Oil companies continually search for the most cost effective way to exploit reserves, particularly those that lie some distance from existing infrastructure. In the past, the only way to exploit these was through investment in new drilling and production facilities. This perception was challenged in the mid ’90s when BP drilled wells that stretched 5 km (3 mi) then 8 km (5 mi) and then 10 km (6¼ mi) offshore from the rig site located onshore at their Wytch Farm development. It was anticipated that the technical success of this project would open the “flood gates” and extended reach drilling (ERD) would become “standard practice.”
While it is certainly true that the success of Wytch Farm encouraged a number of similar projects, the anticipated wave of activity did not appear. Indeed some of the Wytch Farm records still stand today, some 10 years later, and it is only relatively recently that renewed interest in ERD has occurred. So the question is why the renewed interest now and why did it take so long?
The definition of what exactly is an extended reach well is a little confused, but current consensus says it is a well in which the horizontal displacement is at least twice the true vertical depth (TVD). The beauty of this definition is that it implies that as this ratio of displacement to TVD increases, the well difficulty increases. There is no doubt this is true. There is no such thing as an “easy” ERD well and it is factual to say that with increased displacement comes increased difficulty. However, the depth of the reserves to be accessed is also critical – the shallower the reserves the more difficult the well. Therefore, displacement to TVD ratio provides an effective measure of total difficulty.
The displacement/TVD ratios at Wytch Farm exceed 6:1 and it is only very recently that wells with higher ratios have been planned and drilled effectively. So while ERD wells continued to be drilled over the last decade, there have been only relatively minor improvements in the extended reach drilling envelope – most of which has come in deeper wells with very long measured depths, but with lower displacement/TVD ratios.
So what makes high ratio ERD wells so much more difficult and why is the drilling of these high ratio wells now possible? The answer is friction and how best to overcome it.
While there are a myriad of challenges in ERD, minimizing torque and drag while drilling, running casing, and during completions continues to be one of the greatest challenges.
As the length and percentage of any well at high angle increases, friction increases due to the increased contact area between any string (drilling or casing) and the wellbore wall. The mass of this string at high angle simply adds to the issue. What does this friction mean? It means that for running drillstring in and out of the hole, we get to an impasse where enough weight (push) from surface cannot be exerted to move the string. Typically the answer is to rotate to reduce drag. While this helps, another impasse is eventually reached where enough torque cannot be generated at surface to turn the string.
So in part, the answer is to construct higher specification rigs to be able to generate enough weight and torque to run strings in and out of the hole and also to rotate. This process is under way and today rig specifications are starting to match those needed for the very high stepout, high ratio wells that are planned. Obviously the configuration of the drill or casing string/completion also has large effect and is another key area of design to consider.
However, this is not the complete answer. How the well is constructed in terms of wellbore profile and how smoothly the well is drilled in terms of minimizing directional changes have a massive effect. It is here that downhole technology has made a real difference.
ERD wells in the mid-1990s were drilled primarily with conventional positive displacement motors. These systems are effective in providing directional control, but need to be pushed rather than rotated to achieve a directional change, (which in ERD wells is a very slow process and in longer ERD wells becomes impossible), to guide the well to the target. They also typically do not provide optimal hole quality due to excessive dogleg generation, which significantly increase torque and drag. This reduces the maximum displacement from the wellhead that can be achieved.
The advent of rotary steerables in the late 1990s and early 2000s changed these dynamics. Today a well can be drilled to plan with minimal doglegs and all in rotary mode. These systems provide a smooth well profile where tortuosity is minimized. However, this game changing technology initially did not have the required reliability to remain operational in the hole for long periods and it is only recently that reliability levels suitable for the effective drilling long, high ratio ERD wells have been achieved.
Coupled with this new found ability to steer the well to the required target is the ability to position the well in the most productive part of the reservoir. Until recently, almost all well placement was reactive, being based on near wellbore geological data. This often meant that desirable changes to the position within the reservoir were almost impossible due to the high dogleg severities required to affect this change. If they were possible, they significantly added to the overall tortuosity of the well path, again reducing the maximum achievable displacement. The development of LWD systems that “see” further into the formations provides operators with the ability to proactively place the reservoir sections of ERD wells into the reservoir sweet spot in a controlled manner. This minimizes dogleg severity and does not reduce the overall well displacement that can be achieved.
The ability to increase this envelope by application of new surface and downhole system developments, (obviously coupled with practical experience of how to use these system effectively), has led to renewed interest in ERD as a means of using existing infrastructure to access remote reserves. A number of areas around the world including Alaska, the Middle East, and Eastern Russia are now employing ERD techniques to access new reserves and it is likely that in the next few years the ERD displacement and ratio drilling envelope will expand like never before.
Mike Williams
Global Sales Manager,
Drilling & Measurements, Schlumberger
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