Underbalanced Drilling And Completion Operations to Minimize Formation Damage-Reservoir Screening Criteria For Optimum Application
Authors: Bennion D.B. , Lunan B. , Saponja J. 
Published: March 2010

Abstract
Underbalanced drilling (UBD) is becoming increasingly utilized worldwide as an alternative technique to conventional overbalanced drilling to reduce invasive near wellbore formation damage problems in oil and gas producing formations. It is also increasingly used as a method for low damage completion of injection or disposal wells. Properly designed and executed underbalanced drilling operations can eliminate or significantly reduce formation damage concerns with respect to such phenomena as mud or drill solids invasion, lost circulation, fluid entrainment and trapping effects, and potential adverse reactions of invaded drilling or completion fluids with the reservoir matrix or in situ reservoir fluids.
Underbalanced drilling and completion operations are not necessarily a panacea for all formation damage problems and, if an underbalanced drilling operation is poorly designed and executed, there is the potential for greater damage than if a properly implemented overbalanced operation had been used.
This paper reviews reservoir characteristics and properties which should be considered to ascertain if a reservoir is a prime candidate for a UBD operation. This would include such items as reservoir lithology, matrix composition, micro or macrofracturing and heterogenous porosity features, such as large interconnected vug systems, permeability variations, heterogeneity, reservoir pressure, reservoir fluid types and other factors. A discussion of optimum operating parameters with respect to the design and execution of an underbalanced drilling operation for a particular reservoir application is provided. The selection of the appropriate type of equipment to be used (jointed pipe vs. coiled tubing), mode of underbalance condition generation (through string injection, parasite string or microannular injection), base fluid selection (oil, water or gas), gasification medium selection (nitrogen, oxygen content reduced air, air, flue gas, natural gas) and surface equipment considerations for a given UBD operation, are reviewed.

Introduction
Underbalanced drilling (UBD) is utilized with increasing frequency as a method to increase oil and gas well profitability by reducing invasive formation damage to maximize production rates, increase ROP to reduce drilling time and costs, and to obviate various technical problems associated with typical overbalanced drilling operations such as differential sticking and lost circulation.
There have been many cases in which underbalanced drilling has been spectacularly successful in providing added value to the operators utilizing the technology. These examples have been highly publicized and have resulted to some degree in the rapid increase in popularity of the technology. Unfortunately, there have also been many instances where the results from underbalanced drilling operations have been less successful. In many cases this is not due to a deficiency in the basic technology of underbalanced drilling, but rather is related to the faulty execution or inappropriate application of the UBD technology.
Not every reservoir is an ideal candidate for an underbalanced drilling operation and, in some cases, distinct disadvantages may exist in trying to execute an UBD operation in comparison to a simpler more conventional overbalanced application. This paper reviews some of the key criteria and design considerations that should be examined when evaluating a well and provides a discussion of screening criteria to be applied to ascertain if a given formation is, in fact, a viable UBD candidate.