In this blog post, we’ll take a detailed look at the subsea drilling process—from drilling wells to extract oil and natural gas buried beneath the seabed to protecting those wells using techniques such as wellbore lining, casing, and cementing, and finally connecting them to production facilities.
In modern society, oil remains a vital energy resource and industrial raw material. From fuel for transportation—such as buses and private cars—and power plants, to raw materials for clothing and various household goods, it is difficult to find an area in our daily lives where oil and petrochemical products are not used. While facilities for producing oil were primarily installed on land for a long time, technological advancements have made it possible to develop not only onshore but also offshore oil and natural gas resources. In particular, advances in offshore drilling and production technology have made it possible to access deepwater and ultra-deepwater resources that were previously difficult to develop economically. Subsea drilling is one of the key technologies that enables the production of offshore oil and natural gas. So, let’s take a look at how subsea drilling is carried out.
First, let’s briefly review the process of producing crude oil. When it is determined that a location contains sufficient crude oil reserves, drilling equipment—such as a drillship—is used to drill a hole down to the target depth underground. The hole created during this process is called a wellbore. After the wellbore is drilled, various types of steel casing are installed and secured with cement to ensure the wellbore’s stability. In the production phase that follows, production tubing and various production facilities are installed to transport the fluid rising from the reservoir to onshore or offshore production facilities. In offshore oil fields, facilities known as FPSOs are also utilized. FPSO (Floating Production Storage and Offloading) refers to a floating facility for the production, storage, and offloading of crude oil; it processes crude oil and natural gas from subsea wells, separates water and gas, stores the crude oil, and then offloads it to tankers and other vessels. The FPSO itself does not refine crude oil into final petroleum products such as gasoline or diesel, as a typical refinery does; instead, the produced crude oil is subsequently refined at onshore refineries and other facilities to be processed into the various petroleum products we use.
Now, let’s take a closer look at the drilling process. Drilling involves a series of operations to drill holes deep into the seabed to extract crude oil and natural gas, and to maintain those holes stably so that production can take place. Simply put, it is the process of creating long underground passages to reach resources buried deep beneath the seabed. Drilling equipment is used to create these holes. Attached to the end of the drilling rig is a device called a drill bit, which rotates to break up and crush rock as it penetrates the strata.
However, various problems can arise while drilling. The first problem is that gas or fluids flowing from the strata can enter the wellbore, posing a safety hazard. The second problem is that rock fragments—known as cuttings—generated during the drilling process can accumulate inside the borehole, hindering the drill bit’s progress and causing wear on the equipment. Furthermore, as the drill advances deeper into the strata, the pressure and temperature increase, and various problems can arise, such as the borehole walls becoming unstable or drilling fluid leaking into the strata. Therefore, simply rotating the drill bit to cut through rock is not sufficient to ensure safe and efficient drilling operations.
Drilling mud is used to address these issues. Drilling mud, known as “mud” in English, is a drilling fluid with properties similar to those of mud. Drilling mud is not simply mud; it is a specialized fluid whose density, viscosity, and chemical properties are adjusted to suit the drilling environment. When drilling mud is forced downward through the drill string under pressure, it flows out around the drill bit and then returns to the surface through the annulus—the space between the borehole and the drill string. During this process, the drilling fluid transports rock cuttings generated during drilling to the surface, cools and lubricates the drill bit, and regulates the pressure inside the borehole to prevent formation fluids from unnecessarily entering the borehole. Therefore, the drilling fluid is a crucial element in maintaining the safety and efficiency of drilling operations.
Although the process of drilling a hole is carried out using the drilling equipment and drilling fluid described above, the drilling operation does not end there. The challenge of safely protecting the drilled hole and maintaining the shape of the borehole remains. To help illustrate this, let’s recall memories of playing on the sandbox as children and digging holes. When the hole was shallow, there was no major problem, but as it got deeper, the surrounding sand would likely have slid down and filled in the hole you worked so hard to dig. Similarly, in drilling operations, the borehole walls can become unstable, leading to rock collapses or issues where formation pressure and fluids interfere with the drilling process. Moreover, since drilling operations involve digging holes to depths of thousands of meters, it is crucial to manage these issues effectively.
To prevent these problems, the drilling depth is divided into several sections from the start. Once the hole is drilled to the target depth of a section, drilling is halted, and a steel casing is installed in that section and secured with cement to reinforce the borehole. Next, the drilling equipment is brought back in to drill deeper, and the process of installing new casing and cementing is repeated. This reinforcement process is known as cementing. Cementing involves filling the gaps between the casing installed inside the borehole and the surrounding rock formations with cement to secure the casing and ensure the stability of the borehole. Casing not only prevents the borehole from collapsing but also plays a crucial role in maintaining the safety and integrity of the well by preventing fluid migration between different rock formations.
Cementing generally follows this sequence: First, after drilling to a predetermined depth, the drilling equipment is removed, and the casing is lowered into the borehole. Once the casing is in the desired position, a centerer or similar device is used to ensure it is centered within the wellbore. Next, cement slurry and a fluid to drive it are injected into the casing to fill the annulus—the space between the casing and the surrounding rock formation—with cement. It is crucial that the cement is distributed evenly between the casing and the rock formation.
First, a control fluid or spacer fluid is injected into the casing to displace residual water and allow the cement to set properly. This fluid removes residual water left on the surfaces of the casing and formation from the drilling process and minimizes direct mixing between the cement and the water, thereby helping the cement bond properly to the formation and the casing. This pretreatment process is crucial because if the formation water mixes with the cement, the cement’s properties may change, making it difficult to form a uniform cement layer.
While the adjustment fluid fills the gap between the casing and the formation, the bottom plug moves inside the casing. When the bottom plug reaches a predetermined position, its membrane ruptures, opening a passage for the cement slurry to pass through. The cement then moves downward along the inside of the casing and is forced out into the annulus outside the casing, filling the space previously occupied by the adjustment fluid with cement. Subsequently, as the upper plug moves and reaches the lower plug, fluid flow between the inside of the casing and the annulus is blocked. Since the upper plug does not have a rupture membrane like the lower plug, it serves to prevent further cement movement once it reaches its designated position.
Drilling crew members monitor the movement of the plugs and changes in pressure to determine when the cement begins to flow into the annulus and when cement injection is complete. Once the cement is placed at the desired location, a period of time is allowed for the cement to set and develop sufficient strength. This process is commonly referred to as “WOC” (Waiting on Cement). It is crucial to keep the casing stable, as any movement or lateral displacement of the casing before the cement sets may prevent the cement layer from forming uniformly. Equipment such as guide shoes and centralizers is used for this purpose. Guide shoes guide the casing smoothly into the wellbore, while a centerer helps the casing maintain its position at the center of the wellbore during the cementing process.
Once the cement has set, drilling and casing installation up to that depth are complete. Subsequently, the drill string and drilling fluid are used to drill down to the next target depth, where casing is installed, and the cementing process is repeated. It is common for the borehole diameter to gradually decrease during this process. Once the final target depth is reached using this method, equipment such as production casing, liners, and production tubing is installed to enable crude oil and natural gas from the reservoir to flow to the production facilities, thereby completing the well.
Once the well is completed for production, the final step is to install equipment capable of controlling the well’s flow. A prime example of such equipment is the Christmas tree. The Christmas tree is an assembly of valves and control devices installed at the top of the well; it regulates the flow and pressure of fluids produced from the well and shuts off the well when necessary. Therefore, while it may seem like a simple device similar to a faucet, it is actually a critical piece of equipment that manages production and safety while withstanding high pressure, high temperatures, and various fluid conditions. In subsea wells, the Christmas tree is sometimes used in the form of a subsea Christmas tree installed on the seafloor, connected to production facilities to ensure that crude oil and natural gas can be transported safely.
So far, we have taken a detailed look at the drilling operations involved in the offshore crude oil production process. Drilling is not simply a matter of drilling a hole; it is a complex operation that combines various technologies to resolve problems that may arise during the process and maintain the stability of the wellbore. It involves not only the technology of fracturing rock with a drill bit but also techniques for transporting rock fragments to the surface using drilling fluid, cooling and lubricating the drill bit, stabilizing the wellbore through casing and cementing, and controlling well pressure and production flow. Furthermore, these drilling technologies are utilized not only for oil and natural gas production but also for the development of other underground resources. For example, developing geothermal energy requires drilling to access heat sources deep underground; in fact, research into next-generation drilling technologies to reduce the costs of geothermal resource exploration and development is ongoing even in 2026.
Meanwhile, while there was once a strong perception that alternative energy sources such as solar and wind power were not yet practical for widespread use, the situation has changed significantly. Renewable energy, including solar and wind power, is already being used on a large scale in many regions around the world and is expanding rapidly; by 2025, the annual new installation capacity of renewable energy generation facilities worldwide is expected to reach approximately 800 GW. Solar power alone accounted for a significant portion of the increase in global electricity generation, and low-emission energy sources—combining renewable energy and nuclear power—met approximately 60% of the increase in global energy demand in 2025. However, challenges such as grid expansion, energy storage systems, supply chains, and investment remain.
This does not mean that oil has lost its importance. Global oil demand continued to rise in 2025, and the International Energy Agency (IEA) projects that, based on current policies and market conditions, global oil demand will approach a peak or plateau around 2030 at approximately 105.5 million barrels per day. Although the role of oil in the transportation and power generation sectors is gradually diminishing due to the widespread adoption of electric vehicles and the growth of renewable energy, oil is still used in various fields—not only as a transportation fuel but also as a raw material for the petrochemical industry. Therefore, even as the energy transition progresses, technologies that ensure the stable management of oil and natural gas production and the efficient securing of necessary resources will inevitably play a crucial role for some time.
In this regard, drilling technology cannot be viewed merely as a means to produce more oil. It also serves as a foundational technology that enables the safer and more efficient development of limited underground resources and provides access to various resources located deep underground. At the same time, given the rapid expansion of renewable energy and the ongoing energy transition, future drilling technology is likely to be increasingly utilized not only for oil and natural gas but also for the development of new energy resources such as geothermal energy. Just as we take an interest in the advancement of renewable energy technologies and the energy transition, we should also pay attention to drilling technology—which enables the safer and more efficient use of current energy resources—and the process by which this technology is expanding into new energy sectors.
