Development and Utilization of a Coiled Tubing Equipment Package for Work in High Pressure Wells
Authors: van Adrichem W.P. , Gordon D.G. , Newlands D.J. 
Published: October 2008

1. Introduction

Cleanouts of deep, high pressure, high temperature gas wells are a common operation in South Texas. Until recently, these cleanouts required the use of snubbing units due to the high pressures encountered. This resulted in time consuming (7- 12 days) and thus expensive operations, Because of this expense, effortshave been made to extend the application of coiled tubing (CT) to operations where wellhead pressures approach 10,000 psi.

Testing of a specially equipped 1-1/4" CT unit in conditions simulating a 10,000 psi South Texas well cleanout proved that the use of a CT unit was a viable alternative to snubbing operations, Since then, some 50 high pressure cleanouts have been successfully performed at an average cost saving of 50% while taking 1-3days to complete. This paper will focus on the operating parameters, the design, the testing and the field implementation of a high pressure CT unit.

2. Background

Beginning in 1990, Shell Western E&P, Inc. (SWEPI) began commingling production from several zones in deep, high pressure, high temperature gas wells in South Texas. These low permeability zones are across long intervals and require multi-stage massive hydraulic fracture treatments. Sand plugs are used to isolate each fractured zone from subsequent fracture treatments. After the final fracture treatment, the sand plugs must be removed in order to commingle production from all thezones. The wellhead pressures (WHP) during these operations often approach and sometimes exceed 10,000 psi, Consequently, snubbing units were utilized for these operations since CT unit's maximum WHPS are normally limited to 3,500 psi. A study to develop a high pressure coiled tubing (HPCT) package capableof servicing these high pressure wells was performed with the objective of reducing the cost and duration of the cleanout operations without compromising safety.

The project was divided into four parts; 1) determination of the required operating limits 2) selection of CT and equipment, 3) yard testing the HPCT equipment package in a simulated high pressure environment and 4) field testing in a South Texas well.

3. Operating Limits

3.1 Non-calculated limits

To expand the operating ranges of CT, examining the existing safety limits was an appropriate starting point. Dowell's standard operating limits, shown in Table 1, were thought to be representative of the CT industry.

Table 1. Conventional CT limits

(Available In Full Paper)

These limits have been in effect since the late 1970's when most CT material had a 55,000 or 60,000 psi yield strength, the largest diameter was 1-1/4", and only two wall thicknesses were available.

These non-calculated limits have been applied irrespective of CT diameter, wall thickness ormaterial type even though these variables affect the actual operating capability of the CT string. With the evolution of CT materials and the multitude of tubing wall thicknesses available, these non-calculated limits have, in most cases, become more conservative.

These non-calculated limits were insufficient for South Texas HPCT applications. The desired HPCT operating limits are shown in Table 2:

(Available In Full Paper).