E. van Oort

The swelling of clays: molecular simulations of the hydration of montmorillonite.

The swelling of clay minerals on contact with an aqueous solution can produce strong adverse effects in the exploration and production of gas and oil. Molecular dynamics and Monte Carlo simulations were used to study the mechanism of swelling of sodium-montmorillonite. The simulations showed that the abundant clay mineral has four stable states at basal spacings of 9.7, 12.0, 15.5, and 18.3 angstroms, respectively. The amount of swelling and the locations of the stable states of sodium-montmorillonite are in good quantitative agreement with the experimental data.

Manipulation of Coupled Osmotic Flows for Stabilisation of Shales Exposed to Water-Based Drilling Fluids

Coupled osmotic flows have been studied as a means of stabilising shales exposed to water-based muds. The prime factor that governs the magnitude of chemical osmotic flow, i.e. the shale-fluid membrane efficiency, was investigated in detail. Its dependence on shale parameters, fluid parameters and external conditions was quantified. Membrane efficiency was found to increase with an increase in (hydrated) solute-to-pore-size ratio, with an increase in the shale`s high-surface area clay content and with a decrease shale permeability when increasing effective confining stress. Moreover, new drilling fluid chemistries for improving the efficiencies of low- and non-selective shale-fluid systems were identified. Induced osmotic flow with optimised shale-fluid membrane efficiencies in water-based environments is presented as a new strategy for improving wellbore stability in shales.

Silicate-Based Drilling Fluids: Competent, Cost-effective and Benign Solutions to Wellbore Stability Problems

New and improved water-based drilling fluids based on soluble silicates have been developed and applied in the field in a unique collaboration between BW Mud, Mobil NSL, BP Exploration and Shell Research. Silicate-based muds are introduced as superior fluids for drilling troublesome formations like intact and (micro-)fractured shales and chalks. In addition, these inorganic systems are environmentally friendly and inexpensive. Details are presented on the wellbore stabilisation mechanism of silicate-based muds and their drilling fluid engineering. In addition, field trial results are presented which demonstrate the excellent cuttings- and wellbore-stabilising capacity of these exciting new mud systems.

Integrated Borehole Stability Studies: Key to Drilling at the Technical Limit and Trouble Cost Reduction

Drilling the Limit (DTL ) is a powerful initiative to drill wells at minimum time and cost using best-in-class technology. Pre-requisite to drilling at the technical limit, however, is that the borehole is perfect, i.e. the borehole is stable enough to eliminate downtime from stuck pipe and lost circulation incidents, the annular clearances are sufficiently free from cuttings to minimize wiper trips etc. This requirement is non-trivial when drilling challenging wells, e.g. wells drilled at high deviation and extended reach in reactive formations, wells in tectonically active areas, wells in deepwater environments with very small drilling margins etc. Achieving the perfect borehole for such wells is facilitated by including an integrated borehole stability (IBS) study in the well planning phase and by implementing its guidelines during well execution. A review of the IBS process is presented in this paper with actual examples of IBS results that were used to enable DTL TM and minimize trouble costs on challenging wells.

Barite Sag: Measurement, Modelling and Management

A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behaviour. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field. Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60-75°. Drill-pipe rotation was particularly beneficial in minimising barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole. The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in HP/HT and extended-reach drilling operations.