Upstream Oil and Gas Drilling Geology

In this lesson, we will discuss the geology and how it relates to drilling decisions.

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Geology

Geology is the study of the solid matter that constitutes the earth. This entails all underground formations:

the soil and rocks
the different layers of the earth’s crust
the core

Geologists play a big part in drilling operations. Their exploration studies determine where oil and gas might be found and what type of well is best used to reach it.

In the early days of drilling, punching holes in the earth with the hope of hitting big was basically a hit and miss prospect.

Today, technology has come a long way in determining where to drill, how deep to drill, and what type of equipment will be needed to extract the oil and gas, leading to global exploration well successes now in the range of 40-50%.

Rocks

The well drilling process must get through various types of rocks in the earth. Rocks are composed of mineral or organic matter, and they lay in subsurface layers or strata throughout the world.

Rocks can contain a single mineral, such as rock salt and certain limestones, or many minerals, such as granite (quartz, feldspar, mica and other minerals).

Drilling encounters three basic types of rock – igneous, sedimentary and metamorphic – each with a different characteristic that requires a different drill bit, drill speed and bit pressure to be most efficiently and safely drilled.

Igneous Rock

Igneous rock such as granite is very hard , and was once molten rock that cooled and solidified.

Sedimentary Rock

Sedimentary rock is somewhat easier to drill. This type of rock is formed by smaller fragments carried by wind or water until they settle. Examples include sandstone and limestone.

Sedimentary rock is the most common. It first settles into horizontal layers called strata. They are often then deformed by movements of the earth’s crust.

Metamorphic Rock

Metamorphic rock is the result of an existing rock being subjected to extreme pressure and temperature. Examples of this type of rock are slate and marble.

Tectonic Plates

The largest movements affecting the rock strata are related to continental drift, called plate tectonics.

The outer layer of the earth’s crust is made up of seven tectonic plates which float on top of the earth’s molten core. These plates are changing position constantly.

The moving tectonic plates have three types of boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transforming boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation all occur along plate boundaries.

Tectonic plates and their movement can greatly affect the drilling process if a well is located near the plate’s collision boundaries, for example the thousands of wells in California, many near the San Andreas fault.

The shifting plates can quickly alter the composition of the subsurface strata, causing drilling equipment damage and faulty reservoir readings.

Reservoirs

Reservoirs come in various shapes and sizes. The most common conventional reservoirs in the early days of the industry were sandstones.

Today carbonate shale reservoirs contain much of the world’s remaining oil. Carbonate formations are complex, heterogeneous, with irregular flow paths and circulation traps.

Many oilfield service companies are focused on developing the cutting-edge technologies required to efficiently exploit these complex reservoir environments.

Formations

Formations is a generalized term for the rock surrounding a reservoir that must be drilled through to reach the ultimate pay-zone.

Formation evaluation refers to the volume of rock analyzed by measurements made in the borehole, usually by a well log or a well test. Formation evaluation helps estimate the well’s production potential and in choosing a well completion method.

Formation evaluation is done by the Geologist. By gathering pressure data and fluid samples from a formation, Geologists and Chemical Engineers can also determine the best chemical formula for the drilling fluids needed to control the well pressure and drive the cuttings to the surface.

Stresses

Rocks buried deep in the earth are not static, but are subjected to ever changing conditions. Layers of rock place tremendous vertical pressure on the wellbore and bit as it gets deeper.

The vertical pressure at any point in the earth is called the overburden pressure. The overburden pressure is a function of the mass of rock above drillbit.

The rock strata in a 10,000 ft well can cause this vertical pressure to exceed 5,000 psi, or pounds per square inch.

Additionally, rocks to the side exert horizontal forces on the borehole. These forces are controlled by the mud system, which we discuss in a later lesson.

The final sub-surface cap rock is often more compact and significantly harder to drill through. It is important to understand these stresses when trying to determine the best way to drill to a pay-zone.