Location and Extraction
Locating Oil Deposits
When it comes to finding oil, geologists are the ones responsible. Geologists use their knowledge of rock formations in order to find the right conditions for an oil trap. In years past, geologists were forced to interpret surface features, surface rock, soil types, and occasionally core samples obtained by shallow drilling in order to predict the location of oil deposits. While modern geologists also examine surface rocks and terrain, they are aided by technology such as satellite imaging, sonar, and seismology when prospecting for oil. They even attempt to measure small changes in the earth’s gravitational and magnetic fields that could indicate that oil is flowing under the surface.
Most commonly, geologists apply seismology for oil location. In this method, shock waves are created that pass through rock layers and are then reflected back to the surface where they are measured using seismic equipment. In some ways, this is similar to the way sonar maps surface structures. The basic premise of the seismological technique is that the numbers of sound waves reflected and the speed at which they travel are affected by the density of rock layers through which they must pass. Oil deposits will reflect sound waves differently than solid rock.
While seismic methods are preferred to magnetometers and gravimeters for oil exploration, there is use for these tools. In the case of magnetometers, the shape of oil basins and potential oil faults can be mapped. Occasionally, hydrocarbons will find their way through fractures in the ground to near the surface. Bacteria that feast on oil will, in some cases, produce magnetite by eating this surface oil. This will create subtle changes in the Earth’s magnetic fields at those locations, which can be mapped using magnetometers.
Drilling and Extraction
Once a potential oil deposit is located, a development plan is created. A development plan considers whether the oil deposit is more liquid or solid and how economically feasible it will be to extract oil from the location. It was once the case that test drilling was a common practice. However, due to the cost of drilling a single well, particularly in deep water, and improved techniques for assessment, test wells are seldom drilled now. If a well is drilled, it is because the deposit is almost guaranteed to produce oil. The cost of a single deep water oil or gas well can cost in excess of 50 million USD just to drill.
It was also once the case that oil wells could be expected to produce oil on their own due to natural pressure. Most natural pressure wells have been found though, so now it is common to drill multiple holes into a single well. Some of the holes will be used for extraction and some will be used to pump water, chemicals, and pressurized steam into the well to aid in extraction. While extraction has become a rather complicated process and relies on a number of technologies, the basic steps are still as follows:
In the primary recovery stage, “reservoir drive” comes from natural mechanisms. Reservoir drive simply refers to the factors that move oil out of a well to the surface once drilling has occurred. Natural reservoir drives include natural gas expansion, gravity drainage of oil from upper parts of a well to lower parts, and displacement of oil by water. For today’s wells, primary recovery accounts for 5-15% of the total amount of petroleum contained within a given deposit.
As wells age, their natural pressures fall. Once the pressure is too low to force oil to the surface, extraction moves into secondary recovery. Secondary recovery depends on mechanical methods to increase pressure in the well. Often times this takes the form of injecting liquids or other material such as natural gas, carbon dioxide, or air into the well to increase pressure.
Pumps are also used in secondary extraction. Because they are prone to problems and are more expensive to operate, submersible pumps are avoided if possible. Most people will be familiar with a pumpjack, which is often mistakenly referred to as an oil derrick. Pumpjacks are alternatively called pumping units, horse head pumps, beam pumps, and thirsty bird pumps. They are composed of a walking beam that is balanced atop a triangular apparatus. One end of the beam is attached to a motor-driven crank and the other end is attached to a pushrod that extends into the oil well.
Recovery through water injection is usually about 30% of a well’s total output. The total recovery from a well after primary and secondary stages is usually between 35 and 45% of the total contained. Note that this is not a fraction of the total amount that will be extracted. Rather, it is a fraction of the total amount in the well. The amount of petroleum extracted from a well is usually 50 to 60% of the total deposit.
In the final stage of extraction, methods intended to increase liquidity of the oil are applied. At this stage, heat and chemicals become necessary.
Some of the principle methods used in tertiary extraction are called thermally enhanced oil recovery or TEOR. In these methods, heat is applied to make the oil flow better. Steam is the most common method of heating oil, but burning is used as well.
Chemicals called surfactants are also used in tertiary extraction. They decrease surface tension of oil, which helps it flow. Surface tension can be thought of in this case as the attraction of hydrocarbons for one another. Solids have greater attractive forces between molecules that make them up than do liquids. Liquids, in turn, have greater attractive forces between molecules than do gases. By decreasing intermolecular attractions, surfactants help to decrease the viscosity of oil. Surfactants are often used in combination with TEOR.
Occasionally, oil-eating bacteria are also used in tertiary recovery. Bacteria that eat petroleum often have appetites only for hydrocarbons of a certain length. If bacteria are used that break larger hydrocarbons into smaller ones, then they will assist in making the oil less viscous since larger molecules tend toward the solid end of the spectrum. Bacterial injection can be more economical than other recovery methods and also more environmentally friendly. This technique is currently incentivized in places like Texas in an effort to increase its use.
In the last section it was mentioned that total recovery from a well is a maximum of 60% of the available petroleum. Tertiary recovery accounts for only 5 to 15% of this. The reason that 100% of the oil is not extracted is that energy returned on energy invested, or EROEI, usually means it is not economical. When the amount of energy that must be used to extract petroleum exceeds that amount of energy that can be gained from utilizing that petroleum, it no longer makes sense to continue extraction efforts. The cutoff for EROEI on most wells is around 50 to 60% of the total petroleum in the deposit.