Since its introduction, over 700 alternate path sand control completions have been implemented around the world ranging from single zone cased hole gravel-packs to multi-zone fracpacks and fibre optic (DTS) enabled open hole horizontal completions. Over the last six years, the alternate path system has been field proven to provide high reliability in achieving complete packs as well as additional features allowing simultaneous cake cleanup, shale bypass and other contingencies that enable doing it right the first time in deep-water/subsea completions where interventions tend to be economically and logistically prohibitive.
This paper provides a critical review of those completions, capturing both successes and failures, along with the lessons learned in their design, execution and evaluation in relation to completion efficiency in sanding reservoirs. Furthermore, it details future development work and enablers that will allow the use of alternate path technology as the mainstay of intelligent well solutions, geometrical design improvements to provide optimal aspect ratios in the wellbore and its applicability in conjunction with water packing.
Alternate Path is a technique that was developed to bypass any annular blocking that may occur during the completion process due to various reasons. Its original design consisted of steel rectangular tubes (also called shunt tubes) welded on screens, where the tubes had holes for exit of the slurry out of the tubes and into the annulus once the blocked annular section is bypassed. This original design was intended primarily for use in cased-hole gravel packs. As such, this design (tubes with relatively small flow area and with holes drilled on) was sufficient for the pump rates, slurry volumes and the interval lengths that are commonly encountered in those applications. During the last six years, various improvements were made to the original design to address the specific needs for a variety of applications as discussed in the next section.
Although the initial applications of the alternate path technique have been directed towards improvement of gravel placement in cased holes, its current applications in fracpacking and open-hole horizontal gravel packing significantly outnumber cased hole gravel pack applications, in line with higher popularity of frac-packs and open hole gravel packs as reliable sand control techniques that yield high-productivities, particularly in deep-water/subsea environments. Thus, the review will focus primarily on alternate path case histories in these two areas, although examples of cased hole gravel packs will also be included for completeness.
The paper is organized as follows. First, we summarize the evolution of the alternate path system, highlighting the reasons for each improvement along with the benefits and potential disadvantages or limitations. We then continue with a discussion of the cased-hole followed by open-hole applications of the alternate path, providing some statistical information, along with a brief review of representative case histories as well as lessons learned for each application and some key considerations to ensure successful applications in the future. This is followed by a summary of further improvements that are currently being worked on. Finally we draw conclusions.