Abstract

A common major challenge in deepwater drilling is the narrow margin between pore pressure and fracture gradient. In many basins, deepwater wells are becoming even more challenging with subsalt and deeper horizons introducing problems that can lead to well abandonment before the target depth (TD) is reached. The key to drilling these wells safely and cost effectively is to use technologies, methods and procedures that allow the mud weight window to be widened.

This paper describes two technologies that, when combined, can widen the effective mud weight window significantly. One is the Micro-Flux Control (MFC) method. This is a closed-loop drilling system that allows the safe use of a mud weight as close as possible to the pore pressure. The other technology is an ultra-low invasion drilling fluid (ULIF) that increases the leak-off pressure of the open hole section, thereby opening the mud weight window. Several field cases involving the drilling fluid are described, with increases of more than 1.5 ppg in the leak-off pressure observed in wells in the Gulf of Mexico (GOM).

Results obtained with the MFC method at a test facility in the US are described. These show the ability of the method to deal very effectively with any influx or loss. Significant operational savings have already been obtained with the drilling fluid. However, the combination of the two technologies will produce a step change in safety, technical capability, and economics in many deepwater operations.

Introduction

Wells with a narrow margin between pore pressure and fracture gradient are becoming more and more common. Not only is the geological environment where new reserves areoften located becoming more complex, but also the depletion of current fields is creating increasing problems in development and infill wells. The situation becomes evenmore complex in deepwater, especially when operators are trying to locate reserves below salt layers, and in HPHT environments.

In many deepwater prospects, several casing strings - including contingency strings - have to be considered. However, many of these wells are still abandoned before reaching TD. These wells are extremely expensive; sometimes costing close to $100 million each. One operator in the GOM abandoned a well after spending more than $80 million. Another spent $120 million before abandoning the well. It is clear that the technical limit of conventional drilling is often being encountered before achieving the ultimate goal of reaching TD in order to access hydrocarbon reserves. Newtechnologies and methods must therefore be introduced to allow these new frontiers to be added to producible reserves in the near future.

This paper focuses on wells with narrow margins between the pore pressure and fracture gradient, highlighting two technologies that can reduce significantly the problems faced when drilling those wells. Even though the technologies can be used separately, the combination of the two in the same well can bring the optimum drilling solution.

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