RIASSUNTO
Abstract
The oil industry's search for new hydrocarbon accumulations has led to the drilling of ever deeper wells, normally under harsh conditions that involve high pressures, temperatures and mechanical pressures, temperatures and mechanical strengths as well as sour gas production. In order to perform production. In order to perform drill-stem tests (DSTs) safely, these constraints must be overcome. PETROBRAS, the Brazilian state owned oil company, has been drilling wells in ever deeper horizons, reaching up to 18,000 ft (5,500 m). During the last five years some important areas have been discovered, especially offshore, of which the Tubarao Field in Southern Brazil is one example. Based on well-testing experiences in the North Sea, as well as the company's own experience, PETROBRAS has developed a new approach for testing deep wells: the use of a simple and reliable test-string assembly, achieved by reducing and replacing components, and the adoption of extremely rigorous safety standards and careful selection of materials suitable for use in the presence of hydrogen sulphide. This paper presents a field-proven well-testing design, whose main concerns are offshore safety and environmental quality.
WHAT IS A HARSH ENVIRONMENT?
As used in this paper, the concept harsh environment combines the definition of the National Association of Corrosion Engineers (NACE) (1), as commented on in depth in the article ""What is a Sour Gas Environment?"" (2) with parameters on minimum well depth and on minimum bottom hole pressure and temperature. Based on this definition, DSTs in Brazil are classified as taking place in a harsh environment when they place in a harsh environment when they are conducted in wells that are deeper than 15,000 ft (4,500 m) and that display a bottom hole pressure of over 8,000 psi (55.2 MPa), a bottom hole temperature of over 285 deg. F (140 deg. C) and H2S concentrations exceeding the conditions shown in Fig. 1.
HISTORY
During the fifties and sixties, many new oil and gas fields were developed onshore throughout the world. During production phases, some of these were found to display higher hydrosulphuric gas contents than had been indicated during evaluation, especially in Canada.