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The purpose of this paper is to review the planning, execution and results of a Seven Leg, Semi-short Radius Multilateral Well drilled underbalanced in a very mature field in Northeast Brazil. It will explain such details as ... why multilateral?? ... why semi-short radius?? ... why underbalanced?? ... in an effort to explain the many thought processes and considerations when designing and planning drilling operations in depleted reservoirs. Introduction The Carmopolis Field is located approximately 60km northeast of the city of Aracaju, in the State of Sergipe, Brazil. It is a very mature field that covers an area of approximately 140 square kilometers, and was discovered in 1963. The field has multiple pay zones with a total of approximately 350 million m3 of oil in place. To date, approximately 62 million m3 of oil has been produced and the field currently produces about 3000 m3/day of oil and 18000 m3/day of water. The field has gone through various phases of production enhancement scenarios over the past 20 years. Enhanced recovery methods such as water flood and steam injection have been implemented in various parts of the field. New techniques and technologies continue to be used in this field in an effort to enhance production and reservoir recovery even further. The combination of underbalanced drilling and ML technology is just one more step in the evolution of this field. Reservoir Description The Carmopolis reservoir is composed of syntectonic conglomerates and associated finer clastic sediments deposited in an alluvial-fan complex within fault bounded grabens(1). There are four major lithofacies which are part of the Cretaceous period. Each of these lithofacies can be distinguished on the basis of grain size, which decreases 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. Porosity and permeability relationships for each of the major lithofacies were obtained from core samples. Analysis has shown that both the porosity and permeability decrease in the coarser grained lithofacies, thus resulting in poorer reservoir quality. Project Goals and Objectives As in any new drilling project, the main objective is always to maximize return on investment. In a field that already has well over 1450 wells drilled in it, on 4 hectare spacings, finding a suitable location can be a challenge. The combination of low permeability (50mD - 1000mD) of the producing formations, and relatively heavy grade (20 - 21 API gravity) of the oil in place, severely limit the production rate from the vertical wells. Couple these two characteristics with the complex geology and faulting characteristics of the field. The combination makes finding economically viable drilling locations even more difficult. As vertical wells were no longer economic in many parts of the field, horizontal wells were the logical next step. A single high angle well was drilled through the CPS-1, CPS-2 and CPS-3 formations (300 m was drilled through the pay zones from top to bottom) for about 1.5 times the cost of a vertical well. After the high angle well was drilled, another location was identified and targeted for drilling. This location was selected as a favorable target because there was trapped oil discovered on a structural high between two faults. However, due to faulting in the immediate area, one long horizontal well could not be drilled and still get the desired reservoir exposure in all the producing zones. In addition only a couple of the zones would actually be produced, so a long well was not the best option. The location could also have been drained with three vertical wells, but in order to drain very near the fault, additional risk would have had to be taken, in case the well actually was drilled on the wrong side of the fault.