An oil exploration company has said it will not inject dilute acid to stimulate flow at any of its wells in the Weald basin in southern England.
UK Oil and Gas plc (UKOG) said the process, which it described as matrix acidisation, could have a “significantly detrimental effect” on oil and gas flows.
The company’s announcement has prompted questions about the previous use of acid at UKOG sites, as well as how acid operations are regulated and UKOG’s plans to improve oil flow in the Weald.
Matrix acidising aims to increase the productivity of a well by pumping dilute acid into oil-bearing rocks surrounding the borehole. The acid enlarges natural fractures and fissures, releasing hydrocarbons more readily. But the pressure is not high enough to fracture the rock.
UKOG is currently targeting the Kimmeridge and Portland formations at Broadford Bridge in West Sussex and Horse Hill in Surrey. It said in a statement:
“We have no intention to use this process, known as matrix acidisation, in the future at Horse Hill, Broadford Bridge or other Weald Basin wells with the same targets.”
In the statement, the company said the decision was a result of what it had learned at Broadford Bridge. Recent studies from the well had shown that:
“because of the chemical composition of the rocks, the acidisation process can have a significantly detrimental effect on the ability of oil and gas to flow into the well.”
The statement said:
“Crucially, the Portland and Kimmeridge rocks also contain significant amounts of microscopic clay particles bound within the rock. Consequently, as the lime dissolves, some of these clay particles are released into flow paths within the rock.
“The ‘free’ clay particles then migrate towards the well and combine, forming a clay “sludge”, restricting or blocking some of the flow pathways, which can significantly reduce the well’s performance. Once blocked, the flow pathways cannot be reopened.
“It makes neither commercial or technical sense for UKOG to utilise this acidisation process, as its future use could seriously reduce the significant natural flow potential and the good flows of hydrocarbons we expect.”
The statement is significant because it provides some more detail about what happened at Broadford Bridge last year. The well was suspended in March 2018 after the company described flows as “sub-commercial”. Link here
But the statement also raises a whole new set of questions.
1 Did UKOG use matrix acidising?
The UKOG statement implies that matrix acidisation may have been used at Broadford Bridge. A coiled tubing unit was used at the site, which could indicate that the process was carried out. But a company review of the Broadford Bridge operation said “acidisation was not selectively administered to any specific limestone horizon”.
DrillOrDrop asked UKOG twice to clarify whether or not it carried out matrix acidising at Broadford Bridge. UKOG did not reply.
2 Did UKOG have permission for matrix acidisation?
UKOG said in its statement that the process that it referred to as matrix acidisation “was fully permitted and approved by the Environment Agency”.
We asked the Environment Agency (EA), which is responsible for regulating the use of acid, whether UKOG had permission for matrix acidisation. It said:
“No such groundwater activity was applied for and was therefore not included within the environmental permit for the site.”
So, if UKOG did use matrix acidisation at Broadford Bridge it did it without permission from the EA.
Residents had told the EA that they believed that matrix acidising was being used at Broadford Bridge. We asked the EA whether the process was used at the site. It replied:
“No. Acid washing was carried out during the well test at Broadford Bridge Well Site.”
Acid washing is a different process, which the EA says uses small quantities of a weak acid at low pressure to clean the well of drilling debris or scale (link). It is not intended to stimulate the flow of hydrocarbons. Crucially, the pressure is enough just to move the acid down the well and a short distance into the formation.
But if UKOG used an acid wash, and not matrix acidisation, it raises more questions.
3 How would the Environment Agency know that only an acid wash was used?
The EA assessed the acid wash at Broadford Bridge as “de minimis” to ground water. This means the input of the acid would be so small that it represented no present or future danger to the quality of groundwater.
Under the de minimis rules, an activity is excluded from an environmental permit and is, at best, loosely regulated.
Based on responses about the de minimis rule from other sites, we know that the Environment Agency does not require an operator to provide the following information:
- Distance the acid goes into the formation
- Number of intervals or zones in the formation that require acid wash
- Total volume of acid wash and additives
- Number of treatments
- Pressures used to pump the acid
- Dates and times when the acid wash was carried out
Hypothetically, without this information, would the Environment Agency know if a company used greater pressure or larger volumes of acid or if it injected the acid further into the rock formation than would be expected for an acid wash?
The EA has told campaigners that it would rely on information from site visits, such as the volume of stored acid. But campaigners argue that the EA does not have enough staff to make many site visits. Last month, the EA said its most recent visit to the UKOG site at Horse Hill was on 23 August 2018. More details on regulatory loopholes at Weald oil sites
4 How did UKOG learn the lessons of matrix acidisation?
If UKOG did not use matrix acidisation, as the Environment Agency told us, how did the company know that matrix acidisation could have a “detrimental effect” on oil and gas flows into the well?
DrillOrDrop put this question to the company. It did not reply.
5 Did acid wash block the flow pathways at Broadford Bridge?
The UKOG statement referred separately to acid wash.
UKOG said of this process:
“a small volume of dilute acid, in our case 5% acetic acid (i.e. domestic strength vinegar) is left to soak for a couple of hours solely over the perforated zone”.
If UKOG used only an acid wash at Broadford Bridge, did this operation cause the problems with the ‘free’ clay particles that formed a sludge, blocking the flow pathways and reducing the well’s performance?
DrillOrDrop also put this question to UKOG. It did not reply.
6 Could flow problems happen elsewhere?
UKOG described acid wash as “standard global oil field practice” used to dissolve lime in concrete debris left over from the drilling process.
If it was acid wash, rather than matrix acidisation, that blocked the flow at Broadford Bridge, how could UKOG be sure this would not happen at other wells in the Weald and elsewhere in the UK?
DrillOrDrop put this question to UKOG. It did not reply.
7 How would UKOG stimulate its wells without matrix acidisation?
A report commissioned by UKOG in 2016 concluded:
“Kimmeridge Limestone oil likely requires “stimulation” to flow to the surface at commercial rates”.
The report’s author, the global professional services company, EY, said:
“The primary stimulation method for wells in limestone rock formations is acidizing.”
Following the flow tests at Broadford Bridge, UKOG concluded that “other reservoir stimulation techniques” would need to be considered in future.
If UKOG has ruled out matrix acidisation, what other options are available to stimulate the flow of oil?
The company has previously said it was not carrying out hydraulic fracturing and did not intend to use this process in future.
But would UKOG also rule out acid fracking? This is defined by the Environment Agency as pumping acid into surrounding oil-bearing rocks at pressures high enough to open new fractures and fissures.
And would UKOG consider the use of fishbones, a new technique designed to deliver super-targeted jets of acid to fracture rocks?
We asked UKOG what other options it was now considering. The company did not reply.
8 Why seek to squeeze?
UKOG’s statement said it would use 5% acetic acid for the acid wash at a proposed new well site at Dunsfold, near Guildford in Surrey.
This is a different acid at a lower concentration than that previously used at other UKOG sites. We know that UKOG had permission from the EA to use 15% hydrochloric acid at both Broadford Bridge and Horse Hill.
There is photographic evidence from Horse Hill of tanks of Protekt 15, the proprietary name for 15% hydrochloric acid. The company told the Environment Agency it anticipated using a total of 95m3 over a maximum of three operations.
The Horse Hill site has permission for acid wash and another process, which UKOG called an acid squeeze. The company’s waste management plan for Horse Hill said 15% hydrochloric acid would be “squeezed into the formation porous spaces”.
The EA does not formally use the term “squeeze”. But the EA said in guidance that acid squeeze was another term for matrix acidisation or fracture acidisation.
As at Broadford Bridge, the Environment Agency decided UKOG’s acid operations at Horse Hill were de minimis for groundwater.
Campaigners against unconventional oil and gas operations have questioned whether the volume of acid and proposed operations at Horse Hill represented more than an acid wash. The campaign group, Brockham Oil Watch, asked:
“If UKOG have no intention to inject diluted acid into rock formations at Horse Hill, why did it seek permission from the EA to inject at pressure 15% hydrochloric acid so it is “squeezed into the formation porous spaces?”
9 Could an acid wash stimulate a well?
Ada Zaffina, of Brockham Oil Watch, said EA guidance does not define clearly the difference between acid wash and stimulation of the rock formation using in acid.
“We are concerned that matrix acidisation seems to be confused with acid wash in the UK, when in places such as California and Florida, it is considered a form of advanced stimulation, commonly referred to as fracking.”
The energy minister, Claire Perry, said “acidisation refers to a number of techniques used to clean wells to improve productivity”.
But Brockham Oil Watch said:
“Acidisation is also used as a stimulation method, to dissolve rocks in order to create pathways for oil and gas to flow.”
Brockham Oil Watch pointed to the decision document for the Horse Hill environmental permit, which said any operations above fracture pressure would require a separate permit. So, did this suggest that matrix acidisation – stimulation using pressure below fracture levels – was allowed?
At Brockham, the EA said the volume of the acid wash should be 45m3, per well, which is relatively small. But the pressure could be anything up to the level needed to fracture rocks. If the interval to be treated was short the operation could easily stimulate the formation, Brockham Oil Watch said.
10 Is acid wash used in water wells?
The UKOG statement said acid wash was standard practice in both global oil fields and limestone water wells.
But campaigner and researcher, Kathryn McWhirter described the comparison as “spurious”.
“Acidising is very rarely used for water wells. I have interviewed engineers working in the water industry. Water wells are typically much shallower than oil or gas wells.
“When, rarely, they are acidised, water wells are treated with only hydrochloric acid, no partner chemicals. And crucially, water wells are drilled into a nice clean aquifer, whereas oil and gas wells are drilled into formations filled with fossil fuel, along with super-salty brine laced with heavy metals and possibly naturally occurring radioactive materials.
“If I were a water engineer, I would object most strongly to this comparison.”