Identification and Qualification of Shale Annular Barriers Using Wireline Logs During Plug and Abandonment Operations

Abstract

It has long been recognised that during and after drilling through certain formations, the rock moves inward and begins to close off the well. Normally this phenomenon is considered undesirable since it can cause problems for drilling and casing running. It can however be put to good use as the mechanism to create an annular barrier behind casing. In order to extend the life of a number of North Sea brown fields many well slots on production platforms and sub-sea templates are being re-used. This process involves permanent plug and abandonment of the old well track prior to sidetrack drilling into a fresh area of the reservoir. Norwegian Continental Shelf (NCS) regulatory requirements dictate that compliant procedures for well abandonment require the establishment of double barriers to avoid leakage from the reservoir. With a shortage of sufficient traditional cement barriers these wells often need costly remedial work in order to meet abandonment requirements. Traditional sonic and ultrasonic azimuthal bond logging provides information on the material immediately behind the casing. Many such bond logs show solid material behind the casing far above the theoretical cement top. Clear correlation of this bonding pattern to shales, known to cause problems during drilling, indicates that the shale has sealed off the annular region and that it is the presence of such formation material that results in a good bond log response Logging and pressure testing sealed off zones in a number of wells allowed the bond log response to be qualified for a certain formation without further pressure testing. In this manner logs can provide a clear answer of whether shale successfully seals off certain zones and consequently provides a natural annular barrier. This technique has been employed successfully on over 40 wells, proving non-destructively that high quality natural annular barriers had formed, resulting in elimination of complex remedial work and substantial cost savings.

An Evidence-Based Approach to Well-Integrity Risk Management

“…Creeping formations, for example, tend to experience more or less fast viscoelastic or viscoplastic deformation that can bridge the annular gap or increase radial stresses on the cement sheath, thereby preventing the occurrence of microannuli or resealing them; some shales, halites, and possibly ice belong to this category and at least one such rock, the Hordaland Green Clay, has been qualified as a well barrier in Norway (Williams et al 2009). …”

Types of Permanent Plugging Materials

“…In addition, performed extended leak off (XLOT) tests show qualified seals [26]. The question is how this could be happened?…”

Shale Gas and Shale Oil

“…The existence of such fracture systems can be excluded for plastic materials. For these types of materials selfhealing effects are known and described in the literature [66][67][68]. Plastic behavior can be assumed for unconsolidated clay layers (in Northern Germany observed for clay layers of tertiary age, e.g., Rupelian) and for salt layers (in Northern Germany observed, e.g., in Muschelkalk, Buntsandstein, and in particular Zechstein) [14].…”