Imprimir

Oil

Formation of black cocoons

The movement of sediments on the sea bottom is the object of a study by a team from Rio Grande do Sul

STÉFERSON FARIA / PETROBRASDeep waters: Petrobras wants to continue leading in petroleum explorationSTÉFERSON FARIA / PETROBRAS

More and more Petrobras is looking to lower costs and risks in its operations of oil prospecting in deep ocean waters, an area in which the company is a world leader in this technology. For this to happen it is invests in partnerships, such as the one initiated three years ago with the Density Currents Studies Center (Necod), of the Hydraulic Research Institute of the Federal University of Rio Grande do Sul (UFRGS). The goal is to attempt to better understand the movements of sediments that give birth to petroleum and to find the most probable areas where they are deposited. “In 1999, technicians from Petrobras came to talk with us, saying that they wanted to create a center of excellence in the company’s Frontier Exploration Program, in order to prospect in deep waters”, says professor Rogério Maestri, the project’s coordinator. “Our challenge is to understand how the rocks that make up the oil reservoirs are formed, where they move to and where they are located.”

The studies are concentrated in the Campos basin in the state of Rio de Janeiro, where almost 80% of national production of crude oil is being extracted, some 1.5 million barrels a day. A brickwork water tank of 1.8 meters in depth, and with an area of 300 m², reproduced within its interior the topography of the Admiral Camara canyon located in the basin. The choice of this location is strategic. As it had already been explored by Petrobras, it was an excellent source of data for research. In professor Maestri’s opinion, the model operates in 3-dimensional geometry – reproducing the large horizontal and vertical irregularities that make up the topography of the canyon -, with data on density currents.

Past eras
The phenomenon of density currents can be seen in the progression of a cold front, in the movement of an avalanche, in the sedimentary currents formed during the explosion of a volcano or in other situations that occur in nature. The scientific definition of this phenomenon is that there is the flow of a fluid of greater density below another of lesser density. According to calculations by specialists in this sector, 98% of Brazilian petroleum has been found in deposits of density currents that occurred in past eras. Deposited in deep marine environments, the compacted organic material, together with the sediment, transformed itself under ideal conditions of pressure, temperature and time, into crude oil.

The process of the accumulation of this material on the Brazilian maritime coast started to occur some 130 million years ago, with the separation of the African and American continents. In these first movements, the heavier material, deposited on the continental shelf, and the organic material, found naturally on the sea bed, were thrown about in the form of density currents, within the maritime basins. At a determined moment, these basins suffered ruptures, forming small ravines, known in geological terminology as canyons, located on the rim of the maritime shelf.

It is these that conducted the mixture of water with sediments, for hundreds of kilometers, in the abyssal bottom, where deposited at strategic points was the material that gave origin to the oil. “These points and the quality of the deposits are the greatest objective of our research”, explains Maestri. If the material is thicker, it is considered to be a deposit of greater quality. If it is thinner, it will not contain as much petroleum.

The majority of the deposits in which petroleum has been found are aged between 30 and 40 million years, dates that coincide with the cycles of the lowering and rising of the oceans. When the sea level dropped, the shelf remained uncovered and the material that had been deposited over millions of years was carried towards the deeper part of the ocean, forming sediments that gave origin to crude oil, which was formed in generating rocks and migrated to reservoir rocks. These are the ones that hold the crude oil, gas and water which have remained in their interior.

In the research carried out by the Rio do Grande do Sul group, currents of a high degree of turbulence in Newtonian fluids (fluids with behavior similar to water and air) are simulated. The particles carried by the current develop composite movement in the interior of the fluid, representing a circular trajectory around the start of the current. The turbulence in the flux pressed the deposited material at the bottom of the ocean into a characteristic structure. A standard deposit is one that has the thicker material at the foot of this structure and the more viscous material at the top. “The genesis of this current signals where one might find the petroleum deposits”, explains Maestri.

According to data from Petrobras, prospecting of an area in deep waters requires investments greater than US$ 30 million. Almost 90% of this total is spent on the drilling of a test well that effectively proves the existence or not of petroleum and natural gas. The petroleum industry considers a good result as one in which 30% of the test drills in the prospecting phase result in the discovery of new fields. For Maestri, any progress, even if it is only 1% in relation to the proposed objective, represents a large time saving, resources and environmental control in oil prospecting. “Research gains are not yet economically measurable, but from the scientific point of view there has already been an increase in knowledge with the development of two masters dissertations on the theme, one doctorate thesis and various scientific papers presented at congresses and academic and scientific meetings”, he explains.

The research results are periodically passed on to Petrobras and to the Petroleum Sectorial Fund of the Ministry of Science and Technology which are participating in the project funding. From the start until now, some R$ 460,000 have been spent for the development of models. The geologist Fernando Del Rey, one of Petrobras’s coordinators with the researchers from Necod, underlines that the research is still in the initial phase, but represents a significant advance. “The center is already being considered by many international researchers as one of the best equipped laboratories to carry out this type of research. It is the only center in Brazil where density currents are being studied with emphasis on the maritime sedimentary processes.”

The latest generation equipment, the specialization in the area and the forty nine years of the institute’s experience are certainly sufficient credentials for the researchers to live up to the challenge of Petrobras’s proposal of turning petroleum exploration in deep waters an activity of lower risk and lower costs.

Republish