A well must withstand not only the rigors of extraction but it must also prevent contamination at the time of construction and in the future. Well construction is a complex process involving both business and engineering decisions. Matching the right well construction with the target subsurface geology leads to effective subsurface containment.
Setting and cementing casing is one of the most important parts of the drilling process from a regulatory perspective. Most wells contain three or more sections of casing with the deepest casing running through the center of larger and more shallow casings. Most casing reaches from the bottom of the well to the surface. In some cases, the casing top is below the surface. That string is called a liner.
Once the casing has reached the desired depth, drillers cement the casing by filling the annulus between the casing and the formation. Although the ultimate goal is to fill the annulus around the well casing with cement, cement is actually pumped through the inside of the casing, in the same way that drilling mud passes through the drill string before rising through the annulus. Since drillers don’t want to end up with the inside of the casing being filled with cement, they carefully calculate how much cement will be needed to fill the annulus.
After drillers complete well construction, they test the well for integrity and install the wellhead and production equipment at the surface. One of our experts describes the entire process in the following video and explains the functions of different types of casing. As part of this explanation, you will learn how to calculate the amount of cement needed to fill an annular space and create a solid, impermeable seal for the well. Pay close attention as you will have a chance to perform the calculation yourself later in the lesson.
Transcript
Well Construction – Alfred William Eustes III – Colorado School of Mines
Well construction is one of the most important aspects of petroleum production. A well must withstand not only the rigors of extraction, but it must also prevent contamination now and in the future. The first stage in well construction is planning. Planning consists of a series of business and engineering decisions based on what was learned during the process of formation evaluation. Key considerations include what extraction techniques will be used, for example will hydraulic fracturing be used or another method? Will artificial lift be required at some point in the well’s life? And, what type of completion technique would be needed? Once a plan has been finalized, the construction process proceeds.
The next stage is tubular and completion equipment design. This encompasses casing, which is the major structural component of a well, and tubing, which is the conduit through which petroleum is brought to the surface. Completion equipment consists of down-hole tools such as packers, seal assemblies, flow couplings, safety valves and many other types of flow control equipment. Most wells contain three or more sections of casing. Conductor casing, which is the largest and outermost pipe, followed by surface casing and production casing. If there are issues with well control such as high or low pressure, or geological conditions such as salt zones or well bore instability, then intermediate casing are set between the surface casing and the production casing. This creates multiple barriers against fluid flow and helps ensure the integrity of the well. Most casing reaches from the bottom of the well to the surface. In some cases, the casing top is below the surface. That string is called a liner.
Casing or liner cementing involves filling the annular outside the casing or liner some distance up the annulus to cover critical spots in the well bore. Sometimes this may mean filling the annular space all the way back to the surface. With surface casing, the top of the cement is required to reach the surface. This is a very important part of the process, because the surface casing becomes the anchor point for the well head and any other pipes that may be placed inside the well. More importantly, this section of casing covers the anticipated fresh water aquifers. If it is constructed improperly, it could fail, allowing hydrocarbons to contaminate the aquifers. To put it simply, imagine drilling a 3/4 inch hole and inserting a 1/2 inch pipe into it. You want to ensure that that pipe will not wobble or move, but more importantly, you don’t want any fluid to be free to move along the annulus. That means you need to fill the annular space with the proper volume of cement and do it in a way that creates a solid, impermeable seal.
So, how does the cement fill the annulus? It’s the opposite of what you might think. You cannot pour cement from the surface and depend on gravity to fill the annulus. You could not get uniform coverage all around the casing from top to bottom. Instead, cement is pumped down the inside of the casing and a wiper plug and a displacing fluid, such as drilling mud, are used to push the liquid cement through the bottom of the casing and back up through the annulus to whatever annular height is needed. To do this, drillers need to calculate how much cement they need to fill the annulus. To determine the annular capacity, we must compute the difference between the anticipated drill hole size and the pipe diameter. This is done with the following formula: First we subtract the square of the pipe diameter from the square of the drill hole size. Then, we multiply the result by 0.005454 to get the annular capacity measured in cubic foot per foot. There are many other units that could be used, but the use of cubic foot per foot is the customary unit. The volume of cement needed is the same as this annular volume.
Once you’ve determined the annular capacity, you need to know the height of the cement. For this example, we want to fill 2,500 feet, so we simply multiply that by the annular capacity. In this case, you would get 433.1 cubic foot. The last step is determining how many sacks of cement do you need? Yield is the volume attained when a sack of cement is mixed with water. You can divide the annulus volume by that value to get the total number of sacks of cement required. For example, if a cement has a yield of 1.18 cubic foot per sack, divide 433.1 cubic feet by 1.18 cubic foot per sack to get 367 sacks. We would round that to 370 sacks of cement needed. Of course, this is an estimate, so frequently an excess volume is added to ensure coverage to the required height. Additionally, various types of cement bond logs can be used. Cement bond logs are generated by instruments that use acoustic waves to evaluate the mechanical integrity and quality of the cement bond. Basically, does the pipe ring when free or thud when covered with cement.
Once the well passes these tests, and the interior of the well is cleaned up, the well is prepared for well stimulation if needed. Stimulation is the opening of new channels in the rock for oil and gas to flow through easily. After stimulation, well bore cleanup removes debris, excess proppant, ball sealers, stage plugs or fine solids that could damage tools or the surrounding formation. Next, production tubing and completion equipment are installed in the well bore. And lastly, final well head and production equipment is installed. The well is now ready to begin production. Each one of these steps must be completed in accordance with best practices and federal, state and local regulations. Proper well construction is essential to ensure efficient operation, the health and safety of operators and the surrounding community and to safeguard the environment.
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