Category Archives : Downhole Tools

Bottom Hole Assembly 101

Bottom Hole Assembly or BHA

The Bottom Hole Assembly (BHA) is a key component of the drilling system, consisting of various components and tools (including the drill bit itself) which operate at the bottom of the wellbore and physically drill the rock. The BHA is connected to the drilling rig by the drill pipe, an extendable hollow tube which conveys the mechanical and hydraulic energy and movement from the surface rig systems to the bottom hole assembly. This complex arrangement of tools varies considerably depending on requirements, and can stack up to 1,000 ft long. The BHA has become more complex over time to suit the needs of drilling operations and to accommodate different tools that have been developed. This article will explain the desirable objectives of the BHA in both vertical and directional drilling. We will also highlight the different key components of a bottom hole assembly and their functions. The main purpose of a BHA is to effectively load and control the drill bit, but it also serves other functions that will be discussed below.

Designing the BHA

When designing the BHA, drillers must consider the operational objectives, characteristics of the rock being drilled, anticipated drilling parameters, and available tools. Operational objectives that should be considered include the angle being drilled, directional and depth targets, the expected ROP, and how to achieve designed build/drop rate. The geological characteristics that should influence the design of the BHA include the abrasiveness and competency of the rock, bed dip angles, and the pressure regime in the hole being drilled. The drilling parameters that must be anticipated to design the BHA include the applied RPM range, desired WOB, torque, and the anticipated shock or vibration pattern.

How an Optimized Bottom Hole Assembly Benefits the Driller

Since drilling cost is so dependent on time, employing drill bits and proper BHA tools is crucial for faster and less expensive drilling. Drilling time is greatly affected by both the rate of penetration (ROP) of the drill bit, and the number of times the BHA has to be retrieved from the bore hole. Every time a new bit is required, the entire drill string must be retrieved section by section to pull the bottom hole assembly to surface and change it out. This process is known as a round trip, or “trip” and requires time, money, and intervention from the drill crew which puts them at risk. If any part of the BHA is defective or fails, the drill string must also be pulled out from the borehole to replace or repair the defective component, an undesirable condition which we must try to avoid through careful selection of components.

It is crucial to review bottom hole assembly components, such as the drill bit, reamers, and stabilizers in terms of functionality and layout in order to limit possible damaging vibrations. Adjustments and additions to the BHA can lower the potential for vibration improving cost-effectiveness and reducing the risk of failure. To configure the bottom hole assembly, design optimization software is used to help in mitigating cost and eliminating ineffective drilling operations. Also, down hole tool measurements and data must be observed continuously to identify and record the behavior of the bottomhole assembly.

Components of the Bottomhole Assembly

The BHA is the business end of the drilling system that attaches to the drill string and is lowered down in to the hole. The threaded male and female ends of the drill pipe allow for more pipe and tools to be attached so that the drill string extends down to the thousands of feet required to drill an oil and gas well. Individual drill pipe segments, drill collars, and tools are all connected on the rig floor and then deployed downhole. These tools enhance the drilling operations and all serve different functions to work together to accomplish drilling objectives. The key structural components of the BHA are as follows:

  • Heavy-weight drill pipe – These pieces of pipe have thicker walls compared to the outer diameter of a regular drill pipe, and are used as a tapered transition between the drill collars and drill pipe while helping to add weight and stiffness. As previously mentioned the drill pipe functions to connect the rig surface equipment with the bottom hole assembly and the drill bit, allowing us to pump fluid to the bit and to move the bottom hole assembly as needed.
  • Drill collars – These are the large diameter and heavy pieces of pipe above the drill bit and below the drill pipe, which constitute the fundamental structure of the BHA. The weight of the drill collars applies compressional force (WOB) directly to the bit while keeping the more flexible drill pipe in tension to prevent buckling. This also conveys momentum and stiffness as the entire drilling assembly rotates, in order to keep the bit drilling smoothly and consistently.
  • Stabilizers – These are short components with larger diameter fins called “blades” which stick out close to the diameter of the hole being drilled and are used to centralize the drilling assembly within the hole. Stabilizers have these blades attached or integrated to their external surface and are distributed from above the bit and through the drill collars depending on what form of stabilization is required. As the components with the largest diameter in the BHA they often interact with the sides of the well, creating friction and a restriction on fluid flow, so their design and positioning is crucial.
  • Reamers – These are tools that enlarge, maintain or trim the side of the wellbore for various reasons, including easier electric logging, improved drilling performance and bit life, and reduced friction and vibration caused by a miss-shaped hole.
  • Various Subs – These are short components that are often used to connect other pieces of the BHA (crossover subs), or carry out specific functions. Some examples of the latter are subs which redirect or control the fluid flow (diverter and float subs), and subs which absorb movement and vibration to protect the assembly (shock subs and vibration dampening tools).

In addition to the main components listed above, the BHA typically includes a downhole motor or a Rotary Steerable System (RSS), and Measurement While Drilling (MWD), and Logging While Drilling (LWD) tools. These are the main drilling, steering, and recording components that, along with the bit itself, do the work of the bottom hole assembly:

  • Down Hole Motors – These motors provide additional power to the drill bit by converting the energy and flow of the drilling fluids to create additional rotation, and torque using a cavity pump system. This tool improves efficiency and power as it is connected directly to the bit. The housing of the motor also contains a fixed bend angle that can be pointed in a required direction to make the bit steer.
  • RSS – This will replace conventional down hole motor directional tools to help control wellbore trajectory in directional drilling. There are many different designs of tools but they all sit directly behind the bit and either push or point it in the required direction to make it steer. A rotary steerable tool is more expensive than a down hole motor but offers more precision and control.
  • MWD and LWD Tools – These tools are drill-collar-like components containing complex electronics and sensors. MWD tools measure and record the physical properties of the drilling process while LWD tools use electronics and sensors to log the properties of the rock and the drilling environment. These tools communicate this data to the drilling team on surface in real time so that they can adjust the drilling process to achieve objectives.

As mentioned earlier, the drilling parameters, and more importantly the direction of the wellbore, determine the necessary tools that make up the bottom hole assembly.

Vertical Drilling

A vertical hole is a hole that is drilled straight down in to the earth’s surface, with some minor vertical deviation allowed. There are many different layouts of BHA that are used to drill vertically, but most use little or no stabilization near the drill bit, combined with the weight of the drill collars, to allow the BHA to constantly drop back to vertical under gravity and maintain a vertical hole. A down hole motor with a straight housing is sometimes used to improve performance in vertical drilling by adding extra RPM and power directly to the bit.

Directional Drilling

Directional drilling is any type of drilling method that is intended to hit a predetermined subsurface target. These targets are precisely pre-planned so that the drilled well goes exactly where needed within the reservoir rock, often when the target reservoir cannot be accessed using a straight vertical hole from surface. This can occur if multiple wells are required from the same surface pad or platform, if the area is inaccessible for various reasons, or simply just to maximize the recovery potential by having more hole through the reservoir rock. A directional bottom hole assembly has almost an infinite number of layouts depending on what targets are required and what tools will be incorporated. Typically though, the BHA for a directional hole will incorporate either a down hole motor or a rotary steerable system in order to point, push, or steer the bit in the correct direction. The directional BHA will also contain an MWD tool which measures the angles and trajectories of the well bore and relays them back to surface to make sure the well is still on target.

Functions of the BHA

In drilling operations, the bottom hole assembly consists of components of the drilling system that act as penetrating, stabilizing, and maximizing tools to help aid in discovering hydrocarbons. The BHA provides the weight exerted on the drill bit to effectively break rock and so it also functions to provide the strength and stability needed to run in compression. The stabilizers that are in the bottom hole assembly help in preventing bit instability from the vibrations and wobbling that occur when shearing through rock formations. The BHA also helps in maximizing directional control by providing both stiffness and the precision tools necessary to steer the bit in the correct direction. Factors including hole shape, direction, and well characteristics are determined by the BHA, and so proper design of the bottom hole assembly must be made to control direction, ensure safety, and increase drilling efficiency.

Located at the end of the bottomhole assembly is the drill bit, which cuts through the rock structure to explore and exploit oil and gas reservoirs. Using a bit that is right for your drilling application is important, and having the right bottom hole assembly provides support, direction, weight, torque, and capabilities to maximize drilling performance. The bit design and the BHA layout should be carefully planned and matched in order to maximize the performance of both. Proper selection of the BHA and the drill bit help deliver drilling functionality, effectiveness, and overall results. Through examination of field results and research, our team works to constantly improve to ensure that our PDC bits and downhole tools are not the limiting factors in drilling operations.


At Ulterra, we develop groundbreaking technology to suit our customer’s drilling operations. Our patent-pending CounterForce® technology is a true advancement in PDC bit design. Unlike conventional PDC bits, CounterForce technology has a unique cutter orientation that allows the cutters to work together and force the bit to grasp the bottom hole pattern. Our CounterForce technology reins-in lateral forces and redirects them from the hole to effective

Downhole Tools, the Technology and Machinery Responsible for Driving Forward the Gas and Oil Industry

The general public has no real concept or understanding of the technical challenges, risks, and difficulties that companies face to discover and then extract oil and natural gas. Companies within the industry need to invest in premium quality tools and remotely controlled equipment to undertake many different jobs that take place many miles below the earth’s surface.

If the equipment proves unreliable or below required standards, it isn’t as simple as calling out the local repair company. Every bit of downtime costs thousands of dollars in lost productivity, which is why the tools and equipment used need to be virtually indestructible.

What Are Downhole Tools?

Downhole tools, as the name suggests, consist of a wide range of different tools that are used in the oil and gas fields to help with the drilling of wells, clearing of blockages, and ensuring that the wells are working to their full capacity. The tools are operated down the hole which forms a part of the well, hence their name. Although the tools themselves tend to be very expensive, they are an excellent investment due to the significant potential to reduce the cost of oil and gas field operations. The conditions under which downhole tools have to perform are hostile and include extreme pressure and high-temperature situations. Let’s examine in greater detail some of the jobs that can be accomplished with the use of downhole tools.

Some of the Different Jobs Where Downhole Tools Prove Invaluable

Well Bottom Communication – For anyone who has ever tried to drill a hole in a wall, the process is relatively easy because you can physically see where you are drilling, and you also receive feedback as you apply or decrease the pressure on the drill. Unfortunately, when you are drilling on the much grander scale of an oil or gas well, this type of personal feedback is not available. That’s where hi-tech communication systems become critically important, as they supply valuable information to the employees and the crew who are operating the drilling system. This information can help to prevent problems, and even warn the crew of impending issues long before they happen, by providing real-time measurements of the drilling process. These type of communication systems have evolved drastically over recent years as technology has advanced. Previously the collected information took a considerable amount of time to travel back to the surface, often only when the drilling assembly was retrieved out of the hole. By the time the feedback was received, there was a potential for the information to be invalid. In recent years these challenges have been largely overcome, with information transmitted in real-time, which has resulted in better knowledge and understanding for the employees, a safer environment, and even the possibility of remote controlling the drilling process from many miles away.

Completions Equipment

The term ‘completion,’ in regards to the oil and gas industry, means the final part of the well’s construction before it is transformed from a basic hole in the ground to an oil or gas producing well. As you might imagine, this final preparation takes a lot of time, energy, and effort. There are a lot of specifications that need to be adhered to for safety precautions, environmental protection, and maximization of production. Various downhole tools are used in the process in order to accomplish multiple complex tasks. If the hole were simply left alone once drilled, it would not take a great deal of time for it to collapse in upon itself. In order to prevent the hole from collapsing, and to maximize production, various steel pipes, screens, plugs, and valves are joined together and then slid into the drilled hole. This allows employees to control and manage the flow of oil or gas into the well from the surrounding rock while also preventing undesired contaminants, such as sand, water, or poisonous gases, from entering the well. All of these operations are conducted by necessity, and downhole tools are required to perform these challenging but essential tasks.

Fishing Tools

The quality of the drilling tools and technology have drastically improved over the years. Regardless of these technological advances, when you are drilling deep down into the earth’s surface, there is always the potential of physically breaking parts of the drilling assembly. It is also possible for the assembly to become stuck or lodged in the hole such that it cannot be easily removed. If and when these issues do occur, a company will have already invested significant sums of money into the well, so it is not a feasible option to simply drill another well. The solution calls for a variety of different tools, known as fishing tools, which have to be individually designed to solve the problem.

Each tool has a specific task and multiple fishing tools might be required to overcome a specific problem or clear a blockage. Some tools will cut or mill specific parts of the drilling assembly, others will sweep or capture loose pieces of junk or debris in the hole while others are designed to spear or grab specific pieces of pipe or the assembly to be able to pull on it. All being well, the tool will remove the broken piece of assembly or other blockage with it out of the well, quickly and effortlessly resolving the problem.

Downhole Drilling Equipment

Obviously, some of the most important downhole tools are the various parts of the drilling assembly that are used to physically drill the hole in the earth including the drill bits which actually cut the rock. It may come as a shock to some people to learn that many oil and gas wells will need to be between 6,000-12,000 feet deep and can be steered and angled at up to 90 degrees in order to hit the right reservoir. There are oil and gas reservoirs that are buried the equivalent of 10 Empire State Buildings stacked one on top of the other under the ground. Hopefully, this will put in to focus the extreme challenges and difficulties that are faced on a daily basis by gas and oil companies as they attempt to find and drill into these reservoirs.

The downhole drilling equipment is controlled from above the surface, by a crew of specialist drilling employees who constantly receive information from the downhole tools and the drilling assembly as it powers its way deeper and deeper into the earth. Drilling holes that are so deep require significant skills and excellent equipment. The high element of risk that field operators face is the reason why it is so essential to use only the best and most innovative tools possible.

Why Choose Ulterra for your Downhole Tools?

Ulterra is one of the leading manufacturers of downhole tools in the world, specializing in the latest technology of drill bits known as PDC. This acronym stands for Polycrystalline Diamond Compact and describes the specialist cutting elements used in this highly engineered and high-tech type of drill bit. Our company philosophy is based on three simple foundations.

• Performance – All of our products are designed and created using only the best components because we understand that when our clients are out in the oil fields in tough and challenging conditions they need products that they can rely on. All of our products deliver time and time again and are trusted by hundreds of companies around the world. When it comes to delivering outstanding performance that simply won’t let you down Ulterra should always be at the top of your list.
• Innovation – The very nature of oil and gas exploration and extraction throws up unusual and unforeseen issues on an almost daily basis. At Ulterra we have some of the best engineers and designers in the industry, who are consistently coming up with new and innovative solutions to the many and varied problems facing these vital industries. We never stop in our desire to make oil and gas extraction safer, more productive and efficient.
• Consistency – Our high-quality tools paired with our outstanding levels of innovation and dedication consistently deliver results that companies can rely on. With so many variables that are unavoidable in oil and gas extraction, it is critical to have as many consistent parts of the equation as possible, and that is another example of the thought process and quality that Ulterra delivers on a daily basis.

Downhole tools are the unsung heroes of the oil and gas industry. Millions of people across the world are totally reliant on gas and oil for fuel, chemicals and plastic products the industry produces; yet they have no understanding or idea of the processes at work to provide them with the things they rely on every day. At Ulterra, we are never content to rest on our laurels. We are determined to keep innovating, and driving the oil and gas industries forward.