The Longest Challenge: Overview of a Ground-Breaking Horizontal Well in Gimboa Field, Angola

Abstract

In February 2008, a national operator set a new record for the longest horizontal drainhole drilled in Angola. The well (drilled in the Gimboa deep water field) measured 1664 meters of total length, and was successfully placed within a multiple-lobed reservoir with highly variable thickness. This paper provides an overview of the well placement process, with particular focus on the aspects of job planning, which played a critical part in the success of the operation.Preliminary investigation in Gimboa offsets wells had revealed the presence of horizontal and vertical permeability barriers within the targeted reservoir. This added considerable complexity to the trajectory planning and execution. Ideally, the borehole path would have to connect as many reservoir lobes as possible, without exiting the main boundaries. The well also needed to drain two reservoir segments, separated by a major fault of uncertain throw.Severe operational constraints were imposed. The formation consisted of unconsolidated, mechanically weak sands. In order to maintain directional control and prevent washouts, the rate of penetration could not drop below a pre-determined value. Dogleg severity had to be controlled, in order to limit torque losses and avoid problems while running completion screens.The geological challenges were tackled using an innovative technology blend, which integrated deep directional boundary detection, interpretation of real-time borehole images, and log correlations against a model based on seismic and geological facies analysis. Formation pressure was also acquired while drilling, to verify the possibility of hydraulic communication with the injector wells. Such combination was an absolute first for Angola. Special drilling practices were applied in order to implement steering decision without exceeding the prescribed limitations.