Designing drill in fluids which can guarantee minimum invasion into the reservoir rock is a must for open hole completion wells. The industry has proposed several ideas to deal with the problem, most of them based on adding bridging agents to the fluid formulation. Such agents would block pores near the well bore and, consequently, prevent additional fluid to invade the rock, but often require acid treatment for their removal.

A different concept is proposed for controlling invasion in this article: designing a polymer based fluid which would generate extremely high friction losses when flowing through a porous media without generating extra losses while flowing inthe well. In this case the fluid would present proper flow and solids transport properties in the well and would not invade the rock formation.

Experimental results obtained with the flow of different polymer solutions through consolidated inert porous media show that the viscoelastic behavior of fluids can be a major restriction to fluid invasion. Based on a viscoelastic resistive force proposition for the porous media, flow characterizarion maps defining viscous flow, elastic flow and viscoelastic flow regions are obtained

This article deals with the optimization of the drilling fluid viscoelastic parameters, such as the first normal stress difference and the extensional viscosity, aimining the minimization of fluid invasion through oil reservoirs. The arrival of this rheology controlled invasion concept brings to the polymer manufacturers a new challenge: the development of a highly viscoelastic polymer with non damaging behavior and adequate wellbore properties.


As the bit penetrates the reservoir rock, the drilling fluid invades the formation due to the positive pressure differential between the well and the reservoir rock. Portions of the liquid phase of the drilling fluid are lost to the adjacent formation while part of the solids present inthe drilling fluid, constituted by particles smaller than the formation pore size, penetrate the rock during the fluid loss period rapidly plugging the region around the well (Fig. 1). Larger particles accumulate on the wellbore walls, initiating an external cake formation.

One of the main problems caused by the filtrate presence in the reservoir zone is the significant decrease in its oil and gas permeabilities, reducing well productivity. The filtration process modeling and the prediction of its effects on the formation permeability and on the reservoir invasion depth is very important to design drilling fluids which can minimize these issues.

Two invasion mechanisms are relevant: the first, named static filtration, occurs when fluid pumping is interrupted and filtration occurs due to the difference between the hydrostatic pressure in the well and the reservoir pressure. The filtration rates are controlled by the continuously increasing thickness of the filter cake. The other invasion mechanism, called dynamic or cross filtration, occurs when the fluid is pumped through the well. In this process, the cake thickness results from the dynamic equilibrium between the solid particles deposition rate and the erosion rate due to the shear stresses generated by the fluid flow through the wellbore. Thus, the filtration rate to the formation tends to stabilize around a certain value while the cake thickness turns constant. Additionally, it is relevant to note that fluid invasion implies in the displacement of the fluids saturating the reservoir rock: hydrocarbons in gas or liquid phase and water.

Two mechanisms of invasion control are identified: one through the plugging solids that promote the external and internal cake formation and other through the liquid phase resistance to the flow in porous medium. The last one is the proposed topic of this study.

The central idea of this work is to identify and quantify the rheological phenomena governing drilling fluid invasion through porous media. The final objective would be to establish rheological parameters that define the non invasive solids free drilling fluid.

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