Abstract

The use of a coiled tubing conduit for the hydraulic fracturing of shallow gas wells in southern Alberta, Canada has increased each year since its beginnings in 1997. The coiled tubing fracturing (CTF) technique has been utilized for both new and old wells as a means of fracture-stimulating multiple reservoir intervals.

This paper will detail a re-stimulation project completed during the summer and fall of 2002, in which the CTF and snubbing-conveyed fracturing (SF) processes were utilized to re-fracture a group of shallow gas wells that were originally completed in the 1970s. The objective is to examine the possible ways of enhancing production of older shallow gas wells by fracture stimulation utilizing tubing-conveyed processes.

Specifically, the paper will outline ways to consider and select wells that are candidates for re-entries, in addition to the essential work and evaluation that has to be done both prior to and following the re-entry, re-perforation, and stimulation techniques. This study will also provide a relative comparison between conventional fracturing techniques used previously and the CTF and SF processes used most recently. Also included are pre- and post-stimulation production data that give a clear indication of how effective the fracturing through tubing process is when used to re-stimulate these older wells.

Introduction

The wells of interest conducted in this study are located in southeastern Alberta, Canada. They are vertical, shallow, gas wells that are 300 to 500 meters (984 to 1641 ft) in depth. The producing formations of interest are the Medicine Hat and the Milk River, layered shaly sandstone formations that fracture easily.

These wells have been operational since the 1970's and were initially stimulated through casing using single-stage fracturing treatments, with ball sealers being used in an attempt to divert the treatment to multiple perforation sets. This paper examines the re-entry stimulation work done on a total of 15 wells. Ten were stimulated through CTF, and five were stimulated through Snubbing Conveyed Fracturing (SF). There were several reasons for using tubing-conveyed stimulation for this set of re-entry wells:

  • Weak casing integrity of the wells, hence precluding stage or segregated zonal stimulation (stage fracturing) as a stimulation procedure.

  • Minimization of production downtime for recompletion operations

  • Economical stimulation of bypassed pay

The economical and practical application of tubing-conveyed stimulation in the shallow gas areas of the Western Canadian Sedimentary Basin has been well documented.1,2,3 This paper will focus on the methods adopted to choose wells that were candidates for stimulation, well preparation work, and some of the obstacles encountered in carrying out the re-entry stimulation.

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