How onshore drilling is done?

ONSHORE DRILLING

                Drilling into the Earth for exploring valuable natural fossil resources is called onshore drilling. At first seismic survey is needed for exploration of the resource. It is typically carried out in order to explore for and subsequently extract petroleum which lies in rock formations beneath or within the earth surface. Most commonly, the term is used to describe drilling activities on the land, though the term can also be applied to drilling in low altitude hilly areas.

TYPES OF ONSHORE DRILLING

There are two main types of onshore drilling percussion or ‘cable tool’ drilling and rotary drilling:-

I. CABLE TOOL DRILLING

Cable tool, consists of raising and dropping a heavy metal bit into the ground, effectively punching a hole down through the earth. Cable tool drilling is usually used for shallow, low pressure formation. It is recognized by many as the first drilling method employed to dig wells into the earth for the purpose of reaching petroleum deposits and water.

The basic concept for cable tool drilling consists of repeatedly dropping a heavy metal bit into the ground, eventually breaking through rock and punching a hole through to the desired depth. The bit, usually a blunt, chisel shaped instrument, can vary with the type of rock that is being drilled. Water is used in the well hole to combine with all of the drill cuttings, and is periodically bailed out of the well when this ‘mud’ interferes with the effectiveness of the drill bit.

Innovations, such as the use of steam power in cable tool drilling, greatly increased the efficiency and range of percussion drilling. Conventional man-powered cable tool rigs were generally used to drill wells 200 feet or less, while steam powered cable tool rigs, consisting of the familiar derrick design, had an average drilling depth of 400 to 500 feet. The deepest known well dug with cable tool drilling was completed in 1953, when the New York Natural Gas Corporation drilled a well to a depth of 11,145 feet.

II. ROTARY DRILLING

Rotary drilling, consists of a sharp, rotating metal bit used to drill through the Earth’s crust. This type of drilling is used primarily for deeper wells, which may be under high pressure.

Despite the historical significance of cable tool drilling, modern drilling activity has shifted mainly toward rotary drilling methods. However, the foundation of knowledge laid by years of cable tool drilling is, in many cases, directly transferable to the practice of rotary drilling.

III. HORIZONTAL DRILLING

Most modern type of drilling, horizontal drilling is flexible in that it allows for the extraction of natural gas that had previously not been feasible.  Although on the surface it resembles a vertical well, beneath the surface, the well inclines so that it runs parallel to the natural gas formation. These legs can go in different directions at different depths and can be more than one mile long horizontally, in addition to the vertical well that can be thousands of feet below the surface.  Horizontal drilling allows one surface well to branch out underground and tap many different natural gas resources.  It also allows the well to make contact with larger areas within productive formations.

Horizontal drilling also permits the development of natural resources with minimal above ground disturbance, reducing the environmental footprint of natural gas operations and the cost and potential disturbance of existing roads or other infrastructure.  Directional drilling and horizontal drilling terms are often used interchangeably.  Directional drilling refers to drilling at a slant or angle to increase contact with the resource.  Horizontal drilling is a type of directional drilling.  Horizontal drilling uses a technique known as hydraulic fracturing in order to extract natural gas from geologic formations.

WHAT IS SEISMIC SURVEY?        

Seismic survey also can be called as Land Seismic Exploration Technique. This is a type of exploration technique used to explore under the earth surface. Most common method is blasting technique.

In this method explosives are used or blasting under the earth surface(at a suitable specified depth) for generating a sound waves or shock waves travelling downward through the subsurface and being partially reflected at each rock interface. The reflected energy is recorded at the surface by the Seismic Recording System via a 2 to 10 km long Seismic Cable to which ground motion sensors called Geophones are attached.

The Geophones are moved by the upward travelling sound waves, generating a small electrical current within the geophone. The small electrical signals are added to improve Signal to Noise Ratio and are digitized to 24 bit accuracy with the digital signal then being transmitted to the Recording Truck connected to the Seismic Cable.

This is a geophysical technique used to map in 2D or 3D, an image of the earth’s subsurface. Reflection Seismic is used by Oil & Gas, Coal Seam Gas, Minerals and Coal Exploration and Production companies to develop a clear understanding of subsurface rock structure and other geologic properties.