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? 

Please feel free to share in the comment section below.

What Factors To Be Considered When to Change Annular Preventer Element

When to Change Annular Preventer Element

An annular rubber element stands as a pivotal component within an annular blowout preventer (BOP), playing a crucial role in safeguarding oil well drilling operations by preventing the uncontrolled release of formation fluids, such as oil, gas, or water, from the wellbore.

When to Change Annular Preventer Element

When to Change Annular Preventer Element

Crafted from a high-performance elastomer compound, these elements are engineered to withstand the demanding conditions of the downhole environment. Subjected to high pressures, extreme temperatures, and exposure to corrosive fluids, they are strategically placed around the wellbore within the BOP body to forge a seal between the drill pipe or casing and the wellbore wall.

Upon activation of the BOP, the element undergoes compression, forming a tight seal that effectively halts the flow of fluids up the wellbore. Available in various sizes and configurations, annular rubber elements cater to diverse wellbore conditions and applications.

Here are some primary functions of annular rubber elements:

  1. Primary Pressure Barrier: The element serves as the primary barrier against the upward flow of formation fluids throughout drilling, completion, and production phases.
  2. Accommodation of Different Pipe Sizes: Designed to adapt to a range of pipe diameters, ensuring a secure seal irrespective of the size of the drill pipe or casing utilized.
  3. Resistance to Wear and Tear: Manufactured from robust materials capable of withstanding the abrasive downhole conditions.
  4. Maintenance of Flexibility: Flexibility is paramount for the element to conform to the irregularities of the wellbore wall and pipe while maintaining a tight seal.

The decision to replace an annular rubber element in an annular BOP is critical for wellbore safety and should be approached on a case-by-case basis, taking into account various factors. Here are key indicators that replacement might be necessary:

This is an example of worn out annular rubber element.

This is an example of worn out annular rubber element.

Visual Inspection:

  • Visible Damage: Any cuts, tears, abrasions, nicks, or physical damage compromise the sealing ability and warrant replacement.
  • Excessive Wear: Significant or uneven wear suggests the end of the element’s useful life.
  • Swelling or Softening: Signs of exposure to incompatible fluids or excessive heat indicate weakening and necessitate replacement.

Performance Issues:

  • Leaks: Even minor leaks around the element necessitate investigation and potential replacement.
  • Increased Activation Pressure: Elevated pressure requirements could signify wear or damage, reducing sealing effectiveness and calling for replacement.

Preventative Maintenance:

  • Manufacturer Recommendations: Adhering to recommended replacement intervals ensures optimal performance and safety.
  • Pre-operational Inspections: Scheduled inspections before each operation enable early detection of potential issues.
  • Records and History: Detailed records of element usage aid in predicting replacement needs.

Additional Factors:

  • Wellbore Conditions: Harsh environments accelerate wear, necessitating more frequent replacements.
  • Drilling Operations: Operations involving abrasive materials or frequent pressure cycling influence replacement decisions.

Replacing an annular rubber element is a critical safety measure. Consultation with experienced personnel, qualified inspectors, and adherence to industry regulations is imperative for informed replacement decisions. Never delay replacement if there are suspicions regarding the integrity or performance of the element.


Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.

Jack Up Rig for Oil Well Drilling: Let’s Get More Understanding about This Drilling Rig for Offshore Drilling

A jack up rig is a mobile offshore drilling platform commonly used for oil and gas exploration and production in shallow waters. It’s a versatile and efficient platform that offers several advantages over other types of drilling rigs.

Component of a Jack-Up Rig:

Barge or Hull: The primary structure of the rig contains machinery space, generators, mud pits, mud pumps, other drilling equipment, and crew living quarters.

Legs: Typically, three or four retractable legs that can be lowered to the seabed, enabling the rig to be elevated above the mean seal level.

Jacking System: Utilizing hydraulic jacks or electric motors, this system raises and lowers the rig’s legs as needed.

Drilling Equipment: This consists of the derrick, drawworks, mud pumps, and other essential tools for drilling oil and gas wells.

Cantilever: An extended platform over the water that facilitates drilling over the production platform or drilling in the open water location for exploration wells.

Advantages of a jack up rig are as follows;

Mobile: Easily transportable from one location to another.

Stable: Offers a reliable platform for drilling operations in shallow waters.

Self-Contained: Operates independently for extended periods without shore support.

Cost-Effective: Relative cost-efficiency compared to other offshore drilling rigs.

Disadvantages of a jack up rig are as follows;

Limited Water Depth: Operational up to approximately 400 feet of water depth.

Weather-Sensitive: Susceptible to the influence of strong winds and waves.

Environmental Impact:  The jacking process may disturb the seabed and marine life.

Exploring Jack-Up Rigs: Additional Facts:

The first jack-up rig was built in 1954! It was a significant milestone in offshore drilling, marking the beginning of a new era of mobility and efficiency for shallow-water operations.

There are a couple of different contenders for the exact title of the “first”:

  • DeLong Rig No. 1: Built by J.H. DeLong in 1954, this rig is often credited as the first true jack-up, with three retractable legs and a jacking system that allowed it to operate in water depths up to 15 feet.
  • McDermott No. 1: Developed by a joint venture between DeLong and McDermott in 1954, this rig also laid claim to the title of “first,” showcasing a similar jacking system and leg design as DeLong Rig No. 1.

The world’s largest jack-up rig is Maersk Invincible: This rig, built by DSME in South Korea and delivered to Maersk Drilling in 2016, has legs measuring 206.8 meters (678 feet) long, making it the rig with the longest legs in the world. It’s designed for year-round operation in the North Sea, in water depths up to 150 meters.

Maersk Invincible

Maersk Invincible

Beyond oil and gas, jack-up rigs find utility in wind farm construction and offshore platform maintenance.


Jack-up rigs emerge as curtail offshore rigs in the realm of oil and gas exploration within shallow water area. Their mobility, stability, self-sufficiency, and cost-effectiveness underscore their value, despite limitations related to water depth and susceptibility to weather conditions. In addition to their primary role in hydrocarbon exploration and production, these versatile rigs continue to contribute to diverse applications, shaping the landscape of offshore engineering.

Why Do We Keep Cement Samples in Oil Well Operations?

We Keep Cement Samples Because of The Following Reasons

Cement samples are kept after pumping cement in oil well operations for several important reasons. The two images below shoes cement samples, one collected while cement is in liquid phase and another one is when cement is set for awhile.

Cement Sample in Liquid Phase

Cement Sample After Set

Cement Sample After Set

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Example of Real Gas Calculation

This example will demonstrate how to calculate the compressibility of real gas in order to determine gas density and specific gravity at a specific condition.

Calculate the following based on the given condition:

1) Density of this gas under the reservoir conditions of 7,500psia and 220ºF,

2) Specific gravity of the gas.

Gas component is shown in Table 1

Table 1 - Gas Component

Table 1 – Gas Component

Average density of air = 28.96 lb/cu-ft


  • Determine critical pressure and temperature of gas mixtures using Kay’s rule
Table 2 - Critical Pressure and Temperature

Table 2 – Critical Pressure and Temperature

Note: critical pressure and temperature can be found from this link –

Pc’ = Σyipci = 660.5 psia

Tc’ = ΣyiTci = -46.2 F = -46.2 +460 F = 413.8 R

Table 3 - Pc' and Tr' by Kay's Rule

Table 3 – Pc’ and Tr’ by Kay’s Rule

  • Calculate Tr and Pr

Tr = T ÷Tc

Tr = (220+460) ÷ (-46.2+460)

Tr = 1.64

Note: temperature must be in Rankin.

Rankin = Fahrenheit + 460

For the critical temperature calculation, it can be converted the critical temperature from F to R before calculating Tc’. This will still give the same result. 

Pr = P ÷ Pc

Pr = 7500 ÷ 660.5 = 11.4

  • Read the compressibility factor (z) from the chart.

z = 1.22

Figure 1-z-factor from the Standing and Katz Chart

Figure 1-z-factor from the Standing and Katz Chart

  • Calculate average molar mass

Average Molar Mass = Σyi×Mi = 22.1 lb

Table 4 - Average Molar Mass of Gas

Table 4 – Average Molar Mass of Gas

  • Calculate density of gas from the equation below;

gas density

Gas Density = 18.6 lb/cu-ft

  • Calculate gas specific gravity from the equation below;

SG = Gas Density ÷ Air Density

SG = 18.6 ÷ 28.96

SG = 0.64


The answers for this answer are listed below;

Gas Density = 18.6 lb/cu-ft

SG = 0.64

We wish that this example will help you understand to determine z-factor and use it to calculate any related information.


Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.