Modelling of Both Near-Wellbore Damage and Natural Cleanup of Horizontal Wells Drilled With a Water-Based Mud

Abstract

Abstract Prediction of formation damage that occurs in horizontal wells, often open hole completed, is a critical point for optimising an oil field development. The economic impact of near wellbore induced drilling-damage and cleanup efficiency has led to make significant progress in both experimental and numerical studies in order to assess the wellbore flow properties during oil production. There is a fundamental difference between water-based and oil-based mud invasions. In an oil-bearing formation, the displacement of the oil in place with an oil-based mud (OBM) filtrate is a miscible displacement process, while the displacement with a water-based mud (WBM) filtrate is a two-phase flow process (imbibition), generating high wetting phase saturations in the invaded zone. Then, during oil back flow, a portion of the wetting phase is trapped, leading to residual wetting phase saturations greater than the initial ones. Even in the absence of chemical interactions between filtrate and fluids in place, this induces an adverse water/oil relative permeability effect, which is an additional permeability impairment. In a previous paper, a methodology combining both experimental and numerical approach was presented to evaluate the natural cleanup of horizontal wells drilled with an oil-based mud (OBM). This paper presents an extension of the methodology for simulating both i) near wellbore invasion and permeability damage generated with a water-based mud (WBM), and ii) natural cleanup during oil back flow when the well is put under production. This paper describes a numerical approach to model the formation damage with WBM and predict well performance for natural clean up when the well is subject to a pressure drawdown. The kinetics of mud filtrate invasion, the filter cake properties and the filtrate/oil relative permeability curves in imbibition and drainage, together with damaged and return permeabilities are obtained from specific mud damage laboratory tests. Using these data, the mud filtrate invasion during the drilling phase is simulated, leading to a cone-type invasion depth along the horizontal well. This approach has allowed us to study the impact of various parameters related to fluids or cake properties, drilling conditions and natural cleanup process on the well performance.