Today petroleum is used not only as a fuel but as a raw material for many industrial materials such as paint, plastic, rubber, lubricants, and so forth.
What is Petroleum? Petroleum is a mixture of naturally occurring hydrocarbons which may exist in the solid, liquid, or gaseous state, depending upon the pressure and temperature to which it is subjected. Virtually all petroleum is produced from the earth in either liquid or gaseous form, and commonly these materials are referred to as either crude oil or natural gas, depend upon the state of hydrocarbon mixture.
Since the vast majority of oil and gas bearing formations are several hundred meters to several thousand meters beneath the earth’s surface, the oil well offers the only tool for accessing the reservoir from the surface. The well-bore region and the collected core (if any) offer a snapshot of the reservoir properties, in a fashion similar to a line drawn in a three dimensional volume. In other words, results from core tests do not describe the reservoir accurately but they do help describe the physics.
Up until the late 1980s there was practically no variability in the manner in which a well was drilled. However, horizontal drilling (a Soviet invention of the 1920s) revolutionized the drilling industry as it allowed for one well to access a formation at several horizontal locations (Figure 1‑2).
Figure 1-2: Simplified Illustrations of Vertical and Horizontal Wells
Origin of Oil
Many theories of the origin of petroleum have been advanced. The theories of the origin of petroleum can be classified as either organic or as inorganic. The inorganic theory attempts to explain the formation of petroleum by assuming chemical reaction between water, carbon dioxide and various inorganic substances such as carbonates, in the earth. The organic theories assume that petroleum evolved from the decomposition of vegetable and animal organisms that lived during previous geological ages. Organic theories are commonly acceptable.
Source beds as organic rich formations are the necessity of petroleum generation. Petroleum migration occurs after formation from source beds toward the reservoir or storage beds. Reservoir rocks have void spaces and are permeable to fluids, in other words they have interconnected void spaces.
Reservoir rocks are categorized as either sandstone or carbonate. Sandstones are formed from grains that have undergone sedimentation, compaction and cementation. The major characteristics of sandstone reservoirs are as follows:
- Composed of silica grains (mainly quartz and some feldspar),
- Consolidated or unconsolidated formations,
- May contain shale,
- May contain minerals (such as iron oxide and iron sulfides),
- May include clays (Note: Clays have a negative effect on the reservoir quality).
Carbonates are formed from the remnants of hard-shelled organisms that existed in coral reef environments. The major characteristics of carbonates are:
- Limestone (CaCO3) and/or dolomite (CaMg(CO3)2),
- May contain shale,
- Minerals, such as pyro-bitumen and anhydrite
- Pore space is comprised of areas of dissolution (vugs), fractures and inter-crystalline spaces.
About 60% of the conventional oil reservoir rocks are sandstones and about 39% of them are carbonates.
External forces such as buoyancy which force the petroleum to migrate from source rock to reservoir rock could push oil to reach the surface. So presence of a barrier over the reservoir formation is vital in accumulation of oil in the reservoir rocks. This barrier is known as “trap” in petroleum engineering. Traps associated with oil fields are complex. Different reservoir according to type of their trap can be classified as follows (Figure 1‑3):
- Convex Trap reservoirs which are surrounded by edge water and the trap is due to convexity alone,
- Permeability trap reservoir that the barrier is due to the loss of permeability in reservoir rock,
- Pinch out trap reservoir, which the periphery partly defined by edge water and partly by the margin due to the pinch out of reservoir bed,
- Fault trap reservoir that has a fault boundary.
Figure 1-3: Elementary Trap in Sectional View
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