Perforation, injection, debottlenecking underway on Pemex's Cantarell field

The central jewel to Pemex's offshore production is the Cantarell Complex. Alone, Cantarell accounts for 38% of the country's total crude production. Located 75 km northeast of Cuidad del Carmen in the Bay of Campeche, the complex is made up of four adjoining fields: Akal, Chac, Kutz, and Nohoch.

Cantarell is one of the largest oil basins in the world and Mexico's single most important petroleum asset. The 21 platforms currently in service in the Cantarell Complex are all located in shallow water - 35-40 meters. Over the 15 year life of the field, Cantarell's production output has ranged between 900,000 b/d and 1.2 million b/d. In 1996, the Cantarell fields produced 1.1 million b/d. The complex also produces 443 MMcf/d of natural gas.

Over a four-year period, Pemex had planned to upgrade the field by investing $5 billion in renovating, modernizing, and increasing the surface infrastructure, according to Faruk Fayed Zellek, Pemex's Department Chief for Information.

In 1997 alone, Pemex spent more than $1 billion initiating four broad programs. The first of these was designed to maintain pressure in the field through gas injection. Pemex also was to initiate a program of perforations through existing well workovers, to optimize the existing infrastructure to reduce bottlenecks, and model the progression of oil movment in the reservoir. By optimizing the infrastructure, Pemex hopes to reduce the volumes of gas flared in the northeast marine region.

Pemex planned to drill a total of 164 new wells in the Cantarell area and add a total of 18 new platforms over the four years of this plan. Additional pipelines were to be laid for the collection and transportation of crude. New floating structures were to be built for storage and loading of the crude to supplement the capacity of the Cayo Arcas and Dos Bocas terminals already in place, Zellek said.

Pemex had planned to install compression and pumping equipment as well as plants for gas dehydration and liquids separation. In the future, a computerized system will monitor production from the complex as well as the satellite platforms.

Reservoir potential

The original volume in situ of Cantarell was estimated at 40.7 billion boe. Of this, to date, only 6.567 billion has been recovered. Of the original 40.7 billion boe in place, Pemex was able to prove up 12.33 billion boe in 1997.

Recently, as a result of extensive exploration work and improvements in infrastructure, 46% of the original volume of hydrocarbons were proved. This figure contrasts with a theoretical factor of recovery by gravitational segregation in the Akal field, which could reach 65%. This difference, equal to 7.5 billion bbl, highlights the opportunity for Pemex to prove further reserves through its ongoing upgrade program.

In addition, further exploration drilling could increase these original estimated volumes. Pemex said the calculations used to estimate these reserves take into account the opportune implementation of a program to maintain pressure in the Cantarell fields.

The key to Cantarell production is the Akal field, an extremely large development. This field is the source of 90% of Cantarell production. Akal was discovered in 1977 and came on production in June of 1979. Chac and Nohoch are smaller fields and the Kutz has yet to begin production.

The producing zones of the Cantarell Complex vary from 1,000 meters to 3,200 meters below the seabed. The zones are 65-900 meters thick. The high permeability of this massively fractured field allows the hydrocarbons to be recovered by gravitational segregation. Cantarell has thick, high structural relief, and a secondary gas cap, which accounts for a recovery efficiency of 55-60% of the original volume.

Putting on the pressure

Pemex has designed a pressure maintenance program to increase the Cantarell Complex's production capacity and the total recovery of hydrocarbons from its fields. In deciding to increase formation pressure at Cantarell, Zellek said, Pemex considered several options, including water and gas injection. Pemex decided that gas injection would be the optimal method for this field because it takes advantage of the naturally occurring gravitational segregation in the formation.

On the other hand, Pemex was concerned that the injection of water would invite unnecessary risk. It is possible the water would channel through the formation's systems of fractures causing a premature eruption of water in the field, trapping the oil in areas of primary porosity.

Once the options were narrowed to gas injection, Pemex had to choose the type of gas. Both natural gas and nitrogen have been used for other injection programs. Given the volumes required to maintain pressure in the Cantarell Complex and the relative values of thetwo gasses in Mexico, nitrogen was selected.

Again, the goal of this program was to maintain pressure, not displace oil. Over the life of the field, Pemex plans to inject an average of 1.3 bcf /d of nitrogen. By generating and injecting nitrogen, Pemex can produce the natural gas from the Cantarell Complex for Mexico's growing domestic market. Natural gas not consumed domestically could be exported for sale by pipeline.

The Pemex plan

The Cantarell Complex upgrade program initially focused on eliminating existing bottlenecks in the surface infrastructure. This will make the overall system more reliable and, at the same time, reduce the volumes of gas being flared in the Region Marina Noroeste. By eliminating diverse infrastructure restrictions, Zellek said, Pemex hopes to realize the system's full production capacity.

Existing wells will be restored and repaired in an effort to maximize the production from these assets. Once these wells are repaired, Pemex can begin injecting the natural gas it currently flares. This is an interim solution to increase reservoir pressure while the nitrogen facilities are being constructed. This short-term plan covers a period through the end of 2000.