Casing While Drilling

Abstract

Conventional drilling methods have faced significant operational and financial challenges, such as the cost of purchasing, inspecting, handling, and transporting drill equipment, and, most importantly, tripping in and out of the drill string whenever the Bottom Hole Assembly (BHA) requires replacement, a wiper trip, or total depth is reached. Tripping the drill string in and out not only contributes to Non Productive Time (NPT), but also causes well control issues such as wellbore instability and lost circulation. All of this has prompted the oil and gas sector, as well as any other engineering industry, to look for innovative techniques and approaches to address these issues. A new drilling method has emerged as a result of technological developments and continuous improvements to conventional drilling methods. Casing when drilling has been established as a result of technological developments and continuous improvements to traditional drilling processes. Casing Drilling is the process of drilling and casing a well at the same time, employing active casing to maximize production. This paper provides an overview of the casing while drilling method (CwD) and its practical application in well drilling. The typical drilling method and casing while drilling are also compared. The CwD approach outperforms the standard drilling method by a wide margin.

1. Introduction

The rising need for and reliance on energy resources by mankind, especially those brought on by the discovery and exploitation of new commercial hydrocarbon deposits, entails the utilization of innovative innovations, such as drilling process optimization by lowering the expenses, the hazards, and the wasted time. While drilling, casing while drilling replacing traditional drilling string with casing string both to circulate and to transmit mechanical energy to the bit a well is being drilled with fluid. Casing while drilling has a lot of technical and cultural obstacles to overcome, but the significant advantages of this technology—such as shorter drilling times and fewer issues with the drilling string—make it a more and more attractive option to traditional drilling. Experience with using this technology has shown that it can speed up well execution and, occasionally, lower expenses relative to drilling depth.

According to data from the International Energy Agency [1], the world’s natural gas demand has been rising steadily since 2009, reaching 3757 billion m3, with the potential to climb to a 23–25% share of the world’s energy consumption by 2040. Since natural gas is the primary fuel substitute for coal in the electricity generation industry, despite the higher risks associated with the explosive nature of natural gases, the world’s increased demand for this fuel is primarily due to its lower environmental impact when compared to other fossil fuels, particularly with regard to air quality and greenhouse gas emissions [2]. Accordingly, 22% of the world’s electricity supply in 2014 was made up of natural gas. This may vary from 17 to 32% by 2060, representing a 300–1500 bcm absolute increase [3].

Gas producers develop drilling programs for both production and exploration in order to meet this demand. Production drilling aims to boost the rate of gas production from existing reservoirs. Exploration drilling seeks to find new gas sources [4]. Due to difficulties and technological mishaps that may occur during drilling that may increase the wells’ final cost, sizeable sums are frequently allotted to achieve such investment plans. However, these sums are frequently wasted before the program is completed [5]. More and more businesses experiment with and use novel drilling techniques and technology that decrease downtime, mitigate risk, and prevent technical mishaps during drilling in an effort to lower such unanticipated expenses of well drilling operations. Casing while drilling, also known as CwD (Continuous Bottom hole Pressure), Pressurized Mud Cap Drilling, and Dual Gradient Drilling, are alternatives to controlled pressure drilling [6]. Casing while drilling is an alternative to standard drilling that involves drilling the well and casing it at the same time [7].

Even though the casing while drilling method was developed in the 1920s, widespread use wasn’t conceivable until the last 10 years as a result of technical developments [4, 8]. By minimizing drilling time and issues with the drilling string, the strategy was therefore shown to be successful in lowering overall drilling costs.

1.1 The tools for casing while drilling

Casing during drilling can be done with either standard drilling rigs that require little additional equipment or drilling rigs that are specifically designed for this usage.

The drilling rig, which consists of the three vital systems of drive, rotary, and circulation, must achieve the principal parameters of the drilling regime and consolidation of the well bore [9]. The conventional links are operated remotely in order to remove the derrick man from the monkey board, and the surface casing drive systems have been modified and improved to allow for casing while drilling. This allows the casing to be run in the hole, the drilling fluid to be circulated, and the casing to rotate safely on the derrick in place of the conventional tongs.

Surface Casing Drive Systems (Figure 1) can be used for a wide range of casing sizes because they can be automatically controlled by PLC from the driller cabin and can be externally clamped for small casings (from 3 12 in to 9 5/8 in) or internally clamped for casing larger than 9 5/8 in.

Figure 1.

Casing drive system.

Casing while drilling can be done primarily in two ways:

1. Rotate the casing from above to provide torque to a bottom hole assembly that drills and cements; or

2. Run a retractable bottom hole assembly inside the casing that includes a bit and a reamer.

In order to drill by rotating the casing from the surface, which eliminated the ability to remove the bit, it was necessary to develop specialized drilling bits that had comparable performance to normal bits. Since they are simple to mill with PDC bits after cementing, these may act as casing shoes once the appropriate depth has been reached—Figure 2.

Figure 2.

Milling the drilling bit used for CwD.

In order to cement immediately after hitting TD, a float collar is typically installed within the casing while drilling.

The steel float collar (Figure 3) almost matches the resistance of the casing, and its valve must withstand drilling fluid erosion as well as pump pressure and pressure from the casing itself. The casing is fitted with centralizers to keep the well on track, to control casing wear, and to line the casing up during cementing (Figure 4). During directional drilling with casing, it is advised to utilize centralizers with rough surfaces and robust, non-rotating blades made of zinc alloy since they are incredibly durable (see Figure 5).

Figure 3.

Float collar.

Figure 4.

Wiper plugs.

Figure 5.

Centralizer sub.

In order to accommodate the unique operating circumstances in the well, casing threads are different from those used for traditional drilling. As a result, the casing makers created a variety of casings to address the difficulties and harsh well conditions that emerged during casing while drilling. These connections must guarantee fatigue resistance, sufficient sealing capability, and torque resistance [10].

When casing while drilling, it may be necessary to use a recoverable/retractable drilling system since damaged equipment must be replaced before the casing depth is reached.

In order to retrieve the costly drilling equipment used for a directional casing, operators must quickly and effectively reach the formations beneath the casing shoe.

A bottom hole assembly (BHA), which can be inserted into and removed by a wireline, is used for drilling while casing. Its basic components are a bit and an under reamer at the bottom of the casing string to drill a hole large enough to allow the casing to pass freely. The BHA is placed in a nipple at the lower end of the casing string that can be retrieved by wireline without removing the casing from the well and is attached to a Drill-Lock Assembly (DLA) engaging axial lock and torsional lock. The releasable DLA transfers compression and torque loading while rotating the drilling and casing strings. It is advised to apply centralizers on the casing to stop sleeves from wearing out [11].

2. CwD benefits

Many issues with traditional drilling are eliminated or much diminished by using CwD. One significant category of these issues is brought on by the drilling string, which poses the following risks:

• Drill collar, Kelly, and drill pipe can twist off;

• Drilling string sub-assemblies can pull out of threads or unscrew;

• Drilling string can become trapped; and

• Drill pipe can shatter or bend.

Another set of issues related to installing casing in wells with bent or collapsed holes can be avoided by doing it while drilling. Additionally, difficulties brought on by crossing unstable formations (borehole collapsing and over pull), crossing formations with loss of circulation, or crossing formations that have deteriorated from prolonged contact with drilling fluids can all be avoided [12].

These problems are solved by the so-called plastering effect, which is produced by spinning the casing string in a small annular region, sealing the formation pores, and fortifying the borehole walls.

According to Figure 6, while drilling with a standard drilling string, the annular gap is bigger. By using casing during drilling, the annular space is reduced to a minimum, creating a wellbore that is more stable and sealed. Furthermore, the stiffness of the casing string creates a less convoluted hole, lowering the possibility of key seats or mechanical sticking, and the high annular rising velocity of the drilling fluid via this annular region enhances debris cleaning from the wellbore.

Figure 6.

Annular space size in conventional drilling vs. casing while drilling.

By removing the time and effort required for casing string tripping, CwD also has the advantage of reducing drilling time overall. Since the float collar was inserted before drilling began, the drilling fluid is changed with cement once the casing-setting depth has been reached. For this reason, operations such as control tripping of the borehole to correct over pulling areas, circulating to remove solids from the fluid, performing electrometric operations, retrieving the bit by decomposing strings, and inserting casing by filling the casing string at each casing section are avoided [13].

2.1 Problems with the CwD casing design

A steel casing that is bolted together for CwD boreholes strengthens the casing design concerns. The commonly utilized casing is 6–12 m long, with diameters of 4–20 in, and wall thicknesses of 5–15 mm.

The goal of casing a well is to: -

• Prevent the wellbore from collapsing; − isolate formations that pose significant drilling challenges (formation overpressure or easily breakable);

• Provide a sturdy support for the surface facilities (preventers, Christmas tree);

• Transfer through the suspension system to the surrounding rocks the axial loading of the next casing sections or tubing and of the surface facilities weight;

• To ensure sealing of layers containing different types of fluids; varying pressures.

Tensile force, compression, outer and inner pressure, and gravity are the forces affecting the casing string (see Figure 7). Additional forces (fatigue, torque, and buckling) act on the casing when drilling because of it. In order to determine the profile of a casing, the strength of the casing to these forces must be understood.

Figure 7.

Forces acting on the casing.

The weight of the casing itself causes tensile force to exist. These will snap at the weakest area if the tensile force exceeds a critical level, which is equal to the tensile strength of the casing.

Given that the tensile strength of the casing’s connection is different from the strength of the casing body, the calculation must be performed for both components in order to take into account the lowest values when determining the casing profile. The fluid behind the casing’s hydrostatic column provides the majority of the outside pressure.

When the pressure exceeds the strength of the casing, the casing may collapse. We must take into account the relationship between the nominal diameter D and wall thickness t in order to calculate the critical pressure for casing made in accordance with API regulations. The ratio (thick wall casing) determines the outer pressure that is exerted on the inner casing wall at the minimal yield point.

The cutting action, as well as the friction of the casing against the borehole wall and/or the previously cased hole, result in torque at the drill bit at the bottom of the hole. Since the drill bit is above the bottom hole, the only source of torque is casing friction. Due to friction forces operating on the point where the borehole walls and casing make contact, the torque applied to the casing during drilling is typically larger than during traditional drilling, resulting in a strength moment whose vector direction is the opposite of the casing’s rotation. Thus, the moment at the bit is substantially lower than the rotating moment at the surface when the casing is rotating in the borehole and exerting a certain weight on the bit [14].

The cyclic load at various stress levels that are significantly below the material’s yield point causes fatigue. A minor fracture begins to grow from the high-stress point along the casing and eventually breaks it under sustained strain.

It becomes unstable due to buckling, which is caused by the bending moments produced by the geometry of the casing borehole and the compressive stress. If the casing is subjected to compressive stress that is greater than a specific threshold, the casing buckles into a sinusoidal or helical pattern.

Following buckling, the casing leans on the borehole walls; as a result, the lateral force from the point of contact may produce wear and raise the moment needed for rotation [15].

The casing string must be sized in stages, taking into account the factors mentioned above:

• including preliminary sizing for cracking and crushing,

• correcting the profile set in the first stage by lowering the casing collapsing pressure under tensile forces, and

• Testing the tensile resistance of the casing and joints.

The following additional loadings must be taken into account:

• bending with tongs during make-up;

• joint pull-out and slip crushing;

• Corrosion and fatigue failure; and

• Pipe wear from using drill strings and wireline instruments.

It is necessary to use safety coefficients during the design phase to execute casing while drilling under safe settings. Tensile forces should be given the strongest safety coefficients possible in order to account for additional stress caused by things like buckling, dynamic forces, sticking trends, and friction with the borehole wall. For the upper joint of the casing, a safety factor of 1.6 to 1.8 is used in relation to tension. The calculation hypotheses for such stress are less likely to be realized when the coefficient for breaking and crushing is smaller, for breaking 1.1 and 1.0, and for crushing 0.9.

3. Casing while drilling-related safety issues versus traditional drilling

Risk is the possibility of an occurrence that could be advantageous or detrimental to a project or activity. It can be defined as a combination of the likelihood that the risk will materialize and the implications of loss or gain. As a result, hazards can be divided into four categories: low, medium, high, and very high. The methodical process of locating, evaluating, and reacting to a project’s possible risks is known as risk management [16].

Drilling risks may be caused by geology, technical-operational problems, or a mix of the two while a project is being carried out. The so-called difficult geological formations are characterized by the physical and chemical properties of the rocks through which the borehole passes and by the properties of fluids contained in pores or fractures; subjective challenges are related to technology and method. In terms of projects, all risks are identified, risk exposure is evaluated by calculating the likelihood of occurrence and the impact, and then such risks are avoided or managed by putting in place an effective risk management system. The primary dangers that could arise during conventional drilling are depicted on maps in Tables 1 and 2. Or during drilling, casing. Each project’s risk occurrence likelihood is individually estimated based on correlation wells, drilling technology, and techniques. Comparing the two tables leads to the conclusion that drilling with casing could result in a reduction or minimize drilling-related hazards, particularly those relating to traditional drilling string, to preserve circulation loss or borehole stability [17].

Cost	Parameter	Conventional drilling costa	CwD costa
Intangible	Rig mobilization	$180,000	$95,000
	Rig dayrate	$462,500	$400,000–450,000
	Fuel	$112,500	$70,000–80,000
	Solid control equipment	$34,375	$20,000–25,000
	Drilling mud	$210,000	$190,000–$200,000
	Cementation	$204,000	$170,000–175,000
Tangible	Bit cost	$40,000	$45,000–50,000
	Drill Pipe cost	$116,000	$0
	Conductor casing	$3200	$3600–4000
	Surface casing	$59,850	$69,825–74,100
	Intermediate casing	$212,000	$240,000–256,000
	Production Casing	$236,000	$262,800–275,940
	Total Cost	$1,870,425	$1,566,225–1,685,040

Table 1.

Drilling cost comparison between conventional drilling and CwD for vertical well.

Additional Equipment	Cost
Hydraulic Catwalk	$450,000
Top drive + cement swivel + casing drive	$4,000,000–5,000,000
Casing drilling wireline winch1	$500,000
Wireline BOPs1	$50,000
Total	$5,000,000–6,000,000

Table 2.

Capital equipment cost required to convert conventional drilling rig into CwD rig.