Abstract

Akal, the main field of the offshore Cantarell complex, is a supergiant with original oil in place of 32 billion stock tank barrels (bstb) and oil column thickness of about 1200 meters (m). Being a highly fractured carbonate reservoir with a large volume of vugs, Akal has produced under full gravity segregation conditions and subject to natural thermal convection; the gas-oil contact has been steadily moving through the years to its current position of 1930 m subsea, (mSS). Water encroachment from an aquifer that is shared with other neighboring fields has also taken place; water-oil contact has moved 480 m from its original position of 3200 mSS. To maximize the economic value of the Cantarell fields, among others, an unprecedented worldwide pressure maintenance project by nitrogen injection was envisioned and recently implemented.

This paper discusses some reservoir management issues of the Cantarell nitrogen injection project: nitrogen channeling, asphaltenes aggregation, number of required injection wells, nitrogen breakthrough time and evolution of its concentration in the associated gas effluents, monitoring of nitrogen injection, and finally, the impact of Cantarell nitrogen injection on neighboring fields, given the evidence of hydraulic communication and production interference of Bay of Campeche offshore fields through a regional aquifer, or aquifers. Details on these issues and the approach used to solve them are presented in this paper.

Introduction

The Cantarell complex, discovered in 1996 and located about 80 kilometers (km) offshore in the Bay of Campeche, Mexico, (seeFig. 1), is the largest oil field in Mexico1. The Cantarell field is composed of four major fields: Akal, Nohoch, Chac, and Kutz, in addition to the reserves in the newly discovered deeper Sihil field. Cantarell's largest reservoir, the Akal field, is considered the sixth largest reservoir in the world.

Main pay zones in Cantarell, hydraulically continuous over an average thickness of 1200 m, correspond to highly fractured and vuggy carbonate formations from Jurassic, Cretaceous, and lower Paleocene geological ages; less important calcarenite and sandstone formations are from the upper Paleocene and middle Eocene.

Oil was initially undersaturated and the pressure at the reference depth of 2300 mSS was 270 kilograms per square centimeter (kg/cm2). Typical total porosity in the reservoirs is 7 percent; up to 25 percent of it may correspond to secondary porosity (fractures, microfractures, and vugs). Typical absolute permeabilities in the primary and secondary porosity media are 0.3 and 5000 milidarcys (mD), respectively.

The production in Akal field started in 1979 through the Cantarell 1-A well, which initially produced 34,000 standard tank barrels per day (stb/d) of 22 °API oil. In April 1981 production reached 1.156 million stb/d (mmstb/d). Since then and up to 1995, the Cantarell complex produced at an almost constant rate of 1 mmstb/d. In 1997, as a result of the implementation of the Cantarell Project, oil production began increasing to reach the current rate of 1.6 mmstb/d, which represents about 50 percent of Mexico's total oil production to date.

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