A new methodology to derive both relative permeability and effective permeability reduction profile from numerical simulations of formation damage experiments

Near wellbore flow properties are affected by mud and mud filtrate invasion during overbalanced drilling operations. The degree of alteration depends on a large number of parameters such as the nature and characteristics of the drill-in fluid, formation properties and operating conditions. Laboratory mud filtration experiments have been conducted for many years to determine the extent and the degree of formation damage due to drill-in fluid invasion. This paper proposes a new methodology to interpret formation damage tests performed with water-based muds. Numerical simulations are performed to quantify independently, on the one hand the impact of mud solids invasion and on the other hand the effect of multiphase flow process on the global permeability damage. Effective oil return permeability profiles are first determined through local pressure measurements along the core at different backflow rates. Then, corresponding relative permeabilities are determined from matching of the pressure differences and cumulative production evolution as a function of time. Results show that a good fit between experiment and simulation is obtained with a unique set of relative permeability curves for a given formation permeability. Their shapes are very similar to what is obtained using standard water/oil displacement experiments. The efficiency of the restoration of effective permeability is highly dependent on the oil backflow rate. The higher the imposed oil flow rate, the better the restoration of initial permeability. The main advantage of the proposed procedure is to provide, from a single laboratory test, a consistent interpretation of both permeability damage mechanisms (mud solids deposition and filtrate invasion). This leads to a better diagnosis of the origin of the damage. Finally some recommendations are given to improve the design of laboratory formation damage experiments and to interpret natural cleanup of open hole completion. Guidelines are also provided to select the least damaging drill-in fluid formulation for a given permeability formation.