Introduction of casing design as a drill string

General Introduction

Casing is a common petroleum equipment widely used in oil transportation and drilling operations .The use of casing and casing liners for drilling is a convention. Many years ago, it was customary in some shallow fields to attach a drill bit to a small-diameter production tubing column, drill a few hundred feet, and leave it at the bottom once total depth was reached. It was widely recognized that if casing could be used as a drill column without making up and down drill pipe, there would be some distinct advantages over conventional drilling. Examples include reducing or eliminating the time to start and stop drilling and eliminating the need for a separate drill column.

The biggest disadvantage is the lack of bit life, and unless there is some significant improvement in bit lifeor some way to retrieve and replace the bit without tripping the casing, there is no reason to consider this process.

Considerable changes have been brought about in recent years by advances in technology and the development of rigs specifically designed for casing drilling. As a result, a number of benefits are likely to be realized.

The casing used in drilling is clearly subjected to additional stresses compared to conventional casing. The main increase is in shear stresses caused by torsion. While rotating the liner during cementing is a common practice, rotating the complete casing column is not common because of the high torque required to do so. The torque (torque and drag in drilling) required to rotate most complete casing columns exceeds the maximum recommended compensating torque for the joint, so little consideration is given to the torsional stresses in the casing body itself.

The current process of drilling with casing typically utilizes four methods.

First, rotation drilling with conventional bits.

Second, rotation drilling with expendable drill-through bits.

Third, rotation drilling with wire line retrievable bits.

Forth, Slide drilling (with or without rotation) using retrievable downhole motors and bits.

Casing Connections

When rotating casing for drilling, the first consideration is the casing connection. The casing connection is usually a one-time addition and was never intended to be drilled up and down inside and outside the wellbore. The standard API 8 threads are especially bad because a large percentage of them go wrong during such a process. Any type of thread that is not properly lubricated with the right lubricant can have wear problems.

The basic premise of drilling with casing is that the casing will not trip, but this cannot be guaranteed in practice. The main problem with casing connections, therefore, is the possibility of damage due to excessive twisting. This is particularly important in connections without shoulders. Therefore, connection damage should be prevented for most drilling applications with casing.

One solution is to use only shoulder connections, but at a much higher cost. Another possibility, is to use a stop collar, in other words, inserted in the coupling so that the pin can reach a specified depth in the coupling, not further. This represents a reliable, inexpensive alternative to the shoulder connection. The BTC with stop collar has proven to be a better choice than the LTC and STC.

Casing wear and fatigue

The rotation of the casing string during drilling clearly causes a certain degree of wear on the outside of the string. Techniques such as calculating contact force can be used to determine the most severely worn area,

However, the casing string is not static relative to the borehole. For safety reasons, it should be assumed that any casing in the bare eye will wear and must be protected. Currently, this is achieved by using hydraulic tools specifically designed for this purpose to press the wear ring onto the casing.

The fatigue of casing during drilling operations has certainly not been studied as much as drill pipes, but the principles are basically the same. In practice, casing fatigue related to drill string harmonics has been observed. The bending of the wellbore in directional applications may also cause some fatigue problems.

Cementing in Casing While Drilling

Cementing the casing string used for drilling is not difficult. There are specialized floating devices that are used during drilling or inserted into the pipe string after drilling is completed. During the cementing process, the casing string can either move back and forth or rotate to assist in mud displacement, which is usually impossible in traditional cementing operations. A crimp type casing centralizer has been developed with rigid, curved blades to aid in mud displacement. During the cementing process, a small amount of mud displacement will not be a problem. The liner used as a drill string, combined with the liner hanger, can be cemented in a similar manner. Advantages

After the casing and liner drilling operations discussed earlier, it can be foreseen that casing drilling has some advantages, including:

Trip related

Time savings.

No drill collar handling.

Less wear on equipment.

lower chance of swabbing.

Less chance of surge fracturing formations.

Crew related

Less crew exposure to pickup and lay-down operations.

No derrick man for numerous trips.

And no casing crews.

No casing elevator or stabbing board with which to contend.

Drilling related

Lower circulating rates and pressures.

Better cuttings transport (reduced annular volume).

Better control of equivalent circulating density.

Overall better well control while drilling.

Less chance of differential sticking in most cases due to pipe movement.

Summarize

Despite the advantages of drilling with casing and liners, it is clear that questions remain unanswered or only partially answered. They are mainly related to equipment development, reliability, availability, etc.; there are also questions about specific issues such as casing fatigue failure,actual casing wear, cementing reliability, different adhesion tendencies of larger diameter casing, and consequences of casing tripping if necessary.

As technology develops, it is foreseen that these problems can be better addressed. A significant portion of the discussion in this paper is experimental and developmental in nature and involves fewer direct economic benefits. However, casing drilling has also been shown to be economically beneficial in practice, as well as having broader application prospects.