Pioneering Large-Scale Nitrogen Injection

Nitrogen injection is one of many methods oil and gas companies have at their disposal for enhancing recovery at mature fields. The technical definition of enhanced oil recovery (EOR) is a reduction of oil saturation below the residual oil saturation. High oil viscosity can often require EOR be used as a primary tool to recover reserves, but for many fields EOR techniques are applied in order to boost secondary recovery.

EOR techniques become more challenging when applied offshore. Firstly, large well spacing compared to onshore poses an interesting logistical challenge of applying EOR techniques. Additionally, any EOR method must take into account the space available on production platforms for equipment installation and the higher investment costs.

Nitrogen injection has been used as an EOR method with great success at the Cantarell field. Pemex realized that nitrogen injection would be needed at Cantarell when the oil-gas contact level moved up to the position of 1,930m, and the water-oil contact moved to 480m from the original position of 3,200m. Reservoir pressure had dropped 60% to less than 1,520 psia from original pressure of 3,800 psia by 1996, and well productivity had dropped to 25%. If reservoir pressure were not stabilized, large amounts of oil would remain unrecoverable.

The strategy for developing nitrogen injection capabilities at the Cantarell field was ambitious, to say the least. In 1997, Pemex awarded the contract for a nitrogen plant to Compañía de Nitrógeno de Cantarell (CNC), a consortium of five companies: BOC Gases, Marubeni, Westcoast Energy, ICA Fluor and Linde, with the former leading the group. Built over 30 hectares, the four processing units each generated 300 Mcf/day of high purity nitrogen. Nitrogen was generated then compressed.

In order to utilize compressed nitrogen at the Cantarell field, it was piped over 85km to Pemex’s nitrogen injection platform, an eight-leg platform that injected nitrogen through seven injection wells drilled to around 1,400m.

The nitrogen injection plant was over ten times the size of the plant it replaced as the world’s largest. From the start of its operating life in 2000, the plant served to boost production at Cantarell from 1 million bbl/day to 1.6 million at its first utilization, reaching 1.9 million bbl/day in 2002. In 2004, production hit its historical peak of 2.1 million bbl/day, at that time the world’s second most productive field. After Cantarell began its decline, some of the nitrogen produced at the plant was piped to Ku-Maloob-Zaap in order to boost production at Mexico’s second-largest producing field.

Nitrogen was originally chosen over water injection for pressure maintenance. Pemex believed that, due to the field’s fractured nature, water injection would risk watering out producing wells in other parts of the field: a process called ‘water fingering’, in which water reaches the well before the oil. As a result, gas injection was considered the best option to restore the reservoir pressure.

Pemex performed a gas injection test at Cantarell in order to confirm the number and location of injection platforms needed. The company also aimed to assess necessary injection pressure at the wellhead, as well as the direction and angle of the new wells to be drilled.

A number of different types of gas can be used for increasing reservoir pressure including: natural methane gas, carbon dioxide, nitrogen and flue gas. Until 1970, natural gas was the primary tool for EOR, but after this date it simply became too expensive to use the gas for reinjection; it could instead be processed and sold. After running tests to ascertain the effects of using natural gas, Pemex determined that it would require 10%-20% more volume than nitrogen to produce the same results. The demand for natural gas in Mexico was an additional factor in deciding against its use for reinjection.

Pemex furthermore decided that CO2 would not be effective at the Cantarell field, as the pressure in the field was so low that the company feared carbon dioxide would not be miscible with the oil. In CO2 injection, gas is collected from reservoirs and transported to the well by pipeline. Unfortunately, there were no reserves of the gas close to the Cantarell field, which made the proposition of CO2 injection costly; the gas would have to be recovered from turbine exhaust or flue gas streams. Because CO2 has a lower compressibility and greater density than nitrogen, Pemex would have required larger injection wells and potentially even more injection platforms.

Nitrogen was eventually chosen for its advantages over other gases, including its non-corrosiveness and the fact that it does not contribute to the greenhouse effect if released into the atmosphere. However, nitrogen injection has also been seen as having a negative impact on Cantarell’s high gas flaring levels, as the nitrogen-rich associated natural gas surpasses Pemex’s processing capacity. The CNH issued regulations in 2009 on reducing gas flaring in Mexico, and especially in Cantarell. Pemex reduced the average gas flaring level in the Cantarell field considerably from 281 Mcf/day in 2010 to 132.6 Mcf/day during the first 11 months of 2011. The cost of the nitrogen being produced for use in the Mexican sector of the Gulf of Mexico is US$0.36/Mcf.

Elsewhere around the world, nitrogen injection will be used for enhanced oil recovery at ACDO’s Habshan oil field in Abu Dhabi. The US$160 million contract for the nitrogen gas injection plant was won by Samsung in February 2012, and will be capable of injecting 600 Mcf/day. The plant is expected to be completed by August 2014.