The invasion of drilling fluid filtrate into the region around the wellbore has long been recognized as one of the major factors contributing to damaging the reservoir formation. The volume of filtrate has a significant effect on the degree of damage, which is usually evaluated in API or HPHT static filtrate. Basically, filtration volume is a function of filter cake implementation, filtrate character, wellbore conditions and formation parameters. Under static conditions, as filtration proceeds a filter cake is formed and its solid content represents the particle size distribution of the whole drilling fluid. Under dynamic conditions, filter cake deposition mechanisms differ from those under static conditions due to the influence of hydrodynamic forces caused by drilling fluids flow. Consequently, the constitution of the dynamic filter cake is varies from that formed under static conditions, giving it different characteristics and filtration behaviour.
The scope of this study was to investigate the influence of the shear rate applied to the cake surface during dynamic filter cake deposition, on the filter cake constituents and dynamic filtration behaviour along horizontal wells. Extensive dynamic filtration experiments were conducted to test the filter cake and filtration parameters at the equilibrium dynamic stage. Three muddles of three different particle size distributions of Calcium Carbonate water-based drilling fluids were tested at different shear rates up to 400 sec-1.
The experimental results showed significant variations in dynamic filter cake parameters and filtration behaviour under different shear rates applied to the horizontal cake surface. Dynamic cake constituents and behaviour were also seen to differ from those of cakes formed under static conditions and one state cannot be taken to represent the other. Moreover, a critical value of shear rate can be detected from the dynamic cake parameters. The results also clearly demonstrated that a better cake quality was obtained at high shear rate values and a poor cake quality at low shear rates
Mud filters into the formation when an overbalanced drilling operation exists downhole. In this type of drilling operation, the hydrostatic head of the drilling fluid is kept greater than the formation pore pressure. This initially causes the wellbore fluids, and solids that are smaller than the pores of the formation, to inter into the formation. Particles larger than the rock pores are filtered out and form the initial layers of an external filter cake. Fine and coarse particles then accumulate on these pre-deposited large particles and the associated fluids pass through the deposited filter cake into the formation. The invading fluids can interact with the formation fluids and matrix and change the formation permeability. Ferguson & Klotz (1954), concluded that if the filtrate damages the permeability of the oil-bearing formation, the resultant damage to the oil well productivity depends upon the distance that the filtrate invades into the oil bearing formation and the mechanical and chemical characteristics of the fluids. Reduction of filtrate volume will, therefore, reduce invasion distance and increase well productivity.