This paper presents an experimental and numerical study of Kuru grey granite impacted with a sevenbuttons drill bit mounted on an instrumented drop test machine. The force versus displacement curves during the impact, so-called bit-rock interaction (BRI) curves, were obtained using strain gauge measurements for two levels of impact energy. Moreover, the volume of removed rock after each drop test was evaluated by stereo-lithography (three-dimensional surface reconstruction). A modified version of the Holmquist-Johnson-Cook (MHJC) material model was calibrated using Kuru granite test results available from the literature. Numerical simulations of the single drop tests were carried out using the MHJC model available in the LS-DYNA explicit finite-element solver. The influence of the impact energy and additional confining pressure on the BRI curves and the volume of the removed rock is discussed. In addition, the influence of the rock surface shape before impact was evaluated using two different mesh geometries: a flat surface and a hyperbolic surface. The experimental and numerical results are compared and discussed in terms of drilling efficiency through the mechanical specific energy.This article is part of the themed issue ‘Experimental testing and modelling of brittlematerials at high strain rates’.
ABSTRACT: The potential for environmentally friendly nano-additives based drilling fluids is extended here in the context of the ongoing EU project ‘ORCHYD’ where a hybrid drilling technology is developed by combining High Pressure Water Jet (HPWJ) and Percussive Drilling to improve the efficiency of deep geothermal drilling. In this work, different types of nanomaterials such as graphene oxide, hybrid nano silica and their composite nanomaterials are used as additives in a water-based drilling fluid. The influence of additives on the rheological properties and the friction of the drilling fluid has been investigated. The laboratory results showed that an increase in the concentration of additives will increase the viscosity of fluids. A significant reduction in the coefficient of friction (during wear testing of steel material) was also revealed with the addition of GO and chemical functionalization. The need for further characterizing and the perspective to upscale the enhanced nano-additives based fluids for HPWJ and percussive drilling application, in deep geothermal conditions, are discussed. 1. INTRODUCTION This study is performed in the context of the EU project ORCHYD, https://www.orchyd.eu/proiect/, where an innovative drilling tool combining (high pressure water jetting and percussive drilling) will be developed to increase hard rock drilling rates, in deep geothermal wells. The potential of new green additives to the drilling fluids is thus investigated to account for the best compromise for efficient jetting, cutting and transport of the rock particles, and the lifetime extension the drilling tools. Deep geothermal reservoirs are being targeted for renewable energy supply worldwide (Huttrer, 2021), however the high pressures and high temperatures in deep geothermal wells impose significant challenges for desiring drilling fluids. Over the past decade, various additives such as polyacrylamide, organic modified clays, poly carboxylic acids, asphaltic compounds, treated lignite have been used in the drilling fluid to optimize the rheological properties, filtration control and reduce the fluid loss, shale inhibitors and thereby improving wellbore stability (Boyou et al., 2019; Vryzas et al. 2017). However, most of the polymer-based materials degraded thermally and chemically at relative low temperature, which lowers their effectiveness.
In pipe-laying and reeling operations, high levels of plastic deformation may occur in the pipe wall. The effect of plastic deformation should be investigated and quantified in order to qualify a steel material for a given pipe-laying process. This paper presents a numerical modeling approach for realistic simulation of ductile fracture, including results from FE welding simulations. A 3D model of a SENT specimen is adopted, where crack propagation is modelled using a Gurson model. An elastic-plastic hardening model is used to capture the global deformation of the specimen. Results compare well with experiments. This work is part of an effort to develop an experimental and numerical framework where the influence of welding (HAZ properties, residual stresses and hardening) in fracture assessment. Preliminary results show that there is no influence of residual stresses on fracture strength of an X65 ductile steel.
This paper presents an experimental and numerical study of Kuru grey granite impacted with a seven-buttons drill bit mounted on an instrumented drop test machine. The force versus displacement curves during the impact, so-called bit-rock interaction (BRI) curves, were obtained using strain gauge measurements for two levels of impact energy. Moreover, the volume of removed rock after each drop test was evaluated by stereo-lithography (three-dimensional surface reconstruction). A modified version of the Holmquist-Johnson-Cook (MHJC) material model was calibrated using Kuru granite test results available from the literature. Numerical simulations of the single drop tests were carried out using the MHJC model available in the LS-DYNA explicit finite-element solver. The influence of the impact energy and additional confining pressure on the BRI curves and the volume of the removed rock is discussed. In addition, the influence of the rock surface shape before impact was evaluated using two different mesh geometries: a flat surface and a hyperbolic surface. The experimental and numerical results are compared and discussed in terms of drilling efficiency through the mechanical specific energy.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.
Abstract Drilling through hard crystalline rocks like granites is challenging and taxing on the overall performance due to reduced rate of penetration (ROP). While efforts have been made in improving polycrystalline diamond compact (PDC) bits to increase ROP in hard rocks, the evidence of their field performance is currently restricted to only a few sites. There exists scope for alternative drilling technology where a significant fraction of hard rocks are present, such as in deep geothermal wells for electricity generation. In the ORCHYD project, two mature technologies – high pressure water jetting (HPWJ) and percussion drilling - were combined. A combination of bit profile and peripherical groove (slotted by HPWJ) creates a stress-relief effect releasing the rock from surrounding geological stresses, requiring lower energy to break the rock using a mud hammer. Furthermore, pressure waves due to percussion are reflected by free surfaces at the groove aiding in rock breakage. In this project, an experimental study on the influence of operating conditions such as HPWJ pressure, bottom hole pressure and surrounding geological stresses on the drilling performance was conducted. Several tests were performed at a dedicated drilling laboratory where the operational parameters can be varied to emulate drilling conditions for depths up to 4 km. As compared to tricone roller bits, ORCHYD technology guaranteed at least 4 times increase in the drilling performance. The performance of HPWJ in slotting a peripheral groove and mud hammer in rock breakage were strongly influenced by the operational conditions, e.g., for a given jet pressure the groove depth decreased significantly with increased bottom hole pressure. In this work, effects of such operating conditions on drilling performance were tested for different types of rocks such as Sidobre, Kuru Grey and Red Bohus. A sensitivity analysis of the influencing parameters on drilling performance of this technology is presented in this work. With increasing geological stresses, the proposed drilling technique is more effective in increasing ROP due to the stress relief effect. A novel technique combining HPWJ and percussion drilling using a mud hammer prototype was developed to show improved drilling performance in deep, hard rocks as compared to conventional drilling technique. Through this work, the performance of this method under different realistic drilling conditions was studied to optimize ROP, especially when drilling hard abrasive formations in deep oil and gas or geothermal wells.
