Abstract

This paper focuses on arguably the most cost-effective application for coiled tubing drilling. The application is the non-steered extension of an existing well in the underbalanced state. This operation can be very fast and efficient with coiled tubing. However, the job design must minimize the associated risk to ensure problem free execution. This paper describes the steps required to minimize risks and therefore costs, and illustrates by way of example these steps in action.

Introduction

Drilling vertical well extensions with coiled tubing, particularly in the underbalanced state, uniquely exploits the inherent strengths of coiled tubing. These strengths are the ability to enter into slim holes, against a live well head, with a small equipment spread that is fast to rig up and down, and the ability to trip quickly and maintain a steady pressure down hole with continuous circulation.

Coiled tubing has successfully been used to deepen hundreds of wells yet this application has received only sporadic technical attention. There are some very important technical considerations when drilling non-directionally with coiled tubing that must be addressed to ensure a commercially successful job.

When applied correctly, non-steered drilling with coil can be a very fast and effective technology. It should not be equated to the much more complex directional and horizontal drilling, conducted largely with big, specialized coil units, using sophisticated down hole equipment at costs comparable or in excess of conventional drill rigs. Drilling vertical extensions can often be done with a standard coil unit, as small as 1ΒΌ", using readily available motors and optimized bits, sometimes taking only hours to complete.

This paper cites a recent vertical extension drilling job carried out in Western Australia to illustrate the critical engineering aspects of design and execution of a coiled tubing, underbalanced, non-directional drilling job. This job was completed for Arc Energy in April 1999 and produced a well that stabilized at 1.1mmscf/day, where three other wells drilled conventionally into these zones had shown only trace amounts of hydrocarbon.

Design Considerations

Planning

In most cases, it is highly desirable to drill through the existing completion so that the cost associated with pulling that completion is avoided. Drilling thru-tubing also has the advantage of not requiring a completion string to be run after the drilling has been completed. This is particularly advantageous when the well is drilled underbalanced and it is counter-productive to kill the well while running a subsequent completion.

Care must be taken not to be too ambitious when drilling into the open formation. Coil is not good for large overpulls and can only be used for slide drilling. It is more susceptible to differential sticking or hole collapse. An understanding of the formations to be drilled through is very important. For example, drilling through over-pressurized shales into a depleted reservoir in the underbalanced state will likely cause hole collapse.

Mud Choice

The choice of drilling fluid is very important. The drilling fluid must be thin enough to pass through the coiled tubing, yet have the ability to lift cuttings back to surface at flow rates compatible with the coil. It must be compatible with the formation even if the drilling is underbalanced as some fluid contact with the formation is inevitable1. It must be safe to handle on surface and must be compatible with the down hole tools, particularly the motor.

Planning

In most cases, it is highly desirable to drill through the existing completion so that the cost associated with pulling that completion is avoided. Drilling thru-tubing also has the advantage of not requiring a completion string to be run after the drilling has been completed. This is particularly advantageous when the well is drilled underbalanced and it is counter-productive to kill the well while running a subsequent completion.

Care must be taken not to be too ambitious when drilling into the open formation. Coil is not good for large overpulls and can only be used for slide drilling. It is more susceptible to differential sticking or hole collapse. An understanding of the formations to be drilled through is very important. For example, drilling through over-pressurized shales into a depleted reservoir in the underbalanced state will likely cause hole collapse.

Mud Choice

The choice of drilling fluid is very important. The drilling fluid must be thin enough to pass through the coiled tubing, yet have the ability to lift cuttings back to surface at flow rates compatible with the coil. It must be compatible with the formation even if the drilling is underbalanced as some fluid contact with the formation is inevitable1. It must be safe to handle on surface and must be compatible with the down hole tools, particularly the motor.

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