A Deeper Understanding of the Role of Salts and Creeping Formations in Well Integrity
Authors: Loizzo M. , Houghton R. D. , Zahmuwl A. H. , Tonning S. A. , Andersen L. W. 
Published: June 2024

Pre-salt basis host vast amounts of oil and gas and offer some of the most challenging drilling and abandonment environments in the world. Abandonment of the pre-salt Schooner Field in the UK Southern North Sea allowed a greater understanding of the effects of salt and creeping formations on cement and casing providing a behavioural insight into salt basins worldwide.

The Schooner Field is the first Carboniferous gas development in the North Sea. The field was discovered in 1987 by well 44/26-2, and gas production began in October 1996; in July 2019, the Department for Business, Energy and Industrial Strategy (BEIS) approved the decommissioning plan for the Schooner Field. The decommissioning scope included the permanent plugging of 12 (11 platform and 1 subsea) wells in the field.

All these 12 wells were successfully abandoned by DNO. Three Zones of Flow Potential were defined and had to be isolated to prevent fluids, either methane or brine, from flowing to the seabed. The shallowest zone, the Bunter Sandstone, was particularly challenging because of a plan to use the regional aquifer for carbon dioxide storage. To enable this, a primary goal of the project was ensuring isolation to 1,000 psi (6.9 MPa) above current pressure.

A key component of the abandonment strategy was the acquisition of high-resolution, multi-physics cement evaluation wireline logs. The use of an advanced ultrasonic imaging tool run together with an array borehole compensated CBL provided a superb dataset from 10 wells, which could be fed to a Bayesian inversion algorithm to characterize defects and annular materials. This approach has reduced uncertainty and allowed barriers to be qualified within 2 hours of the data being uploaded.

The advanced, quantitative analysis of log data allowed us to understand the dynamics of the interaction between cement and evaporites, and the ability of creeping salts to provide durable and robust well integrity – alone or in combination with cement.Firstly, the Rot Halite, a sequence of evaporites ~100 m thick, is an excellent caprock for the Bunter Sandstone, supporting projects that plan to inject CO2 in the regional aquifer.

Closure stress exerted by carnallite, an evaporite mineral that forms in both the Rot Halite and Zechstein Group, is less than the overburden and the far field horizontal stresses. This means the pressure rating of the creeping rock is lower than halite’s. Coupled with its very high solubility, this makes carnallite less suitable as a caprock than halite or polyhalite.

Secondly, high casing ovality in some of the wells across the Rot Halite was triggered by a hydraulic fracture opening along the high side of the borehole, which destabilized the near-wellbore stress distribution. This behavior could explain collapse in uncemented sections of casing strings run through salt, and the rule of cementing engineering that salt layer must be cemented, even though halite can provide an excellent seal around the pipe. The risk of fracturing seems to increase with well deviation, even though this correlation is still poorly understood.

Thirdly, reservoir isolation across the halite layers in the Zechstein Group has always been perfect, because of the joint effect of creeping evaporites and sulfate attack, which densifies cement and drains water layers left behind during cement setting. Log behavior has been consistent across wells in the Southern North Sea, leaving open the question on whether logs are really required to qualify cement across the upper Zechstein, provided fluid-migration chimneys can be excluded.