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 sidetracking in order to drill into a fresh area of the reservoir. North Sea regulatory requirements dictate that proper well abandonment procedures require double barriers to be in place to avoid reservoir leakage. With the 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 correlations of this bonding pattern with shales, known to cause problems during drilling, indicates that the cause of the good bond response is that the shale has sealed off the annular region.
Logging and pressure testing sealed off zones in a number of wells allowed the bond log response to be qualified for a certain formation. In this manner it 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 30 wells, proving non-destructively that high quality natural annular barriers had formed resulting in elimination of complex remedial work and substantial cost savings.
Stephen holds a Masters Degree in Natural Sciences from Cambridge University, England. He worked with Schlumberger wireline from 1984 until 1998as a Field engineer in Norway, the Middle East and the USA, then in Technical management in the USA and Venezuela, and as Country manager in Peru. From 1998 – 2007 he worked with Norsk Hydro as Formation Evaluation Adviser, and is presently discipline leader in Petrophysics with StatoilHydro.