Best Practices for Drilling Coal Formations in Long Tangent Wells Using Water-Based Mud

Drilling through coal formations, especially in long tangent wellbores, presents a unique set of challenges for the oil and gas industry therefore we need best practices for drilling coal formations. Coal seams are notorious for their potential instability, abnormal formation pressures, and propensity for swelling and sloughing when exposed to water-based drilling fluids. These challenges can lead to various drilling problems, such as stuck pipe incidents, lost circulation events, and well control situations, ultimately compromising the safety and efficiency of drilling operations.

Coal over shale shakers

Coal over shale shakers

When drilling extended reach or horizontal wells with tangent sections, the complexities associated with coal formations are further amplified. The increased wellbore exposure to these challenging formations, coupled with the difficulties in maintaining adequate hole cleaning and wellbore stability in long tangent intervals, necessitates a comprehensive approach to mitigate risks and ensure successful drilling operations.

Using water-based muds for drilling coal formations introduces additional considerations, as these fluids can interact with the reactive shale and coal layers, potentially exacerbating wellbore instability issues. Consequently, careful mud design, composition, and treatment are paramount to maintain the desired mud properties and mitigate formation-related challenges.

The best practices for drilling coal formations in long tangent wells using water-based mud systems are listed below;

Mud Weight and Density Control:

Coal formations are often associated with abnormal formation pressures, either overpressured or underpressured. Maintaining the correct mud weight and density is crucial to prevent kicks (influx of formation fluids) or lost circulation events. Regular formation pressure integrity tests (FIT) and careful pore pressure/fracture gradient analysis should be performed to optimize the mud weight.

Mud Composition and Inhibition:

Coal formations are prone to swelling and sloughing when exposed to water-based muds. The mud should be properly inhibited with potassium chloride (KCl) or other shale inhibitors to minimize wellbore instability. Maintaining a slightly alkaline pH (8.5-9.5) can also help mitigate shale/coal instability.

Hydraulics and Hole Cleaning:

Maintaining effective hole cleaning is of paramount importance in long tangent sections to prevent the accumulation of formation cuttings, which can lead to potential wellbore instability issues and compromised drilling performance.

To enhance cuttings removal and mitigate associated risks, operators should consider employing high-viscosity pills or performing wiper trips, which involve circulating a viscous fluid or specialized pills to displace and lift cuttings from the wellbore effectively.

Drilling Fluids Monitoring and Treatment:

Coal formations can release methane, carbon dioxide, and other gases, which can affect the mud properties and potentially cause kick situations. Regular monitoring of gas levels, mud weight, and rheological properties is essential. Appropriate solids control equipment and treatments (e.g., degassers, defoamers) may be necessary to maintain the desired mud properties.

Wellbore Stability and Casing Design:

Coal formations are often associated with unstable wellbore conditions due to their swelling and sloughing tendencies. Proper casing design, including casing setting depths, mud weights, and potential use of expandable casing or liners, should be considered to maintain wellbore stability.

Bit Selection and Drilling Parameters:

Coal formations can be abrasive and challenging to drill, leading to increased bit wear and potential stuck pipe situations. Selecting the appropriate bit type (e.g., PDC, impreg, or roller cone) and optimizing drilling parameters (WOB, RPM, ROP) is crucial for efficient and safe drilling operations.

Real-time monitoring while drilling:

Utilizing formation evaluation tools while drilling is crucial to identify coal seams and other potential hazards, allowing for timely adjustments to mud properties and drilling parameters to mitigate risks proactively.

Continuous monitoring of key drilling parameters, such as torque and drag, is essential to detect early signs of wellbore instability. Prompt corrective actions, such as modifying mud properties, adjusting drilling parameters, or implementing contingency plans, should be taken to prevent further deterioration of wellbore conditions and potential stuck pipe incidents.

Team collaboration:

Successful drilling of coal formations in long tangent wells necessitates close collaboration among the drilling team, mud engineers, and geologists. The drilling team executes operations while working closely with mud engineers to design inhibitive muds that control coal swelling and maintain proper rheology. Geologists provide critical insights into formation characteristics, hazards, and pore pressures to guide drilling parameters and casing design. This multidisciplinary teamwork enables informed decision-making, proactive adjustments, and timely implementation of contingency plans for safe and efficient operations.

What is your experience about drilling through coal? 

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Type of Mechanical Cutters for Fishing Operations

In this article, it describes type of mechanical cutters used for fishing operation. When running fishing tools to retrieve stuck or damaged downhole equipment, the pipe or casing often needs to be cut or parted. This allows the fishing tools to latch onto and retrieve the fish. If wireline tools are available, the pipe can be parted using wireline cutting methods to minimize rig time. However, when wireline is not practical, the pipe must be cut using mechanical cutters run on the end of a workstring.

Internal Mechanical Cutters

Mechanical Internal Cutter

Mechanical Internal Cutter

One common method is an internal mechanical cutter. This tool is mounted on a mandrel with an automatic slips release mechanism that allows it to be set at the desired depth. Friction blocks or drag springs provide backup for the release.

The cutter works by slowly rotating right-hand and applying weight, which feeds out knives on tapered blocks to cut into the inside of the pipe. Springs in the feed mechanism absorb shocks to prevent the knives from gouging or breaking. A bumper jar is usually run above to control the cutting weight.

The knife tips are made of brass to prevent breaking when contacting the pipe wall. Internal cutters are available for most tubing and casing sizes.

Internal Hydraulic Cutters

An alternative is an internal hydraulic cutter designed for single strings. It uses hydraulically-activated knives for a smooth, efficient cut. An indicator signals when the cut is complete by a drop in pump pressure. Stabilizer slips anchor the tool before cutting.

Hydraulic Internal Cutter

Hydraulic Internal Cutter

The tool is run to depth, then rotation and circulation are initiated. Increasing torque indicates cutting has started. When complete, a control dog drops into a recess, reducing pump pressure to signal the cut is done. Straight pickup then retracts the slips and knives.

For multiple strings or open holes, a pressure-activated multiple string casing cutter can be used instead of a hydraulic cutter. Pump pressure forces the knives into the pipe to make the cut.

External Cutter (Washover Cutters)

When circumstances demand cutting from the outside, the washover outside or external cutter steps in as a versatile solution. Ideal for addressing scenarios where internal obstructions impede the use of wireline tools, this cutter operates on the bottom of a washover string, executing cuts from the exterior.

Washover outside cutter

Washover outside cutter

Adaptability is the hallmark of the external cutter, with configurations tailored to catch various tool joints or couplings on the fish. From spring fingers to flipper dog cages, each design caters to specific requirements, ensuring a secure grip. Flush-joint pipe necessitates a hydraulically actuated catcher, with pump pressure activating the cutter’s knives.

With careful calibration of rotation and fluid flow, the cut is initiated, culminating in the retrieval of the fish and subsequent extraction from the washpipe at the surface. Vigilance is paramount throughout the process to prevent surges in pump pressure and ensure a smooth operation from start to finish.


The Guide to Oilwell Fishing Operations: Tools, Techniques, and Rules of Thumb (Gulf Drilling Guides) by Joe P. DeGeare, David Haughton, Mark McGurk

What is an Explosive Jet Cutter for Pipe Severing Operation?

When abandoning an oil or gas well, performing salvage operations, or facing situations with low fluid levels, heavy drilling mud, or cost constraints, operators often turn to a specialized tool called the explosive jet cutter or jet cutter in short. This shaped charge device runs on an electric wireline and is designed to sever pipes in a controlled and efficient manner.

The jet cutter’s plastic explosive features a modified parabolic face with a circular shape that conforms to the pipe it needs to cut. This unique design allows for a precise and focused detonation.

Upon detonation, the shaped charge explosive flares the cut end of the pipe. To facilitate the subsequent fishing or retrieval of the severed pipe section, it is necessary to remove this flared portion. Typically, this can be accomplished during the same operation by employing a mill guide or a hollow mill container with an insert. These tools are run on the bottom of an overshot fishing tool and, through rotation, can dress off the flared or burred end, allowing the overshot to slip over the fish (severed pipe section) easily.

This image below shows how the tubing was cut with the explosive jet cutter.

Tubing cut with the jet cutter

Tubing cut with the jet cutter

It is important to note that there is a slight risk of damaging adjacent strings or casings if they are in contact with the pipe at the cut point during the detonation. Continue reading

The Economics Fishing Operations in Oil Well Drilling: Balancing Efficiency and Economics

In the oil and gas industry, fishing operations refer to the procedures used to retrieve or remove stuck or failed equipment from the wellbore. These obstructions can include stuck pipe, drill collars, parted tubulars, stuck packers, parted or stuck wireline, and other lost or failed equipment. When such conditions develop, drilling, workover, and completion operations must cease until the obstruction is removed, allowing normal operations to resume.

Obstructions in the wellbore can result from various factors, such as human error, unknown hole conditions, metal fatigue in tubulars, junk in the hole, and faulty equipment. These obstructions are generally classified into two categories: fish and junk.

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What is a Chemical Cutter? and How It Works?

Chemical pipe cutting is a wireline operation that utilizes an electric tool and a halogen fluoride chemical reactant to perforate and weaken pipe, enabling its subsequent removal (pipe recovery). Originally introduced in the 1950s as a patented process exclusive to a single wireline company, chemical cutting has now become widely adopted across the industry. Today, most electric-wireline service providers offer this technique, making it the predominant method for cutting pipes due to its efficiency and effectiveness.

Courtesy of Versa-line – Continue reading