Finite difference modeling of formation damage during underbalanced drilling in a tight gas reservoir

Underbalanced Drilling has recently emerged as an important technique for hydrocarbon reservoir drilling to minimize formation damage which is a common mechanism encountered during overbalanced drilling operations. However, underbalanced drilling is not a damage free solution. Tight gas reservoirs exhibit very low porosity and permeability, and sub-irreducible water saturation. Based on these properties, while using water-based mud system as drilling fluid for drilling these kinds of reservoirs, there will be some formation damage due to the water invasion into the formation. This kind of damage relates to a process called Counter-current Spontaneous Imbibition. This paper presents the results of a numerical model analysis of formation damage induced by water invasion during underbalanced drilling in tight gas reservoirs from counter-current spontaneous imbibition. The model assumes radial two-phase flow (water and gas) in a tight gas reservoir solved using finite difference methods. This model provides significant value by evaluating water invasion and the resulting drop in gas permeability, resulting in a better understanding of formation damage and its consequent effects. The output of the developed numerical model is a near wellbore water saturation profile, and skin factor which is predicted based on exposure time for a specific point on the wellbore. The effects of drawdown pressure, exposure time (time of underbalanced drilling), and temporary near balanced and overbalanced conditions which may be encountered during an underbalanced program are discussed to.