The performance of stochastic designs in wellbore drilling operations

© 2018, The Author(s). Wellbore drilling operations frequently entail the combination of a wide range of variables. This is underpinned by the numerous factors that must be considered in order to ensure safety and productivity. The heterogeneity and sometimes unpredictable behaviour of underground systems increases the sensitivity of drilling activities. Quite often the operating parameters are set to certify effective and efficient working processes. However, failings in the management of drilling and operating conditions sometimes result in catastrophes such as well collapse or fluid loss. This study investigates the hypothesis that optimising drilling parameters, for instance mud pressure, is crucial if the margin of safe operating conditions is to be properly defined. This was conducted via two main stages: first a deterministic analysis—where the operating conditions are predicted by conventional modelling procedures—and then a probabilistic analysis via stochastic simulations—where a window of optimised operation conditions can be obtained. The outcome of additional stochastic analyses can be used to improve results derived from deterministic models. The incorporation of stochastic techniques in the evaluation of wellbore instability indicates that margins of the safe mud weight window are adjustable and can be extended considerably beyond the limits of deterministic predictions. The safe mud window is influenced and hence can also be amended based on the degree of uncertainty and the permissible level of confidence. The refinement of results from deterministic analyses by additional stochastic simulations is vital if a more accurate and reliable representation of safe in situ and operating conditions is to be obtained during wellbore operations.Published versio

Prediction of rock alteration patterns: a potential tool in mineral exploration

Hydrothermal alteration of a quartz-K-feldspar rock is simulated numerically by coupling fluid flow and chemical reactions. Introduction of CO2 gas generates an acidic fluid and produces secondary quartz, muscovite and/or pyrophyllite at constant temperature and pressure of 300 degrees C and 200 MPa. The precipitation and/or dissolution of the secondary minerals is controlled by either mass-action relations or rate laws. In our simulations the mass of the primary elements are conserved and the mass-balance equations are solved sequentially using an implicit scheme in a finite-element code. The pore-fluid velocity is assumed to be constant. The change of rock volume due to the dissolution or precipitation of the minerals, which is directly related to their molar volume, is taken into account. Feedback into the rock porosity and the reaction rates is included in the model. The model produces zones of pyrophyllite quartz and muscovite due to the dissolution of K-feldspar. Our model simulates, in a simplified way, the acid-induced alteration assemblages observed in various guises in many significant mineral deposits. The particular aluminosilicate minerals produced in these experiments are associated with the gold deposits of the Witwatersrand Basin

Molecular dynamics study of methane in water: diffusion and structure

We present molecular dynamics simulation results for the diffusion coefficients and structure of water-methane mixtures in constant NPT ensembles, at T 270, 300K and P 8.104 107 Pa. The systems we have studied consist of one, four and eight CH4 molecules and varying H2O molecules per unit cell, which correspond to methane concentration of 0.081, 0.324 and 0.643 mol/l, respectively. The intermolecular potentials used in all the simulations were the four-site TIP4P model of water [1] and the fitted Lennard-Jones 12-6 potential for CH4ZH2O [2]. Our results show that the methane concentration has little impact on the structure of water and the formation of hydrogen bonds (H-bonds) between water molecules. The H-bond numbers, H-bond length and the H-bond angle are independent of the methane concentration at the temperatures and densities examined in this study. We also find that the number of H-bonds and angles are sensitive to the temperature. The rise of temperature produces a decrease in the number and an increase in the angle of the H-bonds. Enhanced structuring of the hydration-shell water molecules is indicated by an increase of the first and second peak in the water oxygen-oxygen radial distribution function as temperature is decreased. The self-diffusion coefficient of water is sensitive to the methane concentration and temperature