Abstract

Fracture propagation is a very commonly seen phenomenon when using synthetic-based drilling fluids (SBM) in areas of narrow drilling windows. Consequently, deepwater drilling has brought lost circulation control to a more critical level. This is further compounded by other deepwater drilling- related problems such as narrow pore pressure/fracture gradient windows, cold drilling fluid temperatures, high equivalent circulating densities (ECD's), high cost per barrel and a higher cost for lost rig time/non productive time.

Induced fractures pose an even more complicated problem due to the various factors involved. These fractures can be influenced by to the nature of formation, drilling and/or by mechanical effects. One condition of paramount importance in sealing induced fractures is having the lost circulation material (LCM) effectively isolate the tip of fracture.

In its deepwater operations, Reliance introduced the use of crystalline graphite in conjunction with sized calcium carbonate into the SBM to strengthen the formation. Graphite is utilized to seal and isolate the near wellbore fractures the tip along with optimizing the rheology of the SBM. The particle size selection of the bridging particles was engineered based on software to optimize the bridging and sealing effect, which employed knowledge of the pore throat sizes and the permeability of targeted formations. This has resulted in elevating the formation integrity test pressure (FIT) and helping drill longer sections, which in normal circumstances would have been a difficult task.

The authors will discuss the design of these drilling fluid additives and their impact. Also outlined are the treatment techniques necessary to ensure minimal formation of induced fractures along with the engineering tips required to ensure successful application of such treatments. The authors will also review the results of actual formation strengthening seen in one of their exploration wells.

Introduction

One of the foremost challenges of deepwater drilling is managing the drilling fluid weight required to drill through the small operational window between pore pressure and fracture pressure without compromising wellbore stability. A great deal of work is underway to strengthen the formation so the section can be drilled deeper and the casing set as deep as possible. Doing so, in turn, will help eliminate slimmer holes through the producing interval, while increasing the possibility of having an additional contingency casing size available if the well has to be drilled deeper. In all the previous wells, which the operator drilled in deep water, 40 lb/bbl of sized calcium carbonate was added to the drilling fluid for bridging the formation. It was found that the use of this sized and graded, calcium carbonate had limited success, as it was unable to seal the formation pores effectively. Also, during circulation the calcium carbonate particles collide with both each other and with other particles. This process breaks the particles of Calcium Carbonate to finer size, making them ineffective in bridging the rock. This results in the occurrence of downhole losses of varying magnitude as observed in most of the deepwater wells.

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