For many decades, drilling methods have been used intensively and extensively in the oil and gas industry. Advances in technology resulted in reduced drilling and completion times, lower average well drilling and completion costs by 25-30% in the 2012-2015 period, and increased well performance.
Historical success and experience in the oil and gas industry may have a potential application in designing, constructing, and monitoring the next generation of DGRs. Therefore, it is worth examining some of the innovative drilling technologies and exploring how changes in the technology side might affect the future geological repositories for SNF and other HLW and their potential impact on nuclear safeguards implementation.
Technology Revolution and Implications
The oil and gas industry revolution has resulted from the need for more petroleum resources and innovative drilling technology. Since the early 20th century, unconventional resources such as shale oil and gas, tight sandstone oil and gas, coal-bed methane, and natural gas hydrates have been gradually exploited due to exhausted conventional resources and advances in directional drilling. Deep drilling in-ground can reach reserves at 15,000 ft. (4500 m) while ultra-deep drilling can go as far as 25,000 ft. (7500 m). Drilling under the seabed is more challenging; yet, industry records show that operators can reach reserves in deep water at 1,000 ft. (300 meters) and ultra-deepwater at 11,156 feet (3,400 meters). To exploit ultra-deep petroleum resources, vertical drilling is needed, mainly due to the ability to reduce down-hole accidents. Unconventional petroleum resources, on the other hand, are exploited by mostly non-vertical wells, such as directional wells, horizontal wells, multilateral wells, and extended reach wells. In terms of geological disposal of spent nuclear fuel (SNF), both vertical and non-vertical boreholes drilling methods can be useful. By studying industrial use cases, we identify relevant technologies and make recommendations for underground geological disposal. SNF disposal in deep water is not considered in this study.
Vertical drilling for conventional boreholes
Vertical boreholes are wells aimed at a target directly below its surface location. Vertical drilling is typically less than 20° deviation between the hole and the vertical, so the boreholes are almost straight down. In 1895, the first vertical well was drilled by the percussion drilling method or cable-tool drilling method to a depth of 65 ft. (20 m) at Titusville in the United States. It is still in use, particularly for shallow oil or gas wells in the Appalachian Basin. In this method, the rock formation is cracked by repeated strikes from a heavy steel bit which is lifted and dropped using a cable. This drilling operation has to be paused after a while to remove the rock fragments, and then resume until a hole is formed at the designed width and depth. However, the cable-tool drilling is not suitable for soft formations of the southern United States, i.e. sedimentary rocks such as sandstones, limestones, clays in Texas. Hence, a new approach was needed.
Starting mid-20th century, operators improved the operation efficiency by applying rotary drilling to penetrate different types of rock formations. This method features a rotating long steel pipe (drill-string) with a sharp bit on the end to cut through underground formations. In contrast to the cable-tool drilling, rock fragments are lifted from the downhole by the drilling fluid circulation system. Rotary drilling has demonstrated its efficiency among many technologies applied in the petroleum industry. As an example, after Shell started applying rotary drilling in the Gulf of Mexico in 1907, other oil and gas companies followed suit, such as Standard Oil of California to drill the hard formations of California. From an economic point of view, innovative rotary drilling started the bloom of oil and gas production with annual production exceeding one billion barrels in 1925 and two billion barrels in 1940. By the 1990s, global supplies recorded 20 billion barrels annually.
Directional drilling of unconventional boreholes
Directional drilling is the method of drilling a borehole along a projected and controlled, non-vertical route to a favored target. This method has transformed the oil and gas industry since the 1920s because of its efficiency in unfavorable surface configurations such as buildings, trees, groundwater above, or a steeply inclined rock fault zone. * In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. * Several models of directional drilling include horizontal, multilateral, extended reach, which will be investigated in the next sections.
These enable natural gas and oil businesses to work more expeditiously, diminish waste, and reach more reserves. Directional drilling also has non-petroleum uses such as a potential application in building DGRs for permanent disposal of spent nuclear fuel. Deep Isolation – a US-based nuclear waste company – proposes to drill horizontal boreholes allowing countries to dispose of nuclear waste more quickly, at a much lower cost, and with a smaller footprint than traditional disposal approaches.
Directional drilling coupled with advanced drilling techniques such as mud motors, rotary steerable systems (RSS), measurement-while-drilling (MWD) sensing, and logging-while-drilling (LWD) has brought two-fold benefits: 1) overcoming prior disadvantages in a slower rate of penetration and higher frequency of checking in the devices, and 2) increasing dramatically natural gas and oil production which helps balance the demand-supply relationship.
Mud motors and Rotary Steerable Systems
Mud motors and RSS are helping directional drilling advance rapidly due to greater ease in changing the drill bit’s directions. In using mud motors, drilling fluid (or mud) is pumped through, making the drill bit rotate continuously. This mud pressure pushes the bit to a different angle. On the other hand, RSS is 3-D controlled from the surface using advanced communication techniques such as a downlink drilling control system. Therefore, RSS tools can either push the bit or point the bit in the required direction in real-time. Mud motors, specifically high-performance motors, are more popular than RSS because they can result in daily cost savings of 50% or more, significantly low lost-in-hole cost ($168,000 versus $1 million), and well suit all bit types and sizes.
Yet, RSS can perform faster, deeper, and more precisely. Mud motors and RSS offer petroleum operators more drilling options, depending on target zones, precision, and time constraint. For future geological disposal, directional drilling coupled with RSS and mud motor can help construct DGRs in a faster, more secure, convenient, and economical way, as compared with the traditional tunneling method.
Logging-while-drilling (LWD) and measure-while-drilling (MWD) are well-logging systems used to acquire and collect wellbore information and transmit data to the surface in real-time. While MWD is a type of LWD, they are not interchangeable due to their specific functions. LWD helps operators study the composition, chemical, and physical characteristics of the rocks. On the other hand, MWD records drilling mechanics data such as drill-bit position, direction, and downhole pressure.
In the construction of a DGR, the use of LWD could give operators up-to-the-minute updates to monitor canister placement and avoid potential hazards. In the same manner, MWD could allow operators to obtain real-time data about the direction and drill steering for more accurate construction of the wellbore. For current and future DGRs, the newest MWD sensors offer higher quality and accuracy data of the boreholes such as trajectory, rock properties, temperature, and pressure.