was assumed for the second plug and overburden formations, a fracture will rapidly develop vertically up to seabed due to the lower stresses at shallower depths in this low leakoff scenario.

However, it was noted that the worst case scenario (uncontrolled fracture growth) was for minimal permeable intervals up to the seabed. In reality the geology using more realistic lithological data showed that overburden has several permeable zones. Accounting for these intervals in the coupled model, the same fracture growth through tight zones was seen, but the growing fracture became vertically contained when reached the permeable intervals. A permeable layer acts as a pressure sink and causes the propagating fracture to lose the energy required for further vertical growth.

Sensitivities with overburden permeability, thickness and the estimated reservoir volume were performed. These illustrated that the fracture lateral extension below the overburden permeable sand is largest when the sand permeability and thickness are smaller in magnitudes, and that permeability is the dominant factor.

The modelling analysis served to de-risk the likely fracture propagation and containment in an abandoned well in the hypothetical case were failure of the first plug exposes high pressure at a second plug. Even if such failure occurs, it will be a safer (and more cost-effective) alternative, to leave the second plug as its current depth rather than to perform a well re-entry to remove and set it deeper.