A Rigorous Formation Damage Skin Factor and Reservoir Inflow Model for a Horizontal Well
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AbstractIn this paper, we present a new analytical model for formation damage skin factor and the resulting reservoir inflow that includes the effect of reservoir anisotropy and damage heterogeneity. The shape of the damaged region perpendicular to the wellbore is based on the pressure equation for an anisotropic medium, and is thus circular near the well and elliptical far from the well. This assumption gives an appropriate skin factor even for a small penetration of damage. The new model can be used for any distribution of damage along the well. The new skin factor model can be easily incorporated into any existing model of reservoir inflow for a horizontal well.We also present a new reservoir inflow equation for a damaged parallelepiped-shape reservoir drained by a horizontal well. This equation shows that the ratio of the reservoir thickness to the drainage length perpendicular to the well is a very important factor for determining the influence of formation damage on horizontal well productivity because the linear flow geometry becomes dominant for a thin reservoir. The larger the ratio of thickness to drainage length perpendicular to the well axis, the larger the influence of nearwell formation damage on well productivity.In examples presented in the paper, a truncated elliptical cone of damage with a larger penetration near the vertical section of the well (at the heel) is presumed to compare our model with an existing horizontal well damage skin factor model. The comparisons show that, for an anisotropic reservoir with shallow-penetrating formation damage, our model avoids the negative skin result calculated by the previous model and predicts higher skin factor than the existing model. For deep damage, the new model predicts lower skin factor than the previous model.The impact of formation damage on the overall production is also shown in examples. In general, the effect of near well formation damage for a horizontal well completion is relatively small compared with vertical wells. However, if the reservoir thickness is large, radial flow becomes dominant and the impact of formation damage on a horizontal well is more significant, more like that of a vertical well. Our model gives a simple, analytical expression for determining this effect.
