_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225471, “The New Frontier of Closed-Loop Geothermal Drilling: Lessons From the Romanian Pannonian Basin Basement, ” by Florian Aichinger, SPE, and Scott Farmer, SPE, Helmerich & Payne, and Georg Ripperger, SPE, OMV. The paper has not been peer-reviewed. _ In the expanding geothermal sector, the push to drill deeper and reach hotter formations is leading to increased basement drilling, which presents different challenges than those posed by sedimentary basins. This study evaluates the feasibility of drilling an advanced geothermal system (AGS), a closed-loop, unconventional geothermal system, in the Pannonian Basin basement of Romania. Key drilling objectives included achieving high instantaneous rate of penetration (ROP) and extended bit life in anticipated hard rock while also identifying and addressing operational limitations through optimized bottomhole assembly (BHA) and drillstring design. Introduction The Pannonian Basin, extending 50–100 km into Romania along the Hungarian border, is a well-documented geothermal anomaly with heat-flow densities ranging from 50 to 130 mW/m². Its abundant geothermal resources have historically been tapped for heating, primarily in greenhouses using classical thermal water production. To reduce reliance on natural reservoirs, two innovative reservoir-independent approaches have emerged: enhanced geothermal systems (EGS) using hydraulic fracturing and closed-loop systems also referred to AGS. EGS adapts oil and gas fracturing techniques to create pathways in impermeable rock, circulating water to extract heat. Though effective in areas lacking aquifers, EGS faces challenges from induced seismicity and high costs. Closed-loop systems, by contrast, circulate a working fluid through sealed boreholes, absorbing heat from surrounding rock. This method minimizes water use and environmental risks. Larger cities in western Romania, with their extensive coal-powered district-heating systems, are potential candidates for transition to geothermal energy. Deep drilling projects in the basement are time- and cost-sensitive to assumptions of drilling performance and bit life. Uncertainties of these parameters in lesser-known basement formations drove the operator’s desire to derisk them by studying them in a test well. An experienced service company was engaged to optimize drilling in very hard rock, targeting high instantaneous ROP, extended bit life, and identification of potential operational limiters in the test well. Preparation Test-Well Plan. The operator reviewed its traditional hydrocarbon fields in western Romania for well candidates that were already drilled down to the basement in a size which allowed further deepening of the well in at least a 6-in. hole size. The vertical Well A, with a 7-in. casing set in the basement at 1, 562-m measured depth (MD) /true vertical depth, was selected for re-entry. The objective was to drill into the basement with minimal concern for the trajectory because the well served no other purpose than testing bits. Bit and BHA Design. The primary configuration featured a well-stabilized BHA with a straight motor and three stabilizers. Stabilizer placement was optimized using the forced-vibration method. This BHA was intended for testing a specific bit, with a plan to switch to a stabilized bent-motor BHA for trajectory corrections, if needed, to prepare for subsequent bit tests. Two providers recommended bit designs that incorporated pointed or conical cutters for enhanced point loading, along with high diamond thickness and quality to improve impact and abrasion resistance.
Chris Carpenter (Sun,) studied this question.