Under balanced drilling (UBD) is a drilling operation in which effective bottom hole pressure caused by circulation of drilling fluid is taken under near well bore formation pressure. As a result influx of water from aqueous drilling fluid into the formation is prevented. Even though UBD is expected to prevent formation damage and improve well productivity, in some low permeability cases with high capillary pressure counter-current imbibition occurs and reduces well productivity specially for open hole completed horizontal wells. In this paper a new approach to model possible spontaneous imbibition during UBD are presented and the related formation damage is taken into account. The impact of this type of formation damage on well productivity is simulated using a commercial reservoir simulator coupled with a developed toolbox. The simulation result is compared against the absence of spontaneous imbibition in reservoir models with lower capillary pressure. Sensitivity analysis shows that formation damage caused by UBD depends on various reservoir parameters and drilling conditions. Different cases of ROP, hydrostatic pressure of drilling fluid, initial water saturation, and porosity in low permeability reservoir caused by capillary pressure are considered. Surface tension can vary with pressure and temperature; however, it is assumed as negligible in this study. Results show this formation damage should be considered during economics and feasibility studies.


In overbalanced drilling (OBD), drilling fluid filtrates porous media and alters its properties. Formation damage caused by drilling fluid acts as a key point in well productivity forecasting and economical analysis in filed development. The most important types of formation damage include "permanent damage" and "mud cake near wellbore". In the former permeability near wellbore is reduced and as a result well productivity is lost. The latter acts as a resistance against the fluid flow near wellbore region.

Under balanced drilling (UBD) can be used to minimize the formation damage. Bottom hole pressure caused by circulating drilling fluid in UBD is lower than the pore pressure of reservoir formation and this prevents filtration of drilling fluid into the formation. Consequently, formation damage is reduced and well productivity is increased compared with OBD techniques dealing with huge mud loss. In addition, UBD technology increases rate of penetration.

Although it is generally believed that UBD improves well production more than OBD, in low permeability reservoirs fluid imbibes into matrix in counter current direction of the hydrocarbon flow when aqueous drilling fluid is used. This imbibition process occurs due to high capillary forces and results in a kind of formation damage called "phase trapping" which reduces the well productivity.1, 2

The main objective of this paper is to present numerical simulation results of this phenomenon and its impact on longtime well productivity. Besides, we intend to predict inflow during UBD which is important to prevent unsteady UBD pressure conditions and design of surface equipment for drilling operations. Accurate prediction of inflow is also needed for safely hydrocarbon production at surface during UBD.

The impact of formation damage caused by imbibition on well productivity is simulated using a commercial reservoir simulator. To simulate this phenomenon, a new approach is presented for simulation of imbibition near wellbore. An equivalent water source is used to model the well filled by drilling fluid. A toolbox is developed which works in parallel with a commercial reservoir simulator. For the purpose of evaluation a numerical reservoir model including single horizontal well with and without spontaneous imbibition is used. Various rates of penetration as well as UBD pressure differentials are applied to take account impact of formation damage on well production.

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