Abstract

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 the pores near the well bore and, consequently, prevent additional fluid invasion into 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 in the well. In this case the fluid would present proper flow and solids transport properties in the well and would not invade the rock formation.

Previous studies by the authors showed that the viscoelastic behavior of fluids can be a major restriction to fluid invasion. This article deals with the optimization of viscoelastic parameters, such as the first normal stress difference and the extensional viscosity, for minimizing fluid invasion through oil reservoirs. The invasion analysis is based on a two phase (viscoelastic fluid + Newtonian oil) radial flow through porous media and supported by a commercial CFD package. Static and dynamic filtration phenomena are represented.

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.

Introduction

In petroleum engineering, well drilling is an area of continuous development in order to improve the current technologies and look for new ones which can be applied to the adverse conditions faced nowadays and enable operations that were only conceptual some decades ago.

One of the drilling fluid basic functions is to exert hydrostatic pressure over the permeable formations to avoid the formation fluid invasion into the well while the drilling operation takes place. The fluid pressure is normally kept above the formation pore pressure to prevent from kick events (formation fluid invasion to the well), that, in some cases, can lead to an uncontrolled influx (blowout). This concept, called overbalanced drilling, is traditionally employed in most of the drilling operations worldwide.

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 in the 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. The invasion of fluid and solid particles during this process may cause damage to formation around the well.

Two invasion mechanisms are notable in the well. The first, called static filtration, occurs when the fluid pumping is interrupted and, from that point on, filtration occurs due to the difference between the hydrostatic pressure in the well and the reservoir pressure. The static filtration rates are controlled by the continuously increasing thickness of the filter cake.

The other invasion mechanism, called dynamic filtration, occurs when the fluid is pumped through the well. In this process, the cake thickness is resultant from the dynamic equilibrium between solid particles deposition rate and the erosion rate due to the shear stresses generated by the fluid flow in the wellbore. Thus, the filtration rate to the formation tends to stabilize around a certain value while the cake thickness turns constant. This process is known as cross filtration in others areas of Engineering. The invasion phenomenon approach must regard both mechanisms.

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.

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