PETROLEUM EXPLORATION AND PRUDUCTIOM (UPSTREAM SECTOR)

Details of Petroleum Industry E & P sector activities

The different sectors  of Petroleum Industry and its activities are:

A. Petroleum exploration Methods

•  Geologic and geochemical methods

•  Geophysical and seismic methods

•   Mapping

B. Well drilling

• Preliminaries

• Drilling techniques

• Types of drilling

C. Testing for oil

•  Types of well tests

•  Well logging

•  Other well surveys

D. Well Completion

•  Types of well completion

•  Well Casing and Tubings

•  Well heads and Christmas Trees

E. Well Production

A.    Petroleum exploration Methods are:

Exploration Methods	APPLICABILITY
Remote sensing and air-photo interpretation	Surface
Geological Mapping (regional and large-scale)	Surface (sometimes in underground mines also)
Geochemical	Surface and subsurface
Geophysical methods (Gravity, magnetic and seismic)	Surface

Applications of remote sensing in oil exploration:

1. In petroleum exploration Remote Sensing studies used to demarcate basin boundaries.
2. To identify large, small and medium sized  surface structural features such as folds and faults
3. To understand  physiography (drainage pattern, altitude) of the area.
4. To know the lithology, weathering and erosional characteristics (whether the rock is resistant or recessive),
5. To locate the lineaments of the area.
6. To reduce the cost/ risk

Geological exploration methods 

The methods and procedures followed in geologic surveying depend upon whether the survey is reconnaissance or detailed (geological, topography and structure mapping).

Reconnaissance surveying: is for the purpose of determining the oil prospects of a relatively large area in a limited space and time. A preliminary survey, usually executed rapidly and at relatively low cost, prior to mapping in detail and with greater accuracy.

Detailed geological survey: Based on these available maps, the geologist must make his own map contains all the geologic and economic data that were obtained. In the search for stratigraphic traps, the geologist must identify the environment of deposition, possible reservoir rocks, but of possible source and seal rocks as well. He also determines the source of clastic sediment and the direction of transport.  The field party should also collect stratigraphic data, as well as the geologic and structural maps.

Topographic maps: Topographic maps are maps which show the topography, or shape, of the surface within an area, usually by means of contour lines. 

• A contour line is a line of equal elevation. 

• The vertical distance between contour lines is called the contour interval. 

• Steep areas will have contour lines close together, and gentle sloped areas will have contour lines far apart.

• Where contour line crosses over a stream or gully on a topographic map, it forms a “V-shape”.

• The V shape will always point upstream. 
• A contour line which forms a circle indicates the presence of a peak.  
• In addition to their value as base maps for geological study, topographic maps may also yield structural information of great value.
• Based on topographic maps, anticlines can be easily recognized.

• Structural contour map:

• Structure contours are lines of equal elevation, just like topographic contours

• These maps are used  to show the elevation of the top of a subsurface sedimentary rock layers (surface of a geologic structure like a dome, a fault, anticline).
• The contours are usually in m/ft below sea level, as most rocks located below sea level.
• An important structural map would one contoured on the top of a potential reservoir rock.

Geochemical  exploration method

Third type of method for exploration is the Geochemical surveys. These surveys though have limited indirect application but  companies are engaging crews to carry out surveys. Geochemical methods of prospecting are classified as direct or indirect.

The indirect methods are based on the detection of any chemical, physical, or microbiological changes in the soils, waters, or rocks associated with the oil and gas deposits.

The outstanding direct indication of the presence of oil or gas is a seep at the surface. 

Oil Seep

A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the earth's atmosphere and surface, normally under low pressure or flow.

Seeps generally occur above either terrestrial or offshore petroleum accumulation structures.

Gas seeps may be detected by:

1. Odor, due to the presence of sulfur compounds;

2. Yellow colors in the rock surrounding the opening, caused by sublimation;

3.  A whistling noise when the gas emerges under pressure opening;

4. “Eternal fire”  when the gas has become ignited by natural or artificial;

Geophysical exploration Methods

Gravity and Magnetic surveys to know the basement rocks and also to know the thickness of the sedimentary cover rocks.

Seismic surveying: The most common geophysical technique for obtaining the detailed geologic structural information needed to pick well locations is, however, the seismic survey.

B. Well Drilling

After identification of a promising geological structure by sophisticated geological and geophysical mapping that has been done and acquired the petroleum rights in exploration stage, it is time to go ahead and drill to find out if there is any oil or gas present underground.

Definition: Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials.

The drill bit is a multipoint, end cutting tool. It cuts by applying pressure and rotation to the workpiece, which forms chips at the cutting edge.

The drilling process is fairly similar whether the well is an exploration well or development well. The main difference is that the oil and gas company will usually take more measurements and try to drill more zones in an exploration well than in a development well.

Drilling rig

Wells are drilled using a drilling rig. Drilling rigs tend to be mobile units that are owned by separate contractor and can be used in a variety of environments:

i.  Land (Rig)

ii. Transition zones (barges/submersible rigs),

iii. Shallow water depths up to 150 meter (jackup rigs) and

iv.  Deep water (semi-submersible rigs and drill ships).

In many wells a single method is not employed throughout the entire work, and in some wells modifications of several methods are successively used. The terminology employed by well drillers throughout the country is fairly uniform. One of the major terms that may be mentioned is the word " rig."

 There are consumables that are required for the drilling process, such as:

i.  Drill bits,

ii. Drilling fluids (muds),

iii. Casing

iv. Cement.

Drilling mud :

which is a mixture of clay, high density solids, water and chemical additives is pumped down through the inside of the hollow drill string. The Mud serves several purposes:

• Bring rock chips (fragments of rock) up to the surface
• Clean and Cool the cone
• Mud weight should balance the downhole pressure to avoid leakage of gas and oil. Often, the well will drill though smaller pockets of hydrocarbons which may cause “a blow out” if the mud weight cannot balance the pressure. .

Once a well is drilled through the prospective reservoir, the operator will analyze the reservoir rock and typically perform a drill stem test (DST) to determine the potential flow rate of the well.

The flow rate is the speed at which hydrocarbons flow through the wellbore to the surface and will depend on the porosity and permeability of the reservoir rock, the reservoir pressure and the viscosity of the oil, and is a crucial determinant of field economics. Flow rates are measured in barrels of oil per day (bpd) or millions of cubic feet per day (MMCF/d).

Logging the Well

Logging the well Logging tools are lowered into the mud- filled hole (usually prior to casing being set in that part of the hole) and measure the electrical, acoustic, and radioactive properties of the different layers of rock. The results of these logging measurements are analyzed to determine which of these layers are porous and permeable, and likely to contain hydrocarbons.

Completion

After a well has been drilled to total depth (TD) and has proven by drill stem test (DST) to be capable of producing oil or gas, it must be completed.

The oil rigs and work over rigs used to drill and complete the well have moved off the wellbore, and the top is usually outfitted with a collection of valves called a Christmas tree or Production trees.

“Completing a well” means installing equipment in the well to allow a safe and controlled flow of petroleum from the well.

In a cased-hole completion, small holes called perforations are made in the portion of the casing which passed through the production zone, to provide a path for the oil to flow from the surrounding rock into the production tubing.

In open hole completion, often 'sand screens' or a 'gravel pack' is installed in the last drilled, uncased reservoir section. These maintain structural integrity of the wellbore in the absence of casing, while still allowing flow from the reservoir into the wellbore. Screens also control the migration of formation sands into production tubulars and surface equipment, which can cause washouts and other problems, particularly from unconsolidated sand formations of offshore fields.

Production

The production stage is the most important stage of a well's life, when the oil and gas are produced. By this time, the oil rigs and work over rigs used to drill and complete the well have moved off the wellbore, and the top is usually outfitted with a collection of valves called a Christmas tree or Production trees. These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work is needed. From the outlet valve of the production tree, the flow can be connected to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations, or oil export terminals.

As long as the pressure in the reservoir remains high enough, the production tree is all that is required to produce the well. If the pressure depletes and it is considered economically viable, an artificial lift method mentioned in the completions section can be employed.

Oil extraction and recovery

i.   Primary recovery

Hydrocarbons come in varying densities and viscosities; reservoir traps also have variations in porosity and permeability, pressures, and temperatures. All of these factors exert an influence on how easily hydrocarbons can be removed from a trap.

The forces include:

a.  Water located below the oil layer may be pressing upward; when this occurs, it is referred to as a water drive system

b.  If the gas cap located above the oil is causing a downward pressure or expansion of the natural gas at the top of the reservoir it is referred to as a gas cap drive system. 

c.  If the primary driving pressure is the result of pressure release from dissolved gas in solution with the oil, then it is referred to as a solution gas drive system. 

d. Buoyancy resulting from the movement of oil within the reservoir from the upper to the lower parts

ii.      Secondary recovery

Over the lifetime of the well the pressure will fall, and at some point there will be insufficient underground pressure to force the oil to the surface. After natural reservoir drive diminishes, secondary recovery methods are applied.

They rely on the supply of external energy into the reservoir in the form of injecting fluids to increase reservoir pressure, hence replacing or increasing the natural reservoir drive with an artificial drive. Sometimes pumps, such as beam pumps and electrical submersible pumps (ESPs), are used to bring the oil to the surface. Other secondary recovery techniques increase the reservoir's pressure by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the bottom of an active well, reducing the overall density of fluid in the wellbore.

iii.    Tertiary recovery

Steam is injected into many oil fields where the oil is thicker and heavier than normal crude oil. Thermally enhanced oil recovery methods (TEOR) are tertiary recovery techniques that heat the oil, thus reducing its viscosity and making it easier to extract. Steam injection is the most common form of TEOR. This form of recovery is used extensively to increase oil extraction in the San Joaquin Valley, which has very heavy oil, yet accounts for 10% of the United States' oil extraction.

In-situ burning is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil. Occasionally, surfactants (detergents) are injected to alter the surface tension between the water and oil in the reservoir, mobilizing oil which would otherwise remain in the reservoir as residual oil.

Another method to reduce viscosity is carbon dioxide flooding. Tertiary recovery allows another 5% to 15% of the reservoir's oil to be recovered.

Microbial treatments is another tertiary recovery method. Special blends of the microbes are used to treat and break down the hydrocarbon chain in oil thus making the oil easy to recover as well as being more economic versus other conventional methods. In some states, such as Texas, there are tax incentives for using these microbes in what is called a secondary tertiary recovery.

Enhanced recovery methods

Enhanced recovery methods such as (a) water flooding, (b) steam flooding, or (c)  CO2 flooding may be used to increase reservoir pressure and provide a "sweep" effect to push hydrocarbons out of the reservoir.

Such methods require the use of injection wells (often chosen from old production wells in a carefully determined pattern), and are used when facing problems with reservoir pressure depletion, high oil viscosity, or can even be employed early in a field's life.

Pressure Maintenance and Artificial Lift

When an oil well first starts producing oil, it can usually flow to surface naturally because of the high pressure in the reservoir formation. As oil is produced over the months or years, however, the reservoir pressure gradually decreases. This phenomena is normally counteracted by establishing a pressure maintenance program which involves injecting water or natural gas into the reservoir to balance the oil removed. Without pressure maintenance, some form of artificial lift may be required to help raise the crude oil to the surface and obtain the maximum production from the field.

Common artificial lift methods are :

1. Surface pump: Sucker rod pump
2. Down hole pumps: Electrical Submerged Pump (ESP)  

3. Injecting water, natural gas reinjection or Gas lift: Air, Carbon dioxide or some other natural gas of an active well.

Sucker rod pump (Surface Artificial Lift)

A common form of artificial lift is to install a pump to pump the oil up to the surface. The figure below shows an example of a widely used type of sucker rod pump.

Downhole Pumps

Downhole pump insert the whole pumping mechanism into the well. In modern installations, an Electrical Submerged Pump (ESP) is inserted into the well. Here the whole assembly consisting of a long narrow motor and a multiphase pump, such as a PCP (progressive cavity pump) or centrifugal pump, hangs by an electrical cable with tension members down the tubing

Offshore Production

Production of oil and gas offshore is similar to production onshore with the following major differences: (1) There is a need to provide some form of production platform; (2) Some of the production equipment must be installed on the seafloor instead of on dry land and; (3) Development wells are more often directionally drilled from the permanent platform in the offshore. Offshore platforms can be of a floating or fixed design. Fixed platforms, as the name suggests, are attached to the seabed. Floating platforms are held in place by some form of anchoring system or by a dynamic positioning system.

Classify oil wells is by land or offshore wells

They can be characterized as:

1. production wells are drilled primarily for producing oil or gas, once the producing structure and characteristics are determined

2. appraisal wells are used to assess characteristics (such as flow rate) of a proven hydrocarbon accumulation

3. exploration wells are drilled purely for exploratory (information gathering) purposes in a new area
4. wildcat wells are those drilled outside of and not in the vicinity of known oil or gas fields.

At a producing well site, active wells may be further categorised as:

1. oil producers producing predominantly liquid hydrocarbons, but mostly with some associated gas.
2. gas producers producing almost entirely gaseous hydrocarbons.
3. water injectors injecting water into the formation to maintain reservoir pressure or simply to dispose of water produced with the hydrocarbons because even after treatment, it would be too oily and too saline to be considered clean for dumping overboard, let alone into a fresh water source, in the case of onshore wells. Frequently water injection has an element of reservoir management and produced water disposal.
4. aquifer producers intentionally producing reservoir water for re-injection to manage pressure. This is in effect moving reservoir water from where it is not as useful to where it is more useful. These wells will generally only be used if produced water from the oil or gas producers is insufficient for reservoir management purposes. Using aquifer produced water rather than water from other sources is to preclude chemical incompatibility that might lead to reservoir-plugging precipitates.
5. gas injectors injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.

Abandonment

A well is said to reach an "economic limit" when its production rate does not cover the expenses, including taxes. When the economic limit is raised, the life of the well is shortened and proven oil reserves are lost. Conversely, when the economic limit is lowered, the life of the well is lengthened. When the economic limit is reached, the well becomes a liability and is abandoned.

In this process, tubing is removed from the well and sections of wellbore are filled with cement. Completely filling the wellbore with cement is costly and unnecessary. The surface around the wellhead is then excavated, and the wellhead and casing are cut off, a cap is welded in place and then buried.

At the economic limit there often is still a significant amount of unrecoverable oil left in the reservoir. It might be tempting to defer physical abandonment for an extended period of time, hoping that the oil price will go up or that new supplemental recovery techniques will be perfected. However, lease provisions and governmental regulations usually require quick abandonment; liability and tax concerns also may favor abandonment. In theory an abandoned well can be reentered and restored to production (or converted to injection service for supplemental recovery or for down-hole hydrocarbons storage), but reentry often proves to be difficult mechanically and not cost effective.

Crude Oil Treatment

The following impurities must be removed from crude oil as soon as it comes out of the ground

Crude oil usually contains impurities such as sediment, water, natural gas and sometimes hydrogen sulphide. Before the oil can be sold or delivered into a tanker or pipeline, these extra constituents must be removed to bring the crude up to the proper quality. To accomplish this, processing equipment must be installed, including such things as separators, to separate oil and gas from water and sediment.

Where hydrogen sulphide (H2S) is present, special chemical treatment facilities are required to remove it. Hydrogen sulphide is a very dangerous poisonous gas which also is highly corrosive to pipelines and tankers. It smells like rotten eggs, and if inhaled in high enough concentrations can cause death within two minutes.

Major Oil fields : Production greater than 1 billion barrels perday

Field	Location	Discovered
Ghawar Field	Saudi Arabia	1948
Burgan Field	Kuwait	1937
Ferdows/Mound/Zagheh Field	Iran	2003
Sugar Loaf field	Brazil	2007
Cantarell Field	Mexico	1976
Bolivar Coastal Field	Venezuela	1917