UBD Well Productivity: A "Stop and Go" Prediction Tool
Authors: Dipeolu Lanre 
Published: December 2007

Abstract
The technical requirements to demonstrate values of underbalanced drilling projects before taking an executive decision could be overwhelming and probably hurt the advancement of a great opportunity. This paper will discuss the recent research results on inflow performance of underbalanced wells with special focus on tools developed to distill the elements and other parameters contributing to the enhanced flow in UBD wells. It will address the inherent technical challenges and solutions in predicting the productivity performance of a low skin, high producer wells. The methodology and critical elements involved in the analytical and numerical solutions offered will be discussed. The range of validity of application of these methods will also be covered. Also discussed, will be the inflow characteristics of horizontal and extended-reach wells and why these categories of wells benefit from newly developed technology. Discussion will also outline the results of the formulation, to illustrate the reservoir-engineering framework necessary to capture the near wellbore effect. Finally, extension of mathematical equations previously prescribed for the response of vertical or horizontal wells, within the drainage volume of any reservoir geometry will be included.

Introduction
The susceptibility of horizontal, high-angle or extended-reach wells to drilling-induced damage has resulted in an increased number and advances in applications of damage-reducing technology such as underbalanced drilling and special recipe drilling fluids.
On the other hand, the process of valuation and business proposals, which vary with organisations and the level of risk antipathy, remain potential issues. The need to have a quick tool for gains assessment and as a first order decision mechanism, has gained prominence. This consequently led to an unflinching search for mathematical methods capable of describing and representing inflow behaviour under a "zero-damage", similar to the UBD characteristics assumption pattern. The research led to an adoption and extension of previous work on semi-analytic model on inflow performance of partially and fully opens wells. (Ref. 1,2,3)
The intentional flow of reservoir fluids into the wellbore, caused by the drawdown gradient during underbalanced drilling, naturally prevents well damage that results from drilling fluid and solid invasion. Field data and simulation results show that this phenomenon improves the skin and well conductivity, kh. (Ref. 4,5,6,7).These define the requirements for the mathematical model necessary for the prescription of such observations. Considerable materials have been published on inflow performance of vertical and horizontal wells, most of which are limited to the knowledge of conventional drilling. (Ref. 8,9,10,11).The underbalanced drilling, especially in an open hole completion, portends the inflow performance in the technical limit range.Figure 1 illustrates the near wellbore damage in an overbalanced well compared to no-impairment from an underbalanced hole condition. The impact of such impairment and its tendency to clean up will depend on the anisotropy, permeability and formation thickness of the individual reservoir members encountered in the wellbore. (Ref. 12).Figure 2, is a schematic of the expected variation in skin (S1, S2 etc) with a permeability distribution that could constitute partial open to flow when damage occurs. Each individual member of the reservoir that is penetrated will exhibit its specific tendency to flow and damage. For example, retained permeability tendency.