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.


Advantages of UBD have been well recognised. They include increasing penetration rates and stimulation of gas production due to the differential pressure between the formation and the wellbore. Real-time formation evaluation allows for production testing and determination of production against geology. Thus, productive reservoir zones and formation properties can be already determined during the drilling process.1–3

UBD is important for preventing formation damage during the drilling. Damage mechanisms have been extensively discussed in the literature4,5 and are caused, e.g., by fines migration, external solids entrainment, reactive clays, mineral dissolution, bacterial damage, polymer adsorption, scaling, formation of stable emulsion and wettability alterations. Almost all of these are affected, to some extend, by the invasion of drilling fluids as a consequence of the conventional overbalanced drilling. This can be particularly detrimental for the productivity of high-permeability, macrofractured and underpressured reservoirs.

However, despite facilitation of UBD technology, formation damage can not be completely excluded in water wet formations when an aqueous mud system is used. This is due to the prevailing adverse capillary forces.4 As soon as the fluid comes in contact with the rock, it rapidly imbibes along the sandface into the matrix due to the imbibition process, which can even occur in countercurrent direction to the gas flow. The problem of the corresponding hydraulic damage can be very pronounced in low permeability tight-gas reservoirs; particularly if the reservoir is subirreducible saturated.4 In the framework of this paper, only hydraulic damage is considered, i.e., mainly the process known as phase trapping.

The severity of the hydraulic damage associated with the imbibition effect depends on the shape of the gas relative permeabilities, exposure time of the fluid to the sandface, as well as on fluid, reservoir and drilling parameters. It should be noted that productivity can be further curtailed in case of (i) additional interactions between the invading fluid and the matrix (such as clay swelling) or (ii) permeability alteration within the near wellbore zone (such as dilatancy effects), related to any of the aforementioned damage mechanisms.

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