Underbalanced technology is typically understood as a drilling technique, not a completion technique. Yet in many fields where underbalanced (UB) technology is applied, the completion aspect can have considerably greater economic impact - after all, the completion is the crucial link between the reservoir and hydrocarbon flow throughout the life of the well.
The advent of new technology and techniques have allowed UB to be applied to completions in both hard rock environments and in wells requiring support from screens or liners. Successful UB-completed wells have been documented in heavy oil, light oil and gas environments, and in horizontal and vertical wells. In every environment, understanding and controlling pressures and fluids are the keys to success.
This paper discusses the elements of the completion process that are unique to underbalanced operations and then examines five field case histories that highlight the role of UB.
To understand how underbalanced operations (UBO) impact completion, it is useful to begin with the elements of a conventional completion: plan, drill, evaluate, complete, produce. Drilling focuses on wellbore construction - getting the hole to the objective depth - rather than factors that may be important in completion. Once total depth (TD) is reached, evaluation via drill-stem tests or wireline logs typically determines how the well will be completed.
In the case of a cased hole completion, it is easy say that completion process begins after the well reaches TD and at the point of running casing. Casing, cementing, perforating, gravel packing, stimulating, and flowback are discrete parts of the completion process, used in various combinations in many wells.
In cased-hole wells where formation damage is intended to be removed in the completion process, the impact of drilling fluids and pressures on the formation is often considered less important during the drilling operation. Instead, the operator may rely on the remedial step of stimulation to prepare the near wellbore region for production: acid to remove near-wellbore damage, and/or hydraulic fracturing to bypass the damage. Perforating may also bypass some damage in addition to creating a conduit through the casing. To minimize the cost of stimulation by mitigating formation damage, an operator may use less damaging conventional fluids; for example oil-base fluid in an oil-wet reservoir. Conventional fluids can to some degree mitigate damage and reduce the need for stimulation by techniques such as sized salt, filtercake buildup, and use of less damaging fluids (oil base fluid in an oil wet reservoir, for example.)
A simpler completion is the openhole completion. Although tubulars are not run across the reservoir, one may argue that the completion process of an openhole completion begins when the wellbore enters the producing formation. The only activity remaining after the well reaches TD may be cleaning up the well and putting the well on production. There may be an intermediate step in some wells of acidization or other treatment.
If we then define the term completion as "preparing the wellbore to produce," that idea covers a wide range of activities that may stabilize the wellbore and prepare the near-wellbore region for hydrocarbon flow. Most of those activities occur after drilling is completed, but as noted above in the case of openhole completions, the completion process begins when the bit enters the reservoir and happens concurrently with the drilling process.
An underbalanced approach to completion embodies many of the same operations and ideas as the conventional approach, with some key differences. The biggest difference is the UBO's emphasis on protecting the near-wellbore region while drilling into the reservoir, instead of remediating damage at a later time. One may argue that the completion aspect, that is, the near-wellbore protection, dominates drilling procedures.