# Dr. Edoardo Rossi

###### Post-Doctoral Associate

Dr. Edoardo Rossi
Geothermal Energy & Geofluids
Institute of Geophysics
NO F 51.1
Sonneggstrasse 5
CH-8092 Zurich Switzerland

Contact
 Phone +41 44 632 2910 Email rossie(at)ethz.ch

 Dominique Ballarin Dolfin Phone +41 44 632 3465 Email ballarin(at)ethz.ch

## Publications

Underlined names are links to recent or past GEG members

### REFEREED PUBLICATIONS IN JOURNALS

7.
Rossi, E. , M.O. Saar The influence of thermal treatment on rock-bit interaction: a study of a combined thermo-mechanical drilling (CTMD) concept Geothermal Energy, 8/16, 2020. [View Abstract]To improve the economics and viability of accessing deep georesources, we propose a combined thermo–mechanical drilling (CTMD) method, employing a heat source to facilitate the mechanical removal of rock, with the aim of increasing drilling performance and thereby reducing the overall costs, especially for deep wells in hard rocks. In this work, we employ a novel experiment setup to investigate the main parameters of interest during the interaction of a cutter with the rock material, and we test untreated and thermally treated sandstone and granite, to understand the underlying rock removal mechanism and the resulting drilling performance improvements achievable with the new approach. We find that the rock removal process can be divided into three main regimes: first, a wear-dominated regime, followed by a compression-based progression of the tool at large penetrations, and a final tool fall-back regime for increasing scratch distances. We calculate the compressive rock strengths from our tests to validate the above regime hypothesis, and they are in good agreement with literature data, explaining the strength reduction after treatment of the material by extensive induced thermal cracking of the rock. We evaluate the new method’s drilling performance and confirm that thermal cracks in the rock can considerably enhance subsequent mechanical rock removal rates and related drilling performance by one order of magnitude in granite, while mainly reducing the wear rates of the cutting tools in sandstone.

6.
Rossi, E., M.O. Saar A combined thermo-mechanical drilling technology for deep geothermal and hard rock reservoirs Geothermics, 85/101771, 2020. [View Abstract]Combined thermo-mechanical drilling is a novel technology to enhance drilling performance in deep hard rock formations. In this work, we demonstrate this technology in the field by implementing the concept on a full-scale drilling rig, and we show its feasibility under realistic process conditions. We provide evidence that the novel drilling method can increase the removal performance in hard rocks by up to a factor of three, compared to conventional drilling methods. From the findings of this work, we conclude that integration of thermal assistance to conventional rotary drilling constitutes an interesting approach to facilitate the drilling process, and therefore increase the access viability to deep georesources in hard rocks.

5.
Rossi, E., M.O. Saar Field test of a Combined Thermo-Mechanical Drilling technology. Mode I: Thermal spallation drilling Journal of Petroleum Science and Engineering, 190/107005, 2020. [View Abstract]Accessing hydrocarbons, geothermal energy and mineral resources requires more and more drilling to great depths and into hard rocks, as many shallow resources in soft rocks have been mined already. Drilling into hard rock to great depths, however, requires reducing the effort (i.e., energy), time (i.e., increasing the rate of penetration) and cost associated with such operations. Thus, a Combined Thermo-Mechanical Drilling (CTMD) technology is proposed, which employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) Thermal spallation drilling, investigated here as a field test and (II) Flame-assisted rotary drilling, investigated as a field test in the companion paper. The CTMD technology is expected to reduce drilling costs, especially in hard rocks, by enhancing the rock penetration rate and increasing the bit lifetime. Mode I of the CTMD technology (thermal spallation drilling) is investigated here by implementing the concept on a full-scale drilling rig to investigate its feasibility and performance under realistic field conditions. During the test, the successful thermal spallation process is monitored, employing a novel acoustic emission system. The effects of thermal spallation in the granite rock are analyzed to provide conclusions regarding the rock removal performance and the application potential of the technology. The field test shows that thermal spallation of the granitic rock can be successfully achieved even when a liquid (water) is used as the drilling fluid, as long as the heat source is appropriately shielded by compressed-air jets. Thermal damage of the surrounding rock is investigated after the spallation test, employing micro-computer tomography imaging and modeling the stability of the cracks, generated by the spallation field test. This study shows that thermally induced damage is mainly confined within a narrow region close to the rock surface, suggesting that thermal spallation only marginally affects the overall mechanical stability of the borehole. Thus, this confirms that, as part of the Combined Thermo- Mechanical Drilling (CTMD) technology, thermal spallation drilling is a promising mode that has a high potential of facilitating the drilling of deep boreholes in hard rocks.

4.
Rossi, E., M.O. Saar Field test of a Combined Thermo-Mechanical Drilling technology. Mode II: Flame-assisted rotary drilling Journal of Petroleum Science and Engineering, 190/106880, 2020. [View Abstract]To enhance the drilling performance in deep hard rocks and reduce overall drilling efforts, this work proposes a Combined Thermo-Mechanical Drilling (CTMD) technology. This technology employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) thermal spallation drilling, investigated in the companion paper and (II) flame-assisted rotary drilling, investigated here as a field test. The CTMD technology is expected to reduce drilling efforts, especially in hard rocks, enhancing the rock penetration rate and increasing the bit lifetime, all of which reduces the drilling costs. The present work investigates Mode II (flame-assisted rotary drilling) of the CTMD technology by implementing the concept in an existing drilling rig and testing the technology under relevant process conditions. This contribution studies the underlying rock removal mechanism of CTMD and demonstrates its drilling performance, compared to conventional rotary drilling methods. Acoustic emission monitoring, and analysis of the collected drill cuttings provide multiple evidences for thermal-cracking-enhanced rock removal during the flame-assisted rotary drilling. This removal mechanism appears to represent an optimal compromise to minimize rock fragmentation and cutting transport efforts during drilling, compared to a less efficient mechanical scraping of the hard granite rock, observed during the standalone-mechanical drill test. The drilling performance, in terms of removal and wear rates, are evaluated for the flame-assisted rotary drilling. This shows that the proposed drilling approach is capable of enhancing the removal process in hard granite rock, by a factor of 2.5, compared to standalone-mechanical drilling. The implementation of this drilling approach into a conventional drilling system shows that integration of thermal assistance to conventional rotary drilling requires marginal technical efforts. Additionally, this technology can profit from established knowledge in conventional mechanical drilling, facilitating its implementation to improve drilling performance in hard rocks. Hence, this study demonstrates that the Combined Thermo- Mechanical Drilling method is feasible and concludes that this technology constitutes a promising approach to improve the drilling process, thereby increasing the viability of accessing deep geo-resources in hard rocks.

3.
Rossi, E., M.O. Saar The Effects of High Heating Rate and High Temperature on the Rock Strength: Feasibility Study of a Thermally Assisted Drilling Method Rock Mechanics and Rock Engineering, 51/9, pp. 2957-2964 , 2018. [View Abstract]In this paper, the feasibility of a thermally assisted drilling method is investigated. The working principle of this method is based on the weakening effect of a flame-jet to enhance the drilling performance of conventional, mechanical drilling. To investigate its effectiveness, we study rock weakening after rapid, localized flame-jet heating of Rorschach sandstone and Central Aare granite. We perform experiments on rock strength after flame treatments in comparison to oven heating, for temperatures up to 650 $$^{\circ}$$C and heating rates from 0.17 to 20 $$^{\circ}$$C/s. The material hardening, commonly observed at moderate temperatures after oven treatments, can be suppressed by flame heating the material at high heating rates. Our study highlights the influence of the heating rate on the mechanism of thermal microcracking. High heating rate, flame treatments appear to mostly induce cracks at the grain boundaries, opposed to slow oven treatments, where also a considerable number of intragranular cracks are found. Herewith, we postulate that at low heating rates, thermal expansion stresses cause the observed thermal cracking. In contrast, at higher heating rates, thermal cracking is dominated by the stress concentrations caused by high thermal gradients.

2.
, E. Rossi, M.O. Saar Demonstration of thermal borehole enlargement to facilitate controlled reservoir engineering for deep geothermal, oil or gas systems Applied Energy, 212, pp. 1501-1509, 2018. [View Abstract]The creation of deep reservoirs for geothermal energy or oil and gas extraction is impeded by insu cient stimulation. Direction and extension of the created fractures are complex to control and, therefore, large stimulated and interconnected fracture networks are di cult to create. This poses an inherent risk of un- economic reservoirs, due to insu cient heat-sweep surfaces or hydraulic shortcuts. Therefore, we present a new technique, which locally increases the cross section of a borehole by utilizing a thermal spallation process on the sidewalls of the borehole. By controlled and local enlargement of the well bore diameter, initial fracture sources are created, potentially reducing the injection pressure during stimulation, initiating fracture growth, optimizing fracture propagation and increasing the number of accessible preexisting frac- tures. Consequently, local thermal borehole enlargement reduces project failure risks by providing better control on subsequent stimulation processes. In order to show the applicability of the suggested technique, we conducted a shallow field test in an underground rock laboratory. Two types of borehole enlargements were created in a 14.5 m deep borehole, confirming that the technology is applicable, with implications for improving the productivity of geothermal, oil and gas reservoirs.

1.
, and E. Rossi An apparatus for thermal spallation of a borehole Patent EP3450675A1, 2017.

### PROCEEDINGS REFEREED

3.
Rossi, E., M.O. Saar Laboratory and field investigation of a combined thermo-mechanical technology to enhance deep geothermal drilling , 81st EAGE Conference & Exhibition 2019, Jun 2019, pp. 1-5, 2019. [View Abstract]The development of deep geothermal systems to boost global electricity production relies on finding cost-effective solutions to enhance the drilling performance in hard rock formations. In this work, we investigate a novel drilling method combining thermal spallation and conventional drilling. This method aims to reduce the rock removal efforts of conventional drilling by thermally assisting the drilling process by flame jets. Laboratory experiments are conducted on the combined drilling concept by studying the effects of flame treatments on the mechanical strength of hard and soft rocks. In addition, investigation on the interaction between the rock and a cutting tool, permits to show that the combined method can drastically improve the drilling performance in terms of rate of penetration, bit wearing and the required mechanical energy to remove the material. As a proof-of-concept of the method, a field demonstration is presented, where the technology is implemented in a conventional drill rig in order to show the process feasibility as well as to quantify its performance under realistic conditions.

2.
Rossi, E., M.O. Saar Experiments on Rock-Bit Interaction During a Combined Thermo-Mechanical Drilling Method , 43rd Workshop on Geothermal Reservoir Engineering, SGP-TR-213, 2018. [View Abstract]The development of deep geothermal systems to boost global electricity production relies on finding cost-effective solutions to enhance the drilling performance in hard rock formations. Conventional drilling methods, based on mechanical removal of the rock material, are characterized by high drill bit wear rates and low rates of penetration (ROP) in hard rocks, resulting in high drilling costs, which account for more than 60% of the overall costs for a geothermal project. Therefore, alternative drilling technologies are investigated worldwide with the aim of improving the drilling capabilities and therewith enhancing the exploitation of deep geothermal resources. In this work, a promising drilling method, where conventional rotary drilling is thermally assisted by a flame-jet, is evaluated. Here, the thermal weakening of the rock material, performed by flame-jets, facilitates the subsequent mechanical removal performed by conventional cutters. The flame moves on the rock surface and thermally treats the material by inducing high thermal gradients and high temperatures, therewith reducing the mechanical properties of the rock. This would result in reduced forces on the drill bits, leading to lower bit wear rates and improved rates of penetration and therefore significantly decreasing the drilling costs, especially for deep-drilling projects. In this work, the feasibility of the proposed drilling method is assessed by comparing the rock-bit interaction in sandstone and granite under baseline and thermally treated conditions. Rock abrasivity, tool penetration and cutting forces are investigated to quantify the rock-bit interaction in granite and sandstone under baseline conditions and after the thermal treatment. The results highlights the dominant mechanisms regulating the rock removal. The removal performance of the tool in the granite material are found to be greatly enhanced by the thermal treatment both in terms of volume removed from the sample and worn volume at the tool’s tip. On the other hand, the sandstone material, after a thermal treatment, yields significantly lower wearing of the cutting tool. Thus, this results allow to draw important conclusions regarding the achievable drilling performances during the combined thermo-mechanical drilling method towards its application in the field.

1.
Rossi, E., M.O. Saar The effects of flame-heating on rock strength: Towards a new drilling technology American Rock Mechanics Association (ARMA) Symposium, Proc. of the American Rock Mechanics Association (ARMA) Symposium San Francisco, USA June 25-28, 2017, Proceedings ARMA 2017, 2017. [View Abstract]The applicability of a combined thermo-mechanical drilling technique is investigated. The working principle of this method is based on the implementation of a heat source as a mean to either provoke thermal spallation on the surface or to weaken the rock material, when spallation is not possible. Thermal spallation drilling has already been proven to work in hard crystalline rocks, however, several difficulties hamper its application for deep resource exploitation. In order to prove the effectiveness of a combined thermo-mechanical drilling method, the forces required to export the treated sandstone material with a polycrystalline diamond compact (PDC) cutter are analyzed. The main differences between oven and flame treatments are studied by comparing the resulting strength after heat-treating the samples up to temperatures of $$650\, ^{\circ}C$$ and for heating rates ranging from $$0.17 \,^{\circ}C/s$$ to $$20 ^{\circ}C/s$$. For moderate temperatures ($$300-450 \,^{\circ}C$$) the unconfined compressive strength after flame treatments monotonously decreased, opposed to the hardening behavior observed after oven treatments. Thermally induced intra-granular cracking and oxidation patterns served as an estimation of the treated depth due to the flame heat treatment. Therefore, conclusions on preferred operating conditions of the drilling system are drawn based on the experimental results.