document preview

Abstract
This paper describes the current studies to define alternatives for the geological storage of the CO2 present in the associated gas to be produced from the Pre-salt reservoirs of the Santos Basin, Brazil.

Recent hydrocarbon discoveries in Santos Basin, offshore Brazil, in the so-called pre-salt reservoirs, brought many challenges for the production development (Beltrao et al., 2009). The reservoirs are heterogeneous microbialite carbonates, located below up to 2,000 m salt layer thickness, in water depths of 2,200 m. The oil is a 28 – 30oAPI, with GOR higher than 200 m3/m3. Besides the unique environment, one additional challenge is the variable CO2 content in the associated gas.

The sustainable hydrocarbon production from the pre-salt reservoirs will, then, require, in line with Petrobras and its partners’ vision, avoiding emissions of the CO2 produced together with the hydrocarbon. The task that would be difficult for onshore oil fields reaches unparalleled complexity in the subsea completion deep water production scenario. Some alternatives are under study for the CO2 capture and storage: reinjection in the producing reservoirs, in salt caves, in salt water aquifers, in depleted gas reservoirs and even transportation and use of the CO2 for industrial purposes.

Although still in the early stages of development, work done so far paved the way for robust and sustainable gas processing and CO2 separation, compression and reinjection in secure sub surface geological horizons. The current analysis indicate that the best alternative seems to be the reinjection in the oil producing reservoirs, with a good perspective of enhanced oil recovery by the association of gas and water injection in the Water Alternating Gas (WAG) process.

Introduction
The area known as the Santos Basin Pre-Salt Cluster (SBPSC) is located in ultra deep waters, between 1,900 and 2,400 m, approximately 290 km offshore the Rio de Janeiro Coast, Southeast Brazil. Figure 1 shows the main blocks of the SBPSC, currently in the Appraisal Phase. The structure was created around 160 millions years ago, when the supercontinent Gondwana began to break apart, giving place for the South American and African continents. The rift phase created the conditions for the deposition of sediments in the trough between the two continents. As the separation continued, the sea water began to fill the space, creating a low energy and high salinity environment, propitious to the growth of special bacterial colonia. The secretion of these microorganisms, together with the precipitation of carbonate salts, created nucleus to form carbonate rocks, known as microbialites. Later on, due to the severe climate change on Earth, the salt dissolved in the sea water in this low energy environment precipitated, forming a thick salt layer that became a perfect seal for the hydrocarbon that migrated to the microbialites.