Bottomhole Pressure Measurements: Indispensable Tool for Optimizing Underbalanced Drilling Operations
Authors: Pérez-Téllez C. , Urbieta-López A. , Miller A. , Banda-Morato R.G. 
Published: October 2008

Abstract
It has been forecast that by the end of the first decade of this millennium, offshore applications of underbalanced drilling (UBD) tools and technology will play a significant role in the petroleum industry, particularly in deep waters. Since implementing the UBD technique in offshore operations requires serious efforts and very high expenses, knowledge gained from UBD onshore operations must carefully be studied before being implemented into offshore applications.
Even though the UBD technique has successfully been used to minimize operational problems and to achieve drilling to the planned depth, without down time and on budget, the petroleum industry recognizes that the greatest advantage of drilling underbalanced is to increase well productivity through the formation damage prevention during the drilling process. The use of bottomhole pressure (BHP) sensors during UBD, represents one indispensable alternative to optimize the UBD process while simultaneously achieving both purposes, drilling to planned depth and avoiding formation damage.
Operational process improvement, actual underbalanced condition, nitrogen injection optimization, actual pressure drop, reservoir pressure while drilling, and hydraulic model evaluation represented some of the benefits obtained with the BHP sensors during their very first application in Mexican UBD operations.
This work presents the recent results obtained by using BHP memory tools during the ongoing Mexican UBD program. Also, it shows a systematic analysis of the potential savings in terms of cost and time factors that could be obtained if the UBD technique were used to simultaneously achieve drilling to the designed depth and avoiding formation damage.

Introduction
Ever since 1992 UBD technology has been recognized as a mean of drilling wells that are not economically viable if drilled using conventional (overbalanced) tools and methods. So far, this technology has been extensively used to drill thousands of onshore wells around the world and offshore applications have consequently been increased.
Considering that a drilling engineer defines a successful well as one that is drilled to depth according to plan without any down time and on budget, and that a production or reservoir engineer defines a successful well as one that yields maximum productivity1, the general acceptance of this technology has predominantly been justified and documented from two different points of view. Mainly, from a drilling engineering point of view, on the bases of improving drilling performance, enhancements such as increasing rate of penetration2–4, reduction in lost circulation5,6, and virtual elimination of differential sticking problems6,7 have considerably been documented with actual field examples. However, from a production or reservoir-engineering point of view there is a comparative lack of documentation on the improvements attainable from production optimization and reservoir engineering aspects. This is due to the fact that very little material has been published on well data enhancements8 and no real evidence of a formation damage evaluation has been reported after a UBD operation.