Numerical Simulation of the Gas Inflow During Underbalanced Drilling (UBD) and Investigation of the Impact of UBD on Longtime Well Productivity

Abstract

Abstract UBD is defined as a drilling operation where the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. The most widely recognised benefit of underbalanced drilling is the reduction of formation damage, in terms of minimising the drilling fluid leak-off and fines migration into the formation. The main targets of this paper are to present the development of a methodology to simulate the gas inflow during UBD processes. The prediction of the inflow during the drilling process is important in order to maintain continuous UBD conditions. Therefore, a multi-phase UBD simulation tool is introduced. The model incorporates discrete consideration of the well with proper, time varying UBD boundary conditions, i.e., pressure and saturation. Capillary forces, which facilitate countercurrent imbibition, are taken into account. The simulation model is applied for a detailed investigation on the impact of UBD on the productivity of gas wells. Reservoir permeability, penetration rate, thickness of the formation as well as the underbalanced conditions are subjects of a sensitivity analysis. The main emphasis is placed on the derivation of the relation between productivity and the multi¬phase flow characteristics of the reservoir rock. The process of countercurrent imbibition is analysed. Technical contributions in this paper include the presentation of a novel UBD reservoir simulation model utilising special boundary conditions. The framework of conventional reservoir modelling using, e.g., an existing geomodel of the reservoir, can still be reverted to. The flexibility of the technical realisation is illustrated by means of a UBD simulation for a horizontal well utilising unstructured grids.