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

摘要

UBD is defined as a drilling operation where the pressure ofrnthe circulating drilling fluid is lower than the pore pressure ofrnthe target formation of interest. The most widely recognisedrnbenefit of underbalanced drilling is the reduction of formationrndamage, in terms of minimising the drilling fluid leak-off andrnfines migration into the formation.rnThe main targets of this paper are to present the developmentrnof a methodology to simulate the gas inflow during UBDrnprocesses. The prediction of the inflow during the drillingrnprocess is important in order to maintain continuous UBDrnconditions. Therefore, a multi-phase UBD simulation tool isrnintroduced. The model incorporates discrete consideration ofrnthe well with proper, time varying UBD boundary conditions,rni.e., pressure and saturation. Capillary forces, which facilitaterncountercurrent imbibition, are taken into account.rnThe simulation model is applied for a detailed investigation onrnthe impact of UBD on the productivity of gas wells. Reservoirrnpermeability, penetration rate, thickness of the formation asrnwell as the underbalanced conditions are subjects of arnsensitivity analysis. The main emphasis is placed on thernderivation of the relation between productivity and the multiphasernflow characteristics of the reservoir rock. The process ofrncountercurrent imbibition is analysed.rnTechnical contributions in this paper include the presentationrnof a novel UBD reservoir simulation model utilising specialrnboundary conditions. The framework of conventional reservoirrnmodelling using, e.g., an existing geomodel of the reservoir,rncan still be reverted to. The flexibility of the technicalrnrealisation is illustrated by means of a UBD simulation for arnhorizontal well utilising unstructured grids.