Field Experience in Gravel Packing the Dourado Field, Offshore Brazil.

Abstract

ABSTRACT This paper describes the completion operations in the Dourado field, with emphasis on four major gravel packing problems:formation taking excessive gravel;failure of the gravel pack screens;severe well productivity decline andan unusual production casing buckling. The problem occurred in a new area of the field where the main reservoir, the Calumbi sandstone, is largely intercalated with plastic, water sensitive shale streaks. Presumably, stresses developed during gravelpacking set off the creepage tendency of these streaks, which might be intensified by hydration upon rock imbibition with water-based completion and gravel carrier fluids. The result was a continuous migration of shale intoadjacent permeable layers (fig. #1), leaving behind progressive void space, which allows for the excessive pumping of gravel. As the well was flowed, check-valve effect naturally ensued as lumps of dispersed shales concentrates around the wellbore; hence the decline on well productivity. Another consequence of the shale behavior was a detrimental effect in the wellbore stability, which reflected in buckling of the casing in front of the perforated interval. Finally, the screen failure, problem, was attributed manufacturing quality. INTRODUCTION The Dourado field, located at a water depth of 26 meters in the offshore platform of the Sergipe-Alagoas basin, northeast Brazil (fig. #2), was discovered in 1970, and production began six years later. At present, there are 11 wells producing a low viscosity oil from the Calumbi formation, a 1,200-meter deep turbidite of the Tertiary age, characterized by a rather complex, heterogeneous structure composed of an argillite matrix intercalated with thin, randomly distributed, lenses of sediments of varying Iithology, such as, siltstone, limestone and sandstone. Formation thickness is around 1,300 m, whereas individual sandstone lenses are much less conspicuous, ranging from about only 1 to 5 m in some areas, and, exceptionally, to 18 m in others. They constitute, however, the main part of the reservoir, and bear much of the oil in-place. In addition, which is flowing properties are quite favorable, congruent with most of the turbiditic reservoirs composed of aggregation of unconsolidated, well sorted, though very fine, sand grains. As a rule, the absolute porosity is 30% and the oil effective permeability, 1 D. Because of the well-known difficulties in managing successfully any sand control method, sand production has long been considered a major concern in the development project of the field. In this way, as the decision to exploit a marginal area was made in 1993, ten years after finishing the development of the field, conceivable efforts toward improvements came out. The result was the following state-of-the-art changes in the former gravel packing procedures:12 shots/ft underbalance perforation;perforation prepacking right after perforation;shearing and filtering of gelled fluid used in perforation prepacking;low density gravel packing fluids, with new blender for better control of the operation;attentive coring and laboratory work, suitable for unconsolidated sand stone;new gravel pack evaluation log;use of API RP-58 gravel pack sand.