Understanding Geological Factors for Predicting Abnormal Pressure in Well Planning

Drilling for oil and gas can be a hazardous endeavor, with unexpected pressure changes posing a significant threat. Fortunately, geological data can be a powerful tool in predicting these risks and ensuring a safe and successful operation. This article explores some of the most common geological features that can lead to abnormal pressures and how to identify them based on available data.

1. Fault Lines:

Faults are fractures in the Earth’s crust where rock layers have shifted. Crossing a fault during drilling can cause sudden pressure changes, leading to uncontrolled releases of fluids known as “kicks” or complete loss of circulation. Directional drilling, which often traverses faults, carries a higher risk of encountering these pressure fluctuations.

2. Anticlines:

These upward-domed structures are often targeted for drilling due to their potential to contain oil and gas accumulations. However, the pressure within an anticline can be significantly higher than expected, especially at its peak. Additionally, the pressure gradient can vary significantly across the structure, making it crucial to carefully consider the wellbore trajectory and potential sidetracking risks.

3. Salt Formations:

Thick salt layers can act as impermeable barriers, trapping fluids beneath them and leading to overpressured zones. Pierced or uplifted formations within these layers can contain even higher pressures due to further migration of oil and gas. Careful evaluation of seismic data and knowledge of the regional geology are essential for identifying and avoiding these high-pressure zones.

4. Massive Shales:

Impenetrable shale layers can restrict the flow of fluids, causing pressure buildup within them. These shales can be mobile or plastic under pressure, refilling the borehole when the drill bit is withdrawn. Recognizing the presence of such shales through seismic data and well logs is crucial for implementing appropriate drilling techniques and fluid densities to control pressure and ensure wellbore stability.

5. Charged Zones:

Shallow formations exhibiting abnormal pressure are often referred to as charged zones. These zones can be naturally occurring due to fluid migration from deeper formations or man-made due to poor cementing jobs, damaged casings, or infield flooding projects. Modern geophysical techniques can help identify these “bright spots” where normal pressures from deeper zones are encountered at shallow depths, posing significant control challenges.

6. Depleted Zones:

Zones previously drained of oil and gas can have subnormal pressures, leading to severe lost circulation when encountered during drilling. Incomplete local data or poor records of previous wells in the area can increase the risk of encountering these depleted zones and their associated hazards.

By understanding the geological features associated with abnormal pressures and analyzing relevant data, we can significantly improve the safety and efficiency of drilling operations. This knowledge can inform wellbore planning, drilling techniques, and fluid selection, ultimately leading to successful exploration and production while minimizing environmental and safety risks.


Richard C. Selley, 2014. Elements of Petroleum Geology, Third Edition. 3 Edition. Academic Press.

Norman J. Hyne, 2012. Nontechnical Guide to Petroleum Geology, Exploration, Drilling & Production, 3rd Ed. 3 Edition. PennWell Corp.

Richard C. Selley, 1997. Elements of Petroleum Geology, Second Edition. 2 Edition. Academic Press.

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