Growing riches

Fertilisers are increasing productivity not only on the farm, but also at offshore oil reservoirs, Thor Haegh tells Cath O’Driscoll

Two hundred kilometres off the coast of Norway and roughly 300km north of Trondheim, an unusual experiment is in progress in the chilly waters of the Norwegian Sea. Here, close to the Arctic Circle, the Norwegian state oil company Statoil is busily extracting oil from an estimated 80m m3 reservoir beneath the seafloor using a mineral fertiliser. The results of the Norne oil field experiment - to inject nitrate-based chemicals such as sodium nitrate along with seawater to pump oil from the underground reservoir - are still a couple of years away from disclosure, but so far all appears to be going well. If nitrates show the productivity gains already seen in earlier laboratory tests, then Norne’s position as Norway’s most cost-effective offshore oil and natural gas recovery project looks unrivalled. 

The idea is a relatively simple one. Adding nitrates encourages the proliferation of nitrate-reducing bacteria in the oil-seawater mixture. When present in the appropriate numbers these bacteria help to loosen oil from the rocks containing the reservoir - as has already been clearly demonstrated in earlier laboratory tests. Only 30 to 50 per cent of the oil entrapped by porous rocks such as sandstone (as at Norne) and limestone is generally recovered. The financial incentive to improve recovery efficiencies by even a tiny amount is huge. Now priced at over $30 (ca ?20) a barrel, oil is an expensive commodity. Recovery of a mere one per cent more oil from the Norne site - probably a gross underestimate - could be worth as much as $100m (ca ?70m) over the production period. By comparison, Statoil injects about 700t of dry-weight sodium nitrate every year into the Norne field, at less than a tenth of this cost overall. 

Norsk Hydro, the supplier of the nitrate-based formulation, has been making nitrate salts since 1905 - most recently at its site in Porsgrunn, Norway. However, the sodium nitrate used at the Norne field comes from Chile, high up in the Andes mountains. The nitrates used by the oil industry are formulated somewhat differently from common fertilisers, Hydro’s project manager for the technology, Thor Haegh, is keen to point out. For this application Hydro supplies Statoil with specially tailored solutions of particle-free solvent, or as bagged quality material that can be readily dissolved. 

But the idea of using ’fertilisers’ in this way is far from new, says Haegh: ’The technology has been known for years and there are several patents in the area, using nitrates in combination with other chemicals such as nitrite and molybdate’. 

The use of nitrate in offshore applications came about as the result of a long-term R&D project, initiated by Statoil and involving specialists from Statoil, Hydro and the University of Bergen. The initial goal was to find a way of addressing the problem of reservoir ’souring’ - caused by the production of sulphides by sulphate-reducing bacteria in seawater. Build-up of sulphides in this way can often lead to corrosion of steel pipelines and is a major headache for the oil industry. 

The solution that the researchers came up with was essentially an extension of another application for Hydro’s nitrate formulations - in sewage cleans up. Patented in 1989, HydroCare technology involves adding carefully measured doses of nitrates to sewage to remove hydrogen sulphide, which is not only notoriously foul-smelling but also poisonous and corrosive. The system is now implemented at several hundred sewage systems in Europe, the US, Asia and Australia. 

The nitrates used in offshore applications work along much the same principle. In changing the bacterial flora of the oil-seawater mixture to encourage the growth of nitrate-reducing bacteria, they also deter growth of deleterious sulphate-reducing bacteria. Statoil began using calcium nitrate at its Veslefrikk field about three years ago, leading to a significant reduction in corrosion of the water injection pipeline between the two platforms. Encouraged by these results, Statoil went on to apply the technology at its Gullfaks field to treat reservoir souring. Statoil is expected to publish the results in the very near future. 

The traditional way of removing sulphides from oil reservoirs involves sterilising the seawater with ultraviolet light, removing oxygen and adding chemical biocides to kill the bacteria. From an environmental standpoint, nitrates are much friendlier, Haegh elaborates: ’Biocides seem to lose their effectiveness some distance from the well bore unless they are injected in very high doses. Nitrates are not only non-toxic but they are also substantially cheaper. Instead of fighting nature they stimulate nature to ’cure’ the problem of sulphide production’. Other oil companies, such as BP, are already trying out the technology for themselves, and have placed orders for large amounts of nitrates. 

At the Norne field, Statoil’s special adviser Egil Sunde has taken the idea of using nitrates a step further. Several researchers had previously been interested in the possibility of injecting sodium nitrate (along with other nutrients such as phosphates) to increase the concentrations of nitrate-reducing bacteria for enhanced oil recovery. Sodium nitrate is preferred to calcium nitrate, which would react with phosphates to form scale that might impede the flow of oil along pipelines, and clog reservoirs. 

Then in 1991 Sunde patented a method for encouraging the growth of high concentrations of nitrate-reducing bacteria without having to remove the oxygen from seawater. Stainless steel pipes carry untreated seawater, along with sodium nitrate, into the Norne reservoir without the typical deaeration requirement. The result is a high concentration of nitrate-reducing bacteria which is expected to loosen the oil from rocks, together with a smaller population of facultative anaerobic bacteria (which can survive with or without oxygen) that should continue to alleviate the problem of sulphides. There is still no substantial proof that this technology itself enhances oil recovery, though the Norweigan Petroleum Directorate has recently awarded a prize to the Norne field and to Sunde for increased recovery. 

But the opportunities for exploiting nitrates as oil field chemicals don’t stop there. Researchers at Norsk Hydro’s R&D centres at Porsgrunn and Bergen, Norway, are currently collaborating with Hydro Gas and Chemicals to develop a drilling fluid based on calcium nitrates. The purpose of drilling fluids is to lubricate the boring, transport drill cuttings out of the well bore and to stabilise the well. They generally include weighting compounds such as barite (barium sulphate, BaSO4) to balance the fluid pressure within the hydrocarbon reservoir and prevent a catastrophic blow-out. 

Instead, Hydro researchers are attempting to blend calcium nitrate (high density) into oil or an ester to produce a so-called inverted emulsion which reduces the need for particulate weighting agents. The advantage of this particle free/low particle containing fluid is reduced ’sag’ problems, and a larger capacity to absorb drilling particles. It is environmentally friendly because all of the constituents are easily degradable. Much work still remains to be done, Haegh concedes, but a fully tested drilling fluid should be on the market within a year. 

Source: Chemistry in Britain

Acknowledgements

Thor Haegh