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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 underbalance 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).


Underbalanced drilling is a technique that is gaining popularity rapidly. The Bottom Hole Pressure is kept below the formation pressure during drilling operations. This underbalance has to be maintained at all times. Advantages over overbalanced drilling are for example the prevention of formation damage and fluid losses. However, because of the restrictions of a continuous underbalance, well testing is complicated, especially while the formation is already producing while drilling. It would be useful if certain parameters could be determined from production data while drilling. Well trajectories could be adjusted instantaneously in case of disappointing results and reliable early reserve estimates could be made. This would mean a time and money saving alternative for well testing in underbalanced drilling operations.

Conventional well testing is based either on a convolution of fundamental pressure and flow functions or deconvolution, which assumes a constant production rate. Both methods provide a reservoir response which is a function of the reservoir characteristics. During underbalanced drilling, determination of this response is not quite as easy as in the ease of a conventionally drilled and completed well. Boundary conditions vary with time because the well is continuously increasing in length. Tools that accurately measure the flow rate distribution in the well during drilling do not exist at the moment. Therefore, local downhole rates should be solved from measurable data. The resulting rate transient holds valuable information about the characteristic reservoir response.

Transient Inflow Model

The pressure distribution due to any source is a solution of the diffusivity equation. For transient flow of a single phase, slightly compressible fluid in a porous medium assuming constant reservoir and fluid parameters this equation reads In underbalanced drilling operations a specific complication exists, concerning the boundary conditions. As a result of the continuous increase in well length the boundary conditions do not only change in value but in type as well. In the drilled part of the reservoir a constant underbalance is applied.

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