A. Searching for Oil and Gas

Most of the money spent by the petroleum industry in exploring for oil is used for geophysics (the physics of the earth, including seismology, gravity and magnetics, etc). Geophysics provides techniques for imaging the subsurface (seeing below the ground) before drilling, and this can be key in avoiding “dry holes.”

Not realized by the general public is that most of the holes drilled are dry and do not yield commercial oil or gas. Locating an oil and gas reservoir and drilling oil and gas wells is very expensive (offshore wells can cost $15 million or more; in fact, some offshore platforms cost more than $4 billion). That is why it is so important to utilize state-of-the art exploration and production technologies to keep costs as low as possible.

Technology plays a critical role in finding, developing and producing oil and gas reserves. Technology enables oil companies to identify eventual drilling targets that would be missed using standard exploration techniques; reduce the risk of drilling a dry hole; complete drilling projects faster, which reduces costs; reduce costs by knowing which production equipment is best suited for use in a drilling area; and get the most oil and gas from a new or formerly drilled oil and gas well by combining the efforts of geoscientists, engineers and scientists.

Geoscientists continually work to develop new ways to combine and use data from diverse sources in order to form a highly accurate picture of an area’s subsurface geology. The results are new discoveries, improved recovery rates, faster development and greater efficiency.

A petroleum prospecting is a feature, which geologists believe may contain petroleum. There are five elements which all have to be present for a prospect to contain hydrocarbons. If any of them fail, then we get a dry hole:

1) a source rock: when organic-rich rock such as oil shale or coal is subjected to high pressure and temperature over an extended period of time, hydrocarbons form;

2) migration: hydrocarbons are expelled from source rock by three density-related mechanisms: the newly-matured hydrocarbons are less dense than their precursors, which causes overpressure; the hydrocarbons are less dense than the ubiquitous water medium, and so migrate upwards due to buoyancy, and the fluids expand as further burial causes increased heating. Most hydrocarbons migrate to the surface as oil seeps, but some will get trapped;

3) trap: hydrocarbons are bouyant and have to be trapped within as structural (e.g anticline, fault block) or stratigraphic trap;

4) seal rock: the hydrocarbon trap has to be covered by an impermeable rock known as a seal or cap-rock in order to prevent hydrocarbons escaping;

5) reservoir: hydrocarbons are contained in a reservoir rock. This is a porous sandstone or limestone. The oil collects in the pores within the rock. The reservoir must also be permeable so that the hydrocarbons will flow to surface during production.

After selecting an area of interest, many different types of geophysical surveys are conducted and measurements performed in order to obtain a precise evaluation of the subsurface formations, including:

· Magnetometric surveys. Magnetometers hung from airplanes measure variations in the earth's magnetic field in order to locate sedimentary rock formations, which generally have low magnetic properties when compared to other rocks.

· Aerial photogrammetric surveys. Photographs taken with special cameras in airplanes, provide three-dimensional views of the earth which are used to determine land formations with potential oil and gas deposits.

· Gravimetric surveys. Because large masses of dense rock increase the pull of gravity, gravimeters are used to provide information regarding underlying formations by measuring minute differences in gravity.

· Seismic surveys. Seismic studies provide information on the general characteristics of the subsurface structure. Measurements are obtained from shock waves generated by setting off explosive charges in small-diameter holes, from the use of vibrating or percussion devices on both land and in water, and from underwater blasts of compressed air. The elapsed time between the beginning of the shock wave and the return of the echo is used to determine the depth of the reflecting substrata. The recent use of super-computers to generate three-dimensional images greatly improves evaluation of seismic test results.

· Radiographic surveys. Radiography is the use of radio waves to provide information similar to that obtained from seismic surveys.

· Stratigraphic surveys. Stratigraphic sampling is the analysis of cores of subsurface rock strata for traces of gas and oil. A cylindrical length of rock, called a core, is cut by a hollow bit and pushed up into a tube (core barrel) attached to the bit. The core barrel is brought to the surface and the core is removed for analysis.

Sophisticated equipment and advances in computer technology have increased the productivity of exploration. Maps of potential deposits now are made using remote-sensing satellites. Seismic prospecting - a technique based on measuring the time it takes sound waves to travel through underground formations and return to the surface - has revolutionized oil and gas exploration. Computers and advanced software analyze seismic data to provide three-dimensional models of subsurface rock formations. This technique lowers the risk involved in exploring by allowing scientists to locate and identify structural oil and gas reservoirs and the best locations to drill. Four-D, or “time-lapsed,” seismic technology tracks the movement of fluids over time and enhances production performance even further. Another method of searching for oil and gas is based on collecting and analyzing core samples of rock, clay, and sand in the earth’s layers.

After scientific studies indicate the possible presence of oil, an oil company selects a wellsite and installs a derrick—a towerlike steel structure—to support the drilling equipment. A hole is drilled deep into the earth until oil or gas is found, or the company abandons the effort. Similar techniques are employed in offshore drilling, except that the drilling equipment is part of a steel platform that either sits on the ocean floor, or floats on the surface and is anchored to the ocean floor. Although some large oil companies do their own drilling, most land and offshore drilling is done by contractors.

Дата добавления: 2015-07-21