Types of Wells

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There are several different types of wells that can be drilled. If a well is drilled in a lease that has had no producing wells to date, it is called a wildcat. If this well is drilled in a large region with no previous oil or natural gas production, it is known as a rank wildcat.

If the wildcat is successful and makes a "discovery," the wells drilled as exploration follow-ups are known as appraisal or delineation wells.

Once the discovery is proved by the appraisal wells to be commercial, the operator then drills a series of development wells, also called field wells. These wells are drilled to tap hydrocarbons at specific places in the reservoir. If wells are drilled into an exisiting field that has been producing for a number of years, these, too, are called development wells. They can also be referred to as infill wells, and the practice of drilling them is known as infill drilling.

One specific category of development drilling is conducted to provide injection of water or gas into the reservoir, hence the name injection wells. The purpose of such wells is to provide a steady flow of injection that maintains or raises the reservoir pressure and thus prolongs production over a period of years.

Some exploration and development wells are deliberately drilled at an angle (particularly offshore or in shale plays) to reach a specific target not accessible via a vertical well. These holes are called directional or inclined wells. Some boreholes are initially drilled vertically, then at increasing angles until they become horizontal, hence the term horizontal wells. These horizontal sections can extend for hundreds and even thousands of feet. Their purpose is to access a larger share of the hydrocarbons in the reservoir at one time, thus increasing the well's productivity and boosting output revenues.

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