The First Underbalanced Multilateral Well Branches Drilled in Brazil, a Field Case History

Abstract

The present work describes aspects related to planning and execution of six multilateral horizontal branches drilled underbalanced (UB) in Carmopolis field, which is located in Sergipe/Alagoas basin, in Brazil's Northeast Region. This field, discovered in 1963, is the country's largest onshore oil accumulation and has its production zones, which consist basically of sandstones and conglomerates, depleted at about 700 m deep (TVD). As the targeted formations are consolidated, UB drilling did not bring any special concern in terms of wellbore collapse. In order to achieve the required bottomhole pressure nitrogen was mixed with a synthetic based mud (SBM) after flowing down through the annular space between the 9 5/8" and 7" casings and passing through a 1" aperture in the 7" casing. After that, the two-phase mixture went up through the annulus between the 7" casing and the jointed pipe drill string. This concentric casing approach for injecting gas made possible the use of conventional MWD, but it brought some concerns, which are addressed and discussed, about the maintenance of the UB condition while operating. The first multilateral well UB drilled in Brazil is an example of the combination of different technologies as multilateral level 2 well design, UB condition and medium radius directional system. In addition, it also involved four different parties where the operating company worked together with two service companies and a drilling contractor to build a team and achieve operational success. Introduction Carmopolis field, which is located in onshore Sergipe/Alagoas basin, in Brazil's Northeast Region (see Fig. 1), is the country's largest onshore oil accumulation at 268 MMm3 OOIP and a current total oil production at about 2,880 m3/d. Discovered in 1963 and promptly brought into primary production, it mainly produces from the sandstone and conglomerate reservoirs of the Carmópolis/Muribeca formation and secondarily from the deeper Barra de Itiuba formation and the fractured methamorfic basement. The Carmopolis/Muribeca formation, which is composed of syntetonic conglomerates and fine clastic sediments, contains four major oil bearing zones, named CPS-1, CPS-2, CPS-3 and CPS-4. As a result, oil quality varies considerably throughout the stratigraphic column, but general reservoir data is given in Table 11. Waterflooding was first implemented in the southern part of the field in 1968. However, after three years of this pioneering implementation, the results were considered inconclusive and the project was abandoned. The combination of adverse fluid mobility ratio, reservoir heterogeneity, and the lack of proper selective injection led to the quick decline of production. Despite those uncertainties, waterflooding was resumed at the main block of the field in 1971 for attempting to revert a 30% production decline. Besides waterflooding, several other technologies have been implemented, as pilot projects, for improving oil recovery. Polymer flooding, steam injection and in-situ combustion have been introduced, tested and evaluated through the years. In summary, the Carmopolis field has been a kind of field laboratory for investigating a wide range of improved oil recovery (IOR) methods. For further details, a comprehensive history of the IOR applications in Carmopolis and the respective results is available in literature1.

Petróleo em águas profundas : uma história da evolução tecnológica da Petrobras na exploração e produção no mar

“…Esse marco histórico consistiu no emprego de fluido aerado em um poço no Campo de Albacora, que demandou o desenvolvimento de diversos procedimentos e equipamentos para essa aplicação, tornando-se referência quanto à história do desenvolvimento dessa tecnologia (Lage, Santos e Silva, 2001). Além dessas aplicações, o poço multilateral dotado de seis pernas, perfurado em Carmópolis na condição sub-balanceada merece destaque pelo arrojo e complexidade do projeto (Lage et al, 2003).…”