Casing Shoe - Why is Casing Shoe the Weakest Point of the Drill String?
During a drilling operation, a casing shoe becomes the weakest point of the drill string. This is because if the casing shoe does not perform properly, then the entire string can suffer a major loss of productivity. For this reason, it is important to understand how to prevent casing shoe failures during the productive hole section of the drilling operation. Here are some tips to help you:
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Maximum vertical height of a gas influx Hmax at the casing shoe
Defining the Maximum Vertical Height of a Gas Influx is a good first step in deciding how much gas is allowed to flow through the casing. This is important for several reasons. Among them, a gas influx at the casing shoe will provide a large boost to casing pressures. A gas influx can also cause the casing to fracture. So, it is important to identify the source of the influx and what can be done to mitigate its effects.
The best way to accomplish this task is to calculate the maximum vertical height of a gas influx using a model. For example, if a ten inch diameter casing joint is used, the maximum height of the influx can be approximated by multiplying the length of the casing by the height of the casing.
Formation fracture pressure increases with depth
During drilling, fracture pressure is a critical parameter. If a wellbore pressure exceeds the fracture pressure, the formation will break down. It can also cause wellbore failures.
The fracture pressure required for fracture formation is called the fracture gradient. It is a ratio of the bottom hole pressure to the annular pressure loss. It is determined by dividing the true vertical depth by the fracture pressure. The fracture gradient can be used as a pressure indicator for the influx volume of fluid in the wellbore.
Formal estimates of the fracture gradient are important for drilling engineers. They are used to develop pre-drill fracture pressure-depth plots. They are also essential for design of casing programmes. The pressure gradient can be calculated by using Eaton's or Bowers' fracture pressure models.
Preventing skin and/or formation damage during treatment of weak points in the productive hole section
Fortunately for those of us who don't live in the petroleum capital of the world, preventing skin and formation damage is a top priority. A good strategy is to have a clear understanding of where you are at any given time. This will allow you to implement a multilayered approach to your mitigation strategy. One segment casing strategy is to have a dedicated team of analysts who are well versed in all areas of your business. Keeping tabs on the latest trends in the industry is a must. This will ensure your company is always one step ahead of the pack. This is not to mention having a slick customer support team. Lastly, it is essential to have a solid QA/QC program in place before you start to drill.
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Maximum allowable borehole fluid pressure at the formation just below the shoe
Having an appropriate level of wellbore pressure is a must if you want to keep the oil flowing. If you don't, wellbore fluid can get sucked up by the annulus and exit the open wellbore. The result is a reduced fluid column in the well and a decrease in the drilling rate. Similarly, a lower static pressure (SP) in the open wellbore translates into a lower rate of drilling.
The highest and lowest pressures you are likely to experience are found within the first ten feet of the hole. A top-down design exercise is recommended before drilling beyond that point. The following table summarizes the top-down design exercise, including the relevant details.
The above table lists the digits for the top-down design exercise. The table's smallest number is the most likely number, based on the results of the exercise.
Calculating kick volume on bottom
Several definitions of kick tolerance have been debated in the drilling industry. One such definition states that kick tolerance is the maximum allowable gas influx volume. This is the maximum amount of gas that can be pumped out of the well to the surface before the casing shoe fractures.
Another definition states that kick tolerance is the maximum allowable pore pressure in the equivalent mud weight. This definition is based on Boyle's Law. It combines the length of the gas column and the mud gradient to determine the kick volume on the bottom of the casing shoe.
Calculating kick volume on the bottom of the casing shoes is an important issue in the drilling industry. This can be done using the kick tolerance tool. It is an interactive tool that allows engineers to define the range of parameters they can use to calculate kick tolerance. This tool can also be used before drilling to help determine if it is safe to continue drilling.
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