Planning, Executing and Analyzing the Productive Life of the First Six Branches Multilateral Well Drilled Underbalanced in Brazil

Abstract

The present work describes not only the main aspects related to planning and executing six multilateral (ML) horizontal branches drilled underbalanced (UB) in Carmópolis field, but also presents an analysis of the initial productive life of this well. All the main engineering information used for making decisions while planning and at well site, drilling performance and production evaluation are included. As the targeted formations are consolidated, UBD 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 base 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 mixture went up through the annulus between the 7" casing and the drill string. This concentric casing approach for injecting gas made possible the use of conventional MWD, but it brought some concerns about the maintenance of the UB condition while operating. The first ML well UB drilled in Brazil successfully combined the following different technologies:ML level 2;UB condition, andmedium radius build-up drilling system. It also involved four different parties where the operating company worked together with three service companies to build a team and achieve operational goals. As the reservoir pressure was not sufficient to sustain the flow of oil to the surface, this paper also addresses the design of the pilot well, which was specifically prepared for operating with a sucker rod pump (SRP). Finally, from the perspective of the reservoir engineers in charge of developing this field, the oil production is presented and evaluated. Introduction Carmópolis 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 OOIP1. Up to now, approximately 62 million m3 of oil has been produced and the current total oil production runs 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 Carmópolis/Muribeca formation, which is composed of syntetonic conglomerates and associated fine clastic sediments deposited in an alluvial-fan complex within fault bounded grabens, contains four major lithofacies that can be distinguished on the basis of grain size2. Those oil zones, named CPS-1, CPS-2, CPS-3 and CPS-4, are part of the Cretaceous period and decrease progressively in the direction of the sediment movement. The larger grained lithofacies have lower porosity and higher bulk density, which allows identification of each lithofacie, based on bulk density and gamma ray logs. Analysis has shown that both the porosity and permeability decrease in the coarser grained lithofacies, thus resulting in poorer reservoir quality. 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 Carmópolis 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 Carmópolis and the respective results is available in literature1.