Formation Testing While Underbalanced Drilling

Abstract

Abstract Underbalanced drilling is more and more considered an option for the drilling of low-pressured reservoirs and a method to prevent formation damage, especially in the case of horizontal wells. The production rate during drilling depends on reservoir characteristics and the degree of undcrbalancc. As such it carries similar information as a pressure transient test. However, the boundary conditions for this particular situation are distinctly more complex. Underbalanced drilling results in boundary conditions which vary with time. Measurements of these boundary conditions downhole would make it possible to de-couple the wellbore physics from the formation. The significance of such measurements can be shown by simulating the formation response to the time-varying boundary conditions. In order to do this, a (semi-)analytical model has been developed. The model describes the dynamic process of producing while lengthening the wellbore. It connects pressure with rate of penetration and can be used to predict production rates as a result. In addition we developed an analytical procedure that allows direct interpretation of the rate data, yielding estimates of reservoir parameters. A Boundary Element Model (BEM) has been developed to model the rate transient response for time varying well parameters (underbalanced drilling).