Mexico's largest discovered oil deposit, the Cantarell complex in the Bay of Campeche, was the subject of an entire technical session at the Offshore Technology Conference earlier this month.
Dominating the session was discussion of a nitrogen immiscible pressure maintenance and redevelopment project at Cantarell, about 50 miles off Ciudad del Carmen on the Yucatan Peninsula.
The world-class pressure maintenance project is expected to result in recovery of large volumes of oil and gas that would not have been possible without nitrogen injection. The project is therefore related to Oil & Gas Journal's special report on enhanced and improved oil recovery in this issue (see p. 68).
Most importantly, authors with Petroleos Mexicanos SA's Pemex Exploracion y Produccion unit stated that nitrogen injection has already stopped Cantarell's production decline and prevented water from trapping more oil that never would have been recovered (OGJ, May 7, 2001, p. 58).
"After 7 months of nitrogen injection, satisfactory results have been obtained that are in accordance with the simulated pressure behavior. Nitrogen has not been observed in the produced gas," they said.
Proyecto Cantarell
Cantarell consists of four fields: Akal, Nohoch, Chac, and Kutz, covering 162 sq km.
Supergiant Akal, with 91% of the 35 billion bbl of original oil in place, produces 19-22° gravity Maya crude from an average 2,300 m below sea level. Original oil column thickness was 1,200 m.
As Pemex produced oil, gas entrained in the oil expanded and was released. It migrated to the top of the structure, forming a secondary gas cap that now occupies 20% of the reservoir space, or 730 m vertically. Water encroachment from the south flank has flooded 22% of the reservoir volume, or 480 m vertically.
Laboratory and numerical studies showed that greater oil recovery is achieved in the gas cap than in the water-encroached zone because of favorable structural conditions and great vertical transmissibility.
Pemex and contractors in December 2000 commissioned the last of four 300 MMscfd nitrogen generation plants at Atasta in Campeche State to serve Cantarell (OGJ, Mar. 12, 2001, p. 41).
Nitrogen injection started in May 2000 at 300 MMcfd and reached the full 1.2 bcfd in December 2000. Nitrogen is injected through seven wells into the top of the Akal reservoir. The nitrogen supply contract is for 15 years.
Akal is considered the sixth largest oil reservoir in the world. It has highly fractured and vuggy carbonate formations from Jurassic, Cretaceous, and Lower Paleocene ages. Less important calcarenite and sandstone formations are from the Upper Paleocene and Middle Eocene.
Cantarell original reserves were more than 17 billion bbl, said Tomas Limon-Hernandez of Pemex E&P. Pemex in 1998 discovered Sihil, a fifth productive block, underneath Akal, and the company is engaged in other production-enhancement projects (OGJ, May 7, 2001, p. 66).
Cantarell production
Pemex discovered Cantarell in 1976 and started production there in 1979. Oil production peaked at 1.156 million b/d in April 1981, then declined.
The 1981 producing rate required only 40 nonassisted wells.
An early gas lift program stabilized production at 1 million b/d from the late 1980s until about 1995. The 1995 rate required 150 wells on gas lift. During 1981-95, the production rate declined to 7,000 b/d/well from 29,000 b/d/well.
With further gas lift efforts in early stages of the Cantarell Project (Proyecto Cantarell), Limon said, production came back to 1.4 million b/d, but reservoir pressure continued to decline.
Another speaker said that reservoir simulation indicated that if natural depletion had continued at Akal, production would have reached 3,200 b/d/well by 2004. At those rates, it would have taken 80 years to produce the field to depletion, requiring facilities replacement.
Production has rebounded since nitrogen injection began. It reached a record 1.68 million b/d in October 2000 and 1.7 million b/d lately, Limon said.
Pemex's goal is for the field to attain 2 million b/d later this year and maintain it for 4 years. After that, oil production and nitrogen requirements would begin to decline, said Pemex E&P OTC speaker F. Rodriguez. At that time, excess nitrogen could be injected in other offshore fields.
Reservoir simulation indicates that recovery will be 2.324 billion st-tk bbl of oil and 875 bscf of gas more with nitrogen injection than without it.
The nitrogen factor
Proyecto Cantarell work is 80% complete overall, and two new production complexes are 60% complete.
The project involves drilling 205 new wells, 124 of which were completed by yearend 2000.
The field represents 50% of Mexico's current production and 60% of its reserves.
Under the project's gas utilization component, Pemex can now handle 1.06 bcfd in the field, up from about 710 MMcfd previously. This will grow to 1.25 bcfd by yearend 2001, ending gas flaring.
Much earlier than expected, Pemex has begun to study how to utilize gas and gas liquids from the gas cap.
