In spite of continued improvements in logging technology, mud-filtrate invasion and related formation damage due to drilling fluids can result in the misinterpreted values of rock and fluid properties in the reservoir. Well planning with accurate information of target reservoir would not only optimize drilling operation and completion but also maximizes production of oil and gas.
To produce hydrocarbons effectively, the wellbore must communicate with formations beyond the altered zone and this can be accomplished by using proper perforation penetration or creating fractures. Thus the prediction of invaded zone is critical and a numerical model can be used for preplanning purposes.
In this study, the dynamic filtration process and the related penetration into the gas and oil bearing reservoirs are studied in a vertical openhole system using a Computational Fluid Dynamics (CFD) software package. The radius of filtrate invasion is determined by the unsteady-state three-dimensional multiphase fluid flow model. This study investigates the communication between fluids and formations during drilling with special emphasis on the effects of formation porosity and permeability, time, and overbalanced pressure. As drilling fluids Non-Newtonians such as Bingham plastic, Power-law, and Herschel-Bulkley fluids also considered in addition to Newtonian fluids. Mud filtrate invasion in multi-layer reservoirs model and effect of hydraulic fracturing operations are also investigated in this study.
The results provided an insight in the formation damage around wellbore and related reduction in the hydrocarbon flow due to altered fluid saturations. Accurate prediction of damaged zone around the wellbore will benefit different operations such as drilling fluids design, log interpretation, hydraulic fracturing and well completion.
During overbalanced drilling operations, hydraulic pressure of the borehole is greater than the pressure of the porous rock.
Therefore, the circulating drilling fluid forces the mud into the permeable horizons. This phenomenon creates a mud cake from slurry, as solid particles are deposited on the walls of the borehole. In hydrocarbon bearing formations, the drilling fluids push hydrocarbons out from the permeable formation near the borehole region thus impairing hydrocarbon productivity. In addition, the flushed zone with the filtrate from the drilling fluids causes misinterpretation of rock and fluid properties due to replacement of formation water and hydrocarbons, particularly formation porosity and permeability when measured by wireline logging methods. Especially, the mud-filtrate invasion affects the shallow investigation devices such as CNL (Compensated Neutron log), LDT (Litho Density log), MLL (Micro Laterolog) when water-base mud penetrates into oil and gas bearing reservoirs.
The flushed zone inside oil or gas bearing reservoirs serves as a blockage to produce oil or gas. Prediction of horizontal extent of the invasion is important, especially for the success of perforation and hydraulic fracturing operations, because the wellbore should communicate with formation, beyond the invaded region to produce hydrocarbon effectively.