Abstract

Conventional applications of cementing through coiled tubing, which has been around over the last decade has now become a standard practice that includes zonal isolation, casing repair, water or gas shut off and zone abandonment. The use of this technique being advantageous both technically and economically, have been extended to include multizone well testing.

This paper describes how the use of good practices of cement through coiled tubing design and placement helps optimize the overall cost of a well testing operation.

Introduction

The cementing through coiled tubing technique is finding more and more applications with the improvement and fine tuning of the design and placement practices.

Apart from its technical advantages of accurate placement, reduced contamination, single continuous pipe string with less risks of leaks, coiled tubing cementing has financial benefits from leaving the completion in the well, requiring small quantities of kill fluids, and not having to mobilize a workover rig for most of the applications.

This paper describes how the accuracy of cement placement and the effectiveness of the perforations isolation, along with the optimization of the number of test strings run and the zones tested separately through the same string contributed, during a multizone well testing program, to a major technical success and cost saving.

The major factors considered in designing the cement slurry. along with the placement techniques used to achieve cement plug accuracy and isolation efficiency are elaborated.

A comparison between the use of the conventional zone abandonment and the coiled tubing technique in multizone well testing is also shown to illustrate the cost savings involved.

Cementing Through Coiled Tubing

When compared with conventional primary cementing operations, cementing through Coiled Tubing clearly requires more stringent design and control considerations. Therefore, exercising a high degree of control and verification on all aspects of the operation is required to reliably achieve the desired results.

Slurry Design

Cement slurry properties must be carefully designed and tailored to the specific wellbore conditions and formation characteristics. The main properties to be closely monitored are slurry stability, rheology, fluid loss, and thickening time.

Stability

The slurry stability can only be obtained by adequately shearing it while mixing Therefore, the mixing energy or shear imparted on the slurry should be sufficient and similar in the lab as well in the field to guarantee its stability (Fig.1). To ensure consistent mixing energy levels between the laboratory and the field, the following equations can be used:

Laboratory Mixing

For a 600 ml laboratory sample mixed at 4000 rpm for 15 sec and at 12000 rpm for "t12000" sec, the energy per mass unit can be calculated by the following equation:

Field Batch Mixing

The mixing energy associated with field batch mixing equipment can be calculated by the following equation:

To match the laboratory mixing energy, the batch mixing time is calculated by:

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