The drilling of oil wells through shales, which constitute the majority of rocks in the stratigraphic column, may present instability problems due to physico-chemicals interactions between the drilling fluids and these rocks. The costs to solve these problems are very high and, depending upon the intensity of these problems, wells can be lost. This paper presents new equipment designed to apply hydraulic and chemical gradients through shale samples in order to evaluate the shale interaction with a given fluid. The proposed test allows the determination of rock permeability, the coefficient of reflectivity (membrane efficiency) and the ionic diffusion coefficient. These parameters are essential to carry out proper wellbore stability analysis when taking into account the shale-drilling fluid interaction. Tests carried out on Brazilian offshore shale using different saline solutions confirmed the good performance of the equipment. Details of the equipment and test results are described.
1. INTRODUCTION
Drilling through shales has presented serious problems of instability and most of them have been attributed to interactions between this rock and the drilling fluid. Normally, long periods of time are necessary to solve some of these problems and that contributes to raise drilling costs. 10-years old data, [1], indicate that nearly 30% of the additional costs during drilling operations are caused by wellbore instabilities and from these, almost 90% occur during drilling through shales. These problems used to consume more than US$ 500 million per year considering the technology available then. Much has been learned since then, and that allowed the successful drilling of highly inclined wells with the use of a new generation of drilling fluids. However, on average, the losses are still quite high and there is much to be done to transform the research results into daily practice. The proper selection of the drilling fluid for a given situation is still an issue to be dealt with in drilling operations.
The ideal drilling fluid concerning stability of wells must keep the confining effective stresses around the well high enough to preclude rock failure. This can be achieved by at least three distinct manners. Firstly, by avoiding pore pressure increase due to fluid penetration, through the use high-entry pressure fluids, i.e., oil-based-like type of fluid. Secondly, by playing with the osmotic effects caused by water-based, saline fluids. Thirdly, by the use of invert-emulsion fluids, [2], that combine the two previous mechanisms.
The pore pressure control, achieved through osmotic effects, can be exercised through the concepts of mass transport due to hydraulic and chemical gradients. In low permeability rocks such as shales, these gradients induce changes in pore pressures due to effects of hydraulic and ionic diffusion and osmotic effects that can change with rock capability to restrict the ion flux through the formation. This is the most studied topic related to shale-drilling fluid interaction.
In order to evaluate the shale-drilling fluid interactions, new equipment capable to simulate in situ pressure conditions was developed. In this equipment, hydraulic and ionic gradients can be imposed in order to estimate the rock permeability, the coefficient of reflectivity (membrane efficiency) and the ionic diffusion coefficient. These parameters are used as input in borehole stability programs that consider the physical-chemical interactions.
