Petroleum Exploration

By Tedy Muslim Haq

Geophysical Engineering, Bandung Institute of Technology

Petroleum is the name for all hydrocarbon related, naturally occurring materials, including oil, natural gas, and tar. It is made up of hydrocarbon molecules (hydrogen combined with carbon). Oil is generally formed in young sedimentary rocks, especially the ones which have not undergone by metamorphic activities. Oil needs heat to be formed, but too much heat will make the oil get driven out.

There are four requirements to form an oil deposit; source rock, trapping mechanism, reservoir rock, and heating. The fluid in the deposit initially is a mixture of oil and water. During a time, because the oil is less dense than water, it separates and goes up to higher position in the reservoir rock. The gas also separates from the oil and takes the highest position available.

Recently, most petroleum resources are contained deep underground, so exploration relies heavily on drilling methods. In support for drilling, there are a variety of geophysical methods, including seismic and gravity.

The duty of petroleum explorer is to find the source rock sediment, reservoir rock sediment, and oil trap. The first method to start the exploration is earth surface mapping and interpreting. From this data, the possible locations of oil deposit can be guessed. The next step for exploration is gravity survey. Gravity surveys are performed to examine bedrock topography under the earth’s surface, map large metallic mineral deposits, and locate subsurface caverns and also contacts between geologic units of differing mass and density. It is a fairly complicated system but it is based on the premise that a target, oil reserve in this case, has a different density from the surrounding geology. Computer models are generated to depict the general area where such gravity changes occur. Once the gravity survey indicates an area where there are density differences, it is time to perform a seismic survey.

When the gravity survey has done and indicates an area that has differences density, seismic survey is performed. The seismic methods are the most widely used of all geophysical methods used in petroleum exploration. The main advantage is that it provides the most accurate rendition of the geometry of subsurface layers. Unfortunately the cost of seismic surveys is much greater than the cost of other types of geophysical surveys. Seismic methods measure seismic velocity of rock layers to detect both lateral and depth variations. The objective is to determine the lithology and geometry of the layers.

A seismic wave can be thought of as shock wave (elastic wave) or vibration traveling through the ground. The rate of travel, or velocity, of the wave is related to the density of the rock. There are two types of elastic waves produced: P-waves, which are primary or compression waves, and S-waves, or shear waves. The procedure used is to lay out a survey line with geophones set at equal spacing along the line. A shock wave is produced at one of the stations by dropping a heavy weight (seismic truck) or detonating an explosion (dynamite) at a shot point. The shot point is the point on the surface directly above the zone of interest. Ground motions caused by the explosion or impact are transmitted in the form of P-waves and S-waves. A seismic timer is used to measure the travel time of the wave from the instant it is generated until the time the wave reflection is detected back at the surface. Times are measured for each of the successive stations along the line. The return waves are measured by sophisticated computers traveling in front of the seismic truck or explosion site. The computer will calculate the timing of the returns and will then be able to draw graphs called, seismographs, of the topography and layers of the crust.

By examining these charts, expertise would be able to identify the traps. By identifying the traps one would be likely to identify an oil reservoir. Many seismographs can be manipulated by the computer to produce 3-D images of the rock formations. Extensive modeling can also be built, all by computer, based on the data that is returned from the seismic waves.

Geologic and geophysical clues are enticing, but drilling is the only way to learn if an oil or gas field really exists. Once a well is drilled, well logs yield data on the types of rock present and what fluids these rocks contain. The information interpreted from the logs is used to decision whether a well should be completed and used to produce oil and gas, or filled with cement and abandoned. The logs are also used to update the geologic models originally used to locate the well.

Petroleum exploration needs a wide earth science discipline such as geological surface mapping, structures, stratigraphy, and geophysical subsurface mapping including gravity, seismic interpretations, and well logging.

PETROLEUM ENGINEERING

Tedy Muslim Haq

16407170

Institut Teknologi Bandung

Petroleum, oil, and gas are hydrocarbons derived from sedimentary rocks which were deposited in productive region with anoxic (low oxygen) bottom waters. Deep burial resulting in high temperature and pressure converted the organic remains into hydrocarbons. Hydrocarbons can be in gas, liquid, and solid form that depends on the pressure and temperature. Oil hydrocarbons formed in temperature between 180^oF and 295^oF while gas hydrocarbon formed in temperature between 295^oF and 450^oF. Pressure forces the oil and gas from the source rock into water-filled porous and permeable strata above. Because oil and gas are less dense than water, they migrate upwards until their path blocked by and impermeable layer. Oil and gas accumulate, forming a large deposit within the pores of the rocks.

The rocks that has pores filled with oil or gas and blocked by an impermeable layer above and another impermeable layer or water below is called reservoir. There are three kinds of reservoir trap; they are anticlinal, fault, and stratigraphic trap. Reservoir has porosity that is a ratio between the pores volume and the bulk volume. Beside porosity, permeability also has an important role for reservoir quality. Permeability is quantized in miliDarcy unit. Good reservoir has porosity around 15-25% and 10-1000 miliDarcy.

The total amount of oil or gas in the reservoir at initial condition is called original oil-place or original gas-in place. The total amount of oil or gas that can be covered with current technology and at current economic conditions is called reserve. Proved reserves are the quantities of oil or gas from known reservoir which is proven by drilling and expected to be recoverable with current technology and at current economic conditions. Potential reserves are those that may be recoverable in the future with advanced technologies or under different economic conditions.

How much oil or gas can be recovered from the rock is a function of rock properties, type of reservoir, technology, an economics. There are several ways of oil recovery technique; Oil Expansion (2-5%), Solution Gas Drive (10-30%), Gas Cap Drive (20-50%), Water Drive (25-50%), Gravity Segregation (30-70%) and Gas Expansion (70-95%).

Petroleum engineer must to be an expert about reservoir, drilling, formation evaluation, and production engineering. Reservoir engineering is mapping and estimating the initial size of hydrocarbons. It characterizes, describes, predicts and modeling the reservoir. Drilling engineering learns and develops science and technology in communication hole making from surface to a fixed, good, and safe reservoir depth so that is to be a good oil, gas, and geothermal production facility. Drilling rigs can be different, it depends on the kind of place it is been. There are several drilling rigs; land rig, submersible, jackup, semisubmersible and drillship. One of important things in drilling is mud. Mud is being injected while the drilling is in progress. The mud removes cuttings from well, prevents kicks, makes bore hole stable, cools and lubricates bit and downhole tools. The other important thing in drilling is casing. Casing protects bore hole, separates high/low pressure zones, as the basement for wellhead system, reduces frictions of drill string and ensures well completion. Formation evaluation determines about porous and permeable layers (by logging method), calculates productive layer thickness, analyzes rock petrophysics (porosity and saturation) and studies reservoir correlations. Production engineering determines oil and gas production technique to surface, solves the production problem and plans the surface production facility.