Uncertainties in productivity loss due to drilling induced formation damage are studied using a near-wellbore numerical simulator, which integrates laboratory data on formation damage testing. To quantify these uncertainties, experimental designs combined with the Response Surface Methodology are used to assess the impact of uncertain parameters on formation damage. This approach allows to identify the most influential parameters on well productivity loss and to estimate the risks of formation damage. A good control of the most sensitive parameters can limit productivity loss. This approach provides key recommendations for the selection of drilling fluid to maximize well performance.
It has been recognized that formation damage due to drilling fluid has a huge impact on well productivities, especially for horizontal wells which are completed with open hole or slotted liner. The degree of formation damage depends upon a large number of parameters such as the nature and characteristics of drilling fluid, formation properties and operating conditions. Inoverbalanced drilling (OBD), formation damage is caused by the presence of filter cakes and invading drilling fluid in porous media1–9. In underbalanced drilling (UBD), even if the pressure gradient is negative, drilling fluid invasion into reservoir can still occur due to the presence of capillary pressure or temporary overbalanced pressure10–12. The economic impact of poor productivity of open hole wells has pushed toward significant efforts in recent years to study laboratory testing techniques10,11 and numerical modelling methods13–21 for assessing drilling induced formation damage.
Although some progress have been made in laboratory experiments and numerical modelling to quantify the impact of formation damage on well productivities, it does not really consider the uncertainties in laboratory measurement and especially the uncertainties in input parameters for formation damage simulations. Due to limited laboratory conditions, the characteristics of filter cake and the effect of damage by filtrate in porous media cannot often be accurately measured. The depth of filtrate invasion depends also on operating conditions (drilling pressure, rate of well penetration ?) as well as formation properties (reservoir permeability, porosity, 1). Generally speaking, we are not sure of the real value of some parameters, but we can estimate their ranges of variation. It is therefore essential to quantify the impact of these data uncertainties on the evaluation of the drilling induced formation damage.
We present here a study aiming at quantifying theuncertainty in productivity loss. Experimental design methodologies are used with the near-wellbore formation damage simulator. Experimental design combined with the Response Surface Methodology (RSM) 22–26 has been recently used in oil/gas industry for uncertainty study in oil reserves, production forecasting, development optimization, etc. In this paper, we use this method to assess the impact of various uncertain input parameters on formation damage. This method allows to identify the most influential parameters on wellproductivity and to estimate the risks of formation damage. A good control of the most sensitive parameters can limit the nearwellbore formation damage. Uncertainty assessment can also help the selection of drilling fluid to maximize well performance.
