document

Abstract

This paper describes the strategy, design and the drilling fluid and cement operations in the first two wells drilled with Coil Tubing on the Gullfaks field. The strategy and design is explained in great detail. Excellent hole cleaning was achieved in both wells.

A serious problem of differential sticking was eliminated by a radical change in drilling fluid strategy and design. It was possible to successfully drill a 3 3/4" hole, log and run/cement a 2 7/8" liner at a maximum inclination of 122°.

A virtually solids free Potassium Formate Brine/Polymer drilling fluid with a density from 1.50 - 1.56 g/cc was used. The flow properties of the drilling fluid is characterised by a very low fluid loss due to a high extensional viscosity, a low viscosity at all shear rates and a low degree of shear-thinning.

Introduction

The Gullfaks field is located in the Tampen Spur Area of the Norwegian sector of the North Sea. The field structure is divided into a number of rotated fault blocks with low angle normal faults and additional smaller scale faults found in most of the wells. The field is therefore classified as a complex reservoir. Due to the structural complexity and the relatively shallow depth, the field presents extraordinary challenges with respect to well profiles.

A number of oil accumulations not suited for recovery with the existing drilling strategies were identified in 1996. About 20 of these “targets” were considered suitable for Coil Tubing Drilling (CTD)¹. A project was established to qualify CTD as a method for cost effective drilling and to drain these accumulations. The wells A-10A and A-19A presented in this paper represent the first and second of the planned CTD sidetracked wells in the Gullfaks field.

Drilling Fluid Strategy and Design

Designing a drilling fluid for CTD changes the relative importance and priority assigned to the various criteria. There are frequently several solutions - each with strengths and weaknesses. For these wells, the solutions adopted were a Mixed Metal Hydroxide/Barite fluid for A-10A and a Potassium Formate Brine/Polymer fluid for the well A-19A, A-19AT2 and A-19AT3.

Hydraulics.

Because small diameter tubulars and narrow annuli can generate high pressure losses, the hydraulics program is the limiting factor in CTD fluid design. Essentially, pressure losses must allow a flow rate sufficient to operate the down hole tools and clean the hole. At the same time one has to keep within the limits of the equipment and the formation Equivalent Circulating Density (ECD).

The pressure losses are determined by the fluid's density and viscosity, and by the flow rate. The attainable flow rate is controlled by the fluid's density and viscosity. The required flow rate is controlled by its viscosity parameters. Thus, after the required density has been determined, the effect of obtaining the other fluid properties on the viscosity, will be the design limiting factor.

Density.

The fluid density is decided on the basis of the formation mechanical properties. It has to be high enough to control reservoir pressures and to prevent formation collapse. It has to be low enough so that the annular frictional pressure drops do not exceed the ECD limitations. Good hole stability is required to ensure free sliding of the pipe and prevent helical lock-up.

Hydraulics.

Because small diameter tubulars and narrow annuli can generate high pressure losses, the hydraulics program is the limiting factor in CTD fluid design. Essentially, pressure losses must allow a flow rate sufficient to operate the down hole tools and clean the hole. At the same time one has to keep within the limits of the equipment and the formation Equivalent Circulating Density (ECD).

The pressure losses are determined by the fluid's density and viscosity, and by the flow rate. The attainable flow rate is controlled by the fluid's density and viscosity. The required flow rate is controlled by its viscosity parameters. Thus, after the required density has been determined, the effect of obtaining the other fluid properties on the viscosity, will be the design limiting factor.

Density.

The fluid density is decided on the basis of the formation mechanical properties. It has to be high enough to control reservoir pressures and to prevent formation collapse. It has to be low enough so that the annular frictional pressure drops do not exceed the ECD limitations. Good hole stability is required to ensure free sliding of the pipe and prevent helical lock-up.