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Abstract

During the drilling and completion of the Huldra field in the North Sea, high temperature and high pressure conditions were expected and encountered in the reservoir section. The difference between the pore pressure and the fracturing pressure is small. Cesium formate had been evaluated as a potential drilling and completion fluid, but technical hurdles could not be completely addressed in time for the first well. As a result of well control problems occurring in the first well, with barite sag in the oil based drilling fluid as a contributing factor, it was necessary to use a drilling fluid with insignificant potential for sag. For the first time worldwide the cesium formate brine was chosen as a drilling fluid. This fluid could be delivered solids free with densities up to 2.2 s.g. The required down hole density in the well was 1.91 s.g. At the same time it was necessary to have as little contribution to the equivalent circulating density (ECD) from the flow as possible.

The paper describes how the cesium formate brine was used successfully as a drilling and completion fluid. The effect of the fluid on well control, hole cleaning, rate of penetration (ROP), torque/friction, ECD, formation damage, casing wear and hole stability are covered. The paper also describes actions required to minimize losses of this very expensive fluid. The challenges acquiring adequate formations logs while drilling are also described. Finally, the use of cesium formate brine during the completing of the wells with open hole sand screens is outlined.

Introduction

Huldra is a gas condensate field stretching from block 30/2 into block 30/3 in the Norwegian sector of the North Sea, operated by Statoil ASA. High temperature and high pressure conditions were expected and encountered in the reservoir sections in this field. Three exploration wells were drilled on the field from 1979 to 1991, which proved gas and condensate.

A jackup rig in cantilever mode over a well head platform is being used to drill and complete the planned wells. The production platform will become unmanned after the last well is drilled. The water depth is 125 m.

A severe well kick was experienced during the completion of the first well. Barite sag in the oil based drilling fluid combined with well geometry were significant factors contributing to the well kick. Additionally, the barite sagging problems increased the level of concern with regards to screen plugging. For these reasons, the ongoing qualification work on the formate brines intensified.

Based on prior industry experience with formate drilling and completion fluids, and with various tests, it was then concluded that cesium/potassium formate was the preferred drilling and completion fluid.

Experience with formate brines

Use of potassium formate based fluids for drilling and completion operations were introduced in the very early 1990's^1,2. These fluids were considered to have insignificant impact on the environment if discharged³. The maximum density for solids free potassium formate is approximately 1.59 s.g. Barite cannot be used for further densification because it dissolves slightly in formates⁴.

Potassium formate was used both as a reservoir drilling and as a completion fluid in the Gullfaks field in Norway in the early 1990's⁵. Because it was necessary to use a drill-in fluid with a higher density than 1.59, manganese oxide was added as a densifying agent. During the completion the density was reduced to 1,57 s.g. No addition of solids was necessary. The whole drilling and completion operation is considered as a success.