Originally, shallow water flow was severely hindering further drilling in a North Sea exploration well. The water depth was 111.5m. The 20" casing was set at 590m; above a shallow gas zone. After mounting the Blow Out Preventer (BOP), drilling the 17 ½" section started and continued for a few 100 meters. The wellhead was routinely inspected using an ROV (Remotely Operated Vessel). After some time, a tiny flow was observed around the wellhead. The flow increased in strength and later a large wash out area was observed, and further drilling had to be terminated. Most likely, the water flow originated from a zone at a depth of 172m MSL.
To cure the shallow water flow it was decided to grout cement on the outside of the casings. Regular well cements was not desirable to use since these cements are somewhat retarded in itself by mineralogical composition. It was decided to use standard construction industry cement with a very short curing time. Before the grouting could start the BOP had to be removed. Because of the shallow gas zone underneath the 20" casing, two barriers had to be included in the well for well control. Two packers were used, one drillable and one retrievable. The BOP was removed and the grouting operation was performed successfully. No water flow or gas flow were observed while the cement was setting. After the flow was cured drilling resumed, first by drilling and retrieving the packers, then continuing to drill the 12 ¼" section.
The lessons learned from this particular well are to focus on using rapid hardening industry cement when cementing the conductor and a foamed cement system around the surface casing. Furthermore, a riserless mud return system has been used on later wells to be able to drill with weighted drilling fluid prior to setting the riser. These solutions to prevent shallow water flow are further discussed and experiences from later wells are presented in the following paper.
Shallow water flow (SWF) is known to be a major and expensive problem for drilling exploration wells. Alberty et al. (1999) indentified SWF to be the result of four different mechanisms: Drilling of geopressured sands in surface casing intervals, transmission of geopressure through cement channels, induced storage and induced fractures. Geopressured sands are the most common cause of SWF, and can simplified be explained as overpressure in a sand generated by two different compaction mechanisms: Compaction disequilibrium and differential compaction. Drilling of geopressured sands is the mechanism most operators are addressing when referring to SWF and it is known to be the most damaging mechanism. Drilling of these zones is subject to high risk of large washouts and caves, formation compaction and collapse, and subsequent buckling of casing because of the common practice to drill conductor and surface casing intervals riserless.
API (2002) recognized three different scenarios that are related to development of transmission of geopressure through flow channels in and around the cement surrounding the casing: Insufficient primary cement job result, flow occurring during cement operation and flow occurring before the cement has hardened. The power of the flow at the mudline will typically increase because particles in the fluid filling the cement channels, usually drilling fluid, settle with time. Alberty et al. (1999) describes this mechanism to be difficult to diagnose due to time delay influenced by the settling solids, and failure may occur long after the cement has set. Induced fractures are related to lost circulation failures as a result of small difference between drilling fluid and formation strength in deepwater areas, and there is no need for any sands to be present. Induced storage typically takes place during surface casing intervals in permeable and porous silts, sands and even shale due to charging of formations generated by pressure from the drilling fluid column.