The operator of the Wressle oil site near Scunthorpe has confirmed it no longer plans to use acidisation to improve production.
Egdon Resources will now use a technique described by regulators as “small-scale hydraulic fracturing activity” as the first option to stimulate oil flows.
For the past six years, Egdon Resources had said it intended to inject dilute hydrofluoric acid into the well.
Acidisation would, the company previously said, dissolve fine particles that had blocked the pores in the oil-bearing Ashover Grit sandstone reservoir, known as skin problems.
But Egdon has now abandoned the process because it may reduce the effectiveness of proppant squeeze, an alternative form of stimulation planned at Wressle.
Proppant squeeze injects a slurry of gelled liquid and proppants under pressure to open fractures in the surrounding rock formation. It was previously a second option if acidisation did not work and was highly controversial with opponents of onshore oil production.
Egdon told local representatives recently:
“ongoing evaluation has indicated that the proposed acidisation process may adversely impact the proppant squeeze operation. Given this, we can now advise that we are not now undertaking the acidisation operation as part of the work to enable production at Wressle.”
Asked why the company had changed its mind, a spokesperson told DrillOrDrop today:
“There is a low risk of the mud acid deconsolidating some of the sandstone close to the perforations which could impact the effectiveness of the proppant squeeze – it has therefore been concluded that the proppant squeeze alone would be the most effective treatment to deal with the skin issue in the Ashover Grit whilst simplifying operations.”
The spokesperson said:
“As part of our detailed pre-operational planning we review all planned activity with our contractors and the proposed change to our operations came to light during early summer as part of this process.”
DrillOrDrop asked Egdon whether it intended to use acid at any stage of production. The spokesperson replied:
“No. Acids are not now intended to be used as part of the operations.”
Egdon is the second onshore company to state publicly it will not use acid to stimulate wells. In March 2019, UK Oil & Gas plc said it would not use acidisation in the Weald Basin in southern England because it said the process could “have a significantly detrimental effect on the ability of oil and gas to flow into the well”.
Opponents of Egdon’s operation at Wressle criticised the use of hydrofluoric acid, which is highly corrosive. They were particularly concerned about the risk of contamination to ground and surface water.
They were also alarmed at the prospect of proppant squeeze, now Egdon’s first option.
They have criticised proppant squeeze as “fracking by stealth” because it uses hydraulic fracturing techniques, while avoiding fracking regulations in the Infrastructure Act. It is not covered by the current moratorium on fracking in England.
The Environment Agency described the proposed operation at Wressle as “a small-scale hydraulic fracturing activity”.
Egdon said proppant squeeze was a “standard enhancement technique”. It planned to inject 150m3 of slurry in the operation at Wressle. This is 15% of the 1,000m3 per stage used to define associated hydraulic fracturing in the Infrastructure Act.
The process needs a hydraulic fracture plan (HFP), to be approved by the Oil & Gas Authority and the Environment Agency. But HFPs for operations with lower volumes require less information than those using larger volumes.
Egdon told DrillOrDrop it still planned to carry out a single proppant squeeze operation at Wressle. It said the volume and composition of the fluid would also remain the same, despite the decision not to use acidisation.
The environmental permit for Wressle said the hydraulic fracturing fluid to be used in the proppant squeeze was mainly a slurry of water, friction reducer and proppant (ceramic beads).
According to the permit, the treatment would comprise 20-30 tonnes of ceramic beads and 80m3-120m3 of gelled liquid. The operation would be carried out at a depth of 1,500m-1,700m.
The slurry would be injected at a surface pressure of 9,000psi, above formation fracture pressure, for 1-2 hours. The slurry would be forced 40m sideways into the formation and 20m above and below perforations in the wellbore. An estimated 50%-70% of proppant fluid would be retained within the formation, the permit said.
If the proppant squeeze does not sufficiently improve oil flow, Egdon said it would continue with its plan to drill a sidetrack of the main wellbore.
Elizabeth Williams, of Frack Free Lincolnshire, welcomed the decision not to use acidisation at Wressle:
“People feel a sense of relief that “hydrofluoric acid squeeze” is no longer intended at Wressle-1. For the last five years campaigners have raised urgent concerns about acidisation in all its forms. And to its credit the EA did listen (to some extent).”
But she said:
“There remains the element of considerable hazard that “proppant squeeze” at 9000psi will be delivered down a deviated “S well”. This is not a simple vertical operation and complex processes and procedures will be carried out directly beneath the town of Broughton. How can any critical incident or accident in the wellbore be responded to? We do not think that local Fire and Rescue Services are fully aware of the extent of this well stimulation. Has proppant squeeze previously been delivered this way onshore UK – with full regulatory scrutiny?
Ms Williams also asked questioned the acceptability of hydrocarbon exploration in the light of climate change science.
“We urge planners and policy-makers to make radical choices and move away from fossil fuel. We urge the Oil and Gas Industry to cut its losses and stop contributing to global ecological disaster.”
Updated 29/9/2020 to include reference and link to UK Oil & Gas plc not using acidisation and on 4/10/2020 to include reaction from Frack Free Lincolnshire