Underbalanced drilling in its various forms has been used in the oil and gas industry for more than 20 years. A growing body of technical publications documents the mechanics and advantages of underbalanced drilling (UBD) technology, including details related to casing sizes, circulation rates, depths, rate of penetration, etc. There are also many mentions in the literature of resultant well productivity improvements. In contrast, there is little written in the literature about what contribution UBD makes to reserves and ultimate recovery.

This paper provides a basis for considering the reserve contributions of UBD technology. The six reservoir case histories presented here document tangible reserves attributable to the three key reservoir-related benefits of UBD: improved reservoir access, reduced skin damage, and better ability to evaluate the reservoir while drilling.

Assessment of UBD — Related Incremental Reserves

In this paper we attempt to put a value on UBD by assessing reserves added using this technology.

Our procedure for estimating reserves attributable to UBD operations is simple. If a conventional development scheme will recover economic reserves of X bbls, and a UBD scheme will recover economic reserves of 1.5X, then the portion of the reserves attributable to UBD is 0.5X reserves.

To calculate the reserves attributable to UBD rigorously, one should look at all reasonable development schemes, the associated well and field recoveries, the costs and associated reserves, and the resulting economics. In the cases below, we have insufficient information to attribute reserves rigorously, but we do have enough information to make reasonable inferences about incremental economic reserves attributable to UBD techniques.

This practical method of considering reserve contribution does not address recovery factor. However, we discuss recovery factor where information is available.

In most of the case histories discussed below, UBD is used in partially depleted fields to revive production. UBD also is often a crucial element of a larger system, as shown by four of the case histories in which UBD is used in conjunction with horizontal drilling. Neither horizontal nor UBD by itself could produce the reserves that the two can when combined.

A more complex example of the use of UBD is seen in enhanced oil recovery (EOR) projects, which are most often undertaken in mature fields with semi-depleted reservoirs. The total system to achieve incremental recovery comprises infill wells and surface facilities to inject a fluid or gas. If wells cannot be drilled successfully in a depleted field without UBD, then UBD becomes an enabling technology crucial to the success of the entire EOR project. As such, it is reasonable to assign UBD a value equal to a portion of the entire project value or project reserves.

Types of reserves additions

The incremental reserves cited in this paper are rooted in three fundamental benefits of UBD:

  1. Reduced formation damage. Reduction in skin may manifest itself in several ways, most of which are difficult to predict accurately with the tools currently available. In a qualitative sense, reduced skin increases recovery by: allowing more intervals within a producing zone to contribute; lowering abandonment pressure; and lowering the net pay cutoff.

  2. Improved access. UBD techniques make it possible to drill wells in circumstances where conventional drilling techniques do not work. This was found to be the single biggest contributor to reserves in the case histories considered here. In many cases, UBD was used in underpressured or partially depleted reservoirs only after conventional wells proved expensive and unsuccessful.

  3. Reservoir evaluation while drilling. Many wellsite geologists can cite cases where productive zones have been missed completely because they were drilled overbalanced. They looked "good" on a log, but were not tested because of lack of shows, or because they produced little or nothing when tested.

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