On Saturday 18 June 2011 at around 6am, the wellhead of a disused brine well on Halite’s land failed. On this page you will find information about how and why the incident happened and what Halite has done in response.
To read an executive summary of the technical assessment report assembled by Halite’s geology consultants MottMacDonald, click here.
To view and download the full technical assessment report, click here.
We have prepared the Q&A below looking at some key questions and responses we have been asked in relation to the incident.
The large rock salt deposits at Preesall have been the target of brine extraction since their discovery in the 1870s. Fresh water was pumped down boreholes which dissolved the rock salt, creating brine which was then brought to the surface for industrial use.
Within the land owned by Halite at Preesall, there are over 100 of these abandoned and historic brine wells. The casings of the boreholes are visible above ground, protruding usually at between 300mm and 1.2m high. They have a bolted flange, or disc shaped top which secures the wellhead.
Brine well caverns in this area are often found at depths of over 150m and vary in their size and shape. The original design and construction monitoring was very basic and salt was removed up to the level of the mudstone above it. This initial method adopted by ICI resulted in collapse of the ground at surface level as the strong rock salt holding it up had all been removed. With time, a method of extracting brine was developed which left a thickness of salt in the cavern roof which would therefore not threaten the stability of the ground. The abandoned brine wells at Preesall therefore have a combination of either mudstone, or ‘marl’ roofs (from early mining activity) or salt roofs from later activity.
Most of the wells have been filled with brine up to ground level in order to reduce the likelihood of the caverns being sealed due to what is known as ‘salt creep’.
In the early morning of 18 June, brine and air was ejected from the area immediately surrounding Brine Well 45 (all historic brine wells are numbered for monitoring purposes) in what is commonly referred to as a ‘blow out’. The brine mixed with the soil around the casing and flowed down hill to Back Lane causing the area to flood and the Lane to be closed. The air and brine flow eroded the soil surrounding the casing causing a void to develop which then filled with brine. At first, air bubbled vigorously to the surface before gradually subsiding until stopping altogether at around 9pm that evening.
As soon as the event was made known to Halite, emergency mitigation and management measures were undertaken to control the flow of the brine and make the area safe and secure. These included erecting security fencing, digging a trench in the field above Back Lane to intercept the brine and soil flow, beginning clean-up operations and arranging for both geological and environmental advisors to be on site. We quickly began working closely and co-operating fully with all of the relevant agencies including the Environment Agency, Lancashire County Council and Wyre Borough Council.
Once it was safe to do so, scaffolding was erected on the site in order to gain access to the wellhead. The void left by the ground collapse was then filled with granular material to allow safe ground access to the wellhead. On 18 July, the wellhead had fully depressurised and a detailed technical assessment was able to be carried out. The investigation was led by senior geologist, Colin Harding, who is a divisional director at Mott MacDonald, an international management, engineering and development consulting firm.
A range of investigation techniques was used. It was vitally important to be able to assess the state of the cavern, the borehole casing and the stability of the surface. The findings of this work were then compared to information gathered on brine well 45 before the incident occurred. Historic records date back many years. Not all of the old records are extensive or easy to interpret however, as part of its ongoing programme of monitoring and maintenance, Halite had undertaken a high quality sonar survey of the brine well in June 2010 which meant that there was an up to date and accurate record of the brine well cavern dimensions and depth of the casing at that time.
The information available on record showed Brine Well 45 to be a 30m high bowl shaped cavern. The borehole casing (which measures 10 inches across) extended well over 20m into the cavern and was within approximately 6m of the cavern floor. Records show that in February 1911, air was pumped down the casing in order to create an air lock which was designed to protect the cavern roof from the brine and therefore help maintain its stability. Twenty-six years later tests indicated that the air lock had a volume of around 1500m³. In 2010, observation showed that the borehole casing was filled with brine up to about 10m below the flange creating a head of brine at least 180m in height and resulting in the development of high pressure in the airlock.
Data taken before and after the incident shows that the important cavern features had not altered. Both the casing depth and the cavern floor remain intact and unchanged. The cause of the incident was not therefore the result of geological changes or cavern instability.
CCTV was used to view the borehole casing. A camera was able to reach 190m below ground to the current brine level. From this activity it was possible to see that a short section of the casing was distorted and, at one point, had ruptured inward as a result of corrosion. This had occurred in the part of the casing in the air lock where the cast iron casing was exposed to air and therefore corrosion was possible. However, an assessment of the likely corrosion environment in the airlock showed that corrosion would be expected to occur at a rate of about 0.01mm per year and only from the outside as the inside of the casing was protected as it was filled with brine. The casing measured 6mm thick and so it would reasonably have been expected to have a life span of some 600 years. For failure due to corrosion to have occurred after only 100 years there would have to have been some damage caused to the casing either when it was being installed, or subsequently when it was in operation. This could have reduced its thickness and therefore made the corrosion process much quicker than calculated.
The ruptured casing in the airlock released high pressure air into the wellhead but the flange was able to withstand the increased pressure – photographs taken after the incident show that the flange was in place and the brine and air bubbling on either side.
The highly pressurised brine and air entered the gap between the inner and outer casings, called the annulus. It is not possible to be certain at exactly where in the casing this occurred. Either, the brine and air travelled back to the bottom of the casing along the annulus and from that moved up to the surface around the casing or, the more likely explanation is that it found a gap in the casing further up at approximately 24m below the surface.
Whilst our investigation was able to identify as the primary reason for the incident occurring the corrosion of the casing in the airlock as a likely result of damage during installation or operation, it has not been possible to identify what caused the casing to rupture further up and become detached. One of the possible explanations for this is that the casing was rotated and/or pulled by third party interference at the wellhead. There was forensic evidence obtained which was consistent with this being the cause of the incident. Safety and security is our number one priority and any information indicating a deliberate attempt to cause a dangerous incident of this kind was and will be treated extremely seriously. The board of Halite felt it right, proper and the responsible thing to do to share information that came to light as a result of this work with the police. The strength of the information provided to the Police by Halite resulted in a case being opened and an investigation being carried out by them.
The Police have stated that they have found no verifiable evidence either way to indicate whether a crime has or has not been committed. We are very grateful to Lancashire Constabulary for their efforts at a senior level to investigate this information and our concerns and appreciate that from their perspective, there is insufficient evidence at this stage to take matters further.
It is not possible to share further details publicly as, should further information come to light, the Police investigations will be re-opened.
Since taking responsibility for the land an ongoing maintenance and monitoring programme was initiated which involved regular checking of wellheads for, amongst other things, build up of pressure. In light of this incident, the programme has been thoroughly reviewed and additional activity will be undertaken including a full risk assessment on all existing brine wells. Further investigations will be undertaken so that all of the caverns with air locks are identified and prioritised.
Onsite security has been stepped up to provide visible reassurance to the local community of Halite’s commitment to security and safety. A 24-hour (check) ranger patrol has been introduced to monitor land in Halite’s ownership, paying particular attention to areas with wellheads from historic workings.
The investigation undertaken into the incident has shown that the cavern at Brine Well 45 is stable and the cavern roof and cavern floor remain intact and unchanged. It can therefore be concluded that cavern instability neither caused, nor was triggered by the incident. There is therefore no reason to be concerned for any future cavern collapse or ground subsidence to occur in the adjacent area.
Halite has now published a report prepared at the conclusion of an internal investigation under the Health & Safety at Work Act 1974. A statement from the company is also available on the news section of this site.
The full brine well 45 HSE report is available here, and as are appendices to the report: Report on the assesment of the brine well 45 incident and the brine well monitoring and risk management strategy.