Simulation of Inflow While Underbalanced Drilling With Automatic Identification of Formation Parameters and Assessment of Uncertainty

Abstract

Summary Underbalanced drilling (UBD) is defined as a drilling operation in which the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. The most widely recognized benefit of UBD is the reduction of formation damage by minimizing the drilling-mud leakoff and fines migration into the formation. It also facilitates the possibility for reservoir characterization during drilling. The purpose of this paper is to present (1) how to use the inflow data for the evaluation of formation properties, and (2) how to cope with the uncertainty of the results. An in-house multiphase reservoir simulator is used for the simulation of the formation reservoir-fluid flow during UBD. The model incorporates discrete consideration of the well with appropriate time-varying UBD boundary conditions. Capillary forces, which facilitate countercurrent imbibition of the drilling mud into the formation, are taken into account. The production rates during UBD depend on the formation properties and the drilling conditions. The inflow information is analogous to transient test data and can be used for the estimation of the reservoir-model parameters. With the progression of the drilling process, the amount of available inflow data increases and allows for a sequential history-match procedure. Statistical analysis of the inverse problem can then be utilized to determine the optimal level of parameterization that is justified by the quality and quantity of the measured data. This paper highlights (1) the use of a reservoir simulator to derive the inflow of fluids in heterogeneous reservoirs, and (2) its coupling with gradient-based optimization techniques for determination of unknown reservoir parameters. The simulation-whiledrilling approach proposed is fully automated and can be set up on a known or totally unknown reservoir model, introducing an iterative and automatic updating process of the reservoir model.