Implementing a Strategy for Shale as Well Barrier in New Wells

Abstract

Shale is an effective barrier material. It has a proven track record of acting as a seal (barrier) for oil and gas reservoirs for millions of years. Shale with high clay content and especially high smectite has low permeability, in the nanodarcy range, compared to standard class G laboratory cement that has permeability in the 10–20 microdarcy range. Weak ductile shales will also have a self-healing behavior should fractures be induced at some point. Shale is approved by regulators to be used as well barriers and part of permanent plug and abandonment (P&A) for oil and gas wells. Examples of regulations are Norsok D-010, 2013 in Norway and O&G UK, 2012. In Norsok D-010, one suggests the formation of shale barriers to happen due to creep in ductile shales. Creep occurs in many materials and is observed as deformation under constant load and is also well described in rock mechanics literature. In a previous paper (Kristiansen et al., 2018), it was discussed how shale can be activated as a barrier to form around the wellbore in some shale types. This can be done by inducing a pressure drop in the open annulus (rapid drawdown), by heating the shale by a couple of hundred degrees Celsius, or by chemical processes. In that paper, the process found most effective and practical at that time was demonstrated: the activation of shale barriers with a rapid pressure drop in the annulus. It was also shown that the barrier can be verified days after by standard verification methods used in the industry (pressure testing and bond logging). The shale barrier verification criteria are analogous to cement barriers. In this paper we share the experience from the implementation of a strategy to use shale as well barriers in new wells at Valhall and a second field, Ula, around 100 km away. The method used to activate the shale barriers has revealed some challenges from a well control point of view, but it has also shown that waiting a couple of weeks, or in some cases a couple of months, shale barriers are forming with the same quality as when they were activated or logged later as part of P&A. From this work it can be concluded that the shale barriers logged during P&A are, in some cases, in place only weeks or months after the wells have been drilled. The activation seems to induce an acceleration of time-dependent deformation that will naturally happen over longer time and is consistent with rock mechanics principles of time-dependent deformations in rocks (like creep and consolidation).