Info about oil

Oil comes from the remains of plankton that died 10-600 million years ago. Their remains sank to the bottom of the sea, decaying over time into different sedimentary layers. With little oxygen present microorganisms broke the remains into carbon-rich compounds that formed organic layers. This organic material mixed with the sediments, forming fine-grained shale, or source rock. The source rock came under intense pressure and heat as new layers were deposited and distilled into crude oil and natural gas. The oil then flowed from the source rock and accumulated in thicker, more porous limestone or sandstone called reservoir rock. Movements in the Earth’s crust then trapped the reservoir rocks containing oil and gas in between layers of impermeable rock, or cap rock, such as granite or marble. “Simples!”

Estimating oil in place and recovery
The term “oil reserves” relates to different classifications of economically recoverable oil. The classifications reflect the degree of certainty about recovery. The total estimated amount of oil in a reservoir, including producible and non-producible oil is called oil in place (OIP). Only a fraction of this will be possible to extract and only the producible amount is called a reserve. The ratio of producible reserves to total OIP in a field is known as the recovery factor. Recovery factors vary greatly, and may change over time based on factors such as technology and economics. The recovery factor may rise if enhanced oil recovery (EOR) techniques such as gas injection or water-flooding are used. In general, early estimates of the reserves of an oil field are conservative and tend to grow with time.

Because actual OIP cannot be seen, other techniques are used to estimate the size and recoverability of resources. Volumetric methods attempt to determine the amount of OIP by using the size of the reservoir as well as the physical properties of its rocks and fluids. A recovery factor is assumed, using assumptions from fields with similar characteristics. OIP is multiplied by the recovery factor to arrive at a reserve number. Current recovery factors for oil fields around the world range from 10% to 60%, or even over 80%. The wide variance is due to diversity of fluid and reservoir characteristics for different deposits.

Global Oil Reserves 
According to US energy information agency the top 4 highest countries proved oil reserves including non-conventional deposits are in Venezuela (298 billion barrels), Saudi Arabia (268 billion barrels), Canada (173 billion), and Iran (155 billion) 

Source US Energy information Agency

Types of oil reserves
All reserve estimates involve uncertainty as they rely on interpreting geological and engineering data. The degree of uncertainty is reflected by describing reserves as one of two types - Proven or Unproven. Unproven reserves are further divided into probable and possible reserves. This reflects the degree of uncertainty about their existence - hence the 3P’s - Proven, Probable, and Possible.

Proven reserves
Proven reserves are those with a reasonable certainty of being recoverable under existing economic and political conditions, and with existing technology. These reserves are referred to as P90 (90% certainty of being produced). Proven reserves are also known as 1P reserves.

Proven reserves are further divided into "proven developed" (PD) and "proven undeveloped" (PUD). PD reserves can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment is required. PUD reserves require additional capital investment (e.g. drilling new wells) to bring the oil to the surface.

Unproven reserves
Unproven reserves are based on similar geological and engineering data as proven reserves - but technical, contractual, or regulatory uncertainty means that such reserves cannot be classified as proven. They are further divided into probable and possible categories

Probable reserves are known accumulations with at least a 50% chance of recovery. They are referred to as P50, or 2P reserves (proven plus probable).
Possible reserves have a less likely chance of being recovered than probable reserves. They have at least a 10% certainty of being produced. These are referred to as P10 or 3P (proven plus probable plus possible). Reasons for classifying reserves as possible include ambiguous interpretation of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas) and projected reserves based on future recovery methods.

Further classifications of Resources
A further system of evaluating resource size was introduced in 2007. It incorporates the above definitions for reserves, but adds categories for contingent and prospective resources.

Contingent resources are quantities of oil estimated at a given date, potentially recoverable from known accumulations, but where the project is not yet considered mature enough for commercial development due to one or more contingencies. For example, there could be no currently viable markets, or commercial recovery is dependent on technology still under development.
Prospective resources are quantities of oil estimated at a given date, to be potentially recoverable from undiscovered accumulations by future development projects. Prospective resources have an associated chance of discovery and a chance of development.

Unconventional resources
Unconventional resources exist in accumulations across large areas. Examples include extra heavy oil, natural bitumen, and oil shale.. Unconventional resources require specialized extraction technology to produce. For example, steam and/or solvents are used to mobilize bitumen for in-situ recovery. The extracted oil may require significant processing prior to sale. The total amount of unconventional oil resources in the world considerably exceeds the amount of conventional oil reserves, but is much more difficult and expensive to develop.

Drilling for oil 
Drilling for oil and gas involves a number of stages:
  • Well Planning
  • Well Design
  • Drilling Operations
  • Formation Evaluation and Testing
  • Well Completion

Well planning
Well planning is designed to create a programme for drilling which is safe, cost effective, and achievable. There are many complications to take into consideration, such as Geology, Equipment, Temperature, Sizing, Budget etc. Drills may be categorised as any one of the following types, all of which have different requirements.
  • Wildcats – no (or little) known geological data for site selection
  • Exploratory holes – site selected on seismic data, surveys etc. No drilling data from prospective area
  • Step-outs – delineating the reservoirs boundaries. Drilled after the exploratory discovery. Site selection usually based on seismic data. Also known as a delineation well.
  • Infill well – drills the known productive portions of the reservoir. 
  • Re-entries – existing well re-entered to deepen, sidetrack, rework or recomplete the well. Various amounts of planning required, depending on the purpose of re-entry. 

Well Design
Plans will be made for where drill holes need to be cased in order for drilling to reach the desired depth. The decision will be based on data such as formation pressures, strengths, and makeup, and is balanced against the cost objectives and desired drilling strategy. With casing depths determined, hole sizes and casing sizes must be decided. The hole drilled for each casing string must be large enough to easily fit the casing inside it, allowing room for cement between the outside of the casing and the hole. Also, the inside diameter of the first casing string must be large enough to fit the second bit that will continue drilling. Thus, each casing string will have a subsequently smaller diameter.

The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing and associated hardware such as packers, gas lift mandrels and subsurface safety valves.

Drilling Operations

There are many types of drill bits, each designed for different kinds of rock. The bit is screwed into the end of a drill string, consisting of individual lengths or joints of hollow steel pipe c.30ft long. As the drilling progresses, additional joints of pipe are added to the string. As the bit drills ahead a specially formulated drilling fluid or mud is continually pumped or circulated from the surface, to the bottom of the well, and then back to surface to cool the bit and remove the cuttings.

Formation evaluation and testing for Oil
Drilling takes place in stages: The crew drills, then runs and cements new casings, then drills again. If rock cuttings from the mud reveal oil sand from reservoir rock the final depth may have been reached. At this point, the drilling apparatus is removed and several tests may be performed to confirm the findings
  • Well logging - lowering electrical and gas sensors into the hole to take measurements of the rock formations
  • Drill-stem testing - lowering a device into the hole to measure the pressures, which will reveal whether reservoir rock has been reached
  • Core samples - taking samples of rock to look for signs of reservoir rock

Well completion
Once final depth has been reached the well is completed to allow oil to flow into the casing in a controlled manner. A perforating gun is lowered into the well. The gun has explosive charges to create holes in the casing through which oil can flow. After the casing has been perforated a small-diameter pipe is run into the hole to enable the oil and gas to flow up through the well. A device called a packer is run down the outside of the tubing. With the packer set at the production level, it is expanded to form a seal around the outside of the tubing. A multi-valved structure called a Christmas tree is connected to the top of the tubing and cemented to the top of the casing. The Christmas tree allows the flow of oil from the well to be controlled.

After the well is completed, the oil is allowed to flow. With limestone reservoir rock, acid is pumped down the well and out of the perforations. The acid dissolves channels in the limestone that lead oil into the well. With sandstone, a specially blended fluid containing proppants is pumped down the well and out the perforations. The pressure from the fluid makes small fractures in the sandstone that allows oil to flow, whilst the proppants hold the fractures open. Once oil is flowing, the rig is removed and production equipment is set up to extract the oil.

Offshore Oil Drilling

Using seismic equipment, oil companies decide where to drill, using mobile offshore drilling units (MODU) to drill the initial well. There are four main types of MODUs:

  • A submersible MODU usually consists of a barge that rests on the sea floor at depths around 10 meters. Steel posts extend above the water line. A drilling platform rests on top of the steel posts. These rigs are typically used in areas with calm water.
  • A jackup is a rig that sits on top of a floating barge. Once the barge is positioned, the jackup can extend legs down to the sea floor. The legs are loaded in such a way that they don't penetrate the floor. Once each leg is secure, the jackup continues to ratchet the legs so that the platform rises above the water level. This keeps the rig safe from tidal motions and waves. Jackups can operate in depths of up to 525 feet (160 meters).
  • Drill ships are ships that have a drilling rig on deck. The drill operates through a hole in the hull. Drill ships can pilot to the drill site and use a combination of anchors and propellers to correct for drift as the rig drills for oil. They can operate in deep water conditions.
  • Semisubmersibles float on the surface of the ocean on top of huge, submerged pontoons. Some have propulsion systems, which allow them to navigate to drilling sites under their own power while others require a second vessel to tow them to the right location. Some convert from drilling rigs to production rigs, reducing the need for a second rig once oil is found.

The MODU's job is to drill into the ocean's floor to find oil. The part of the drill that extends below the deck and through the water is called the riser. The riser allows for drilling fluids to move between the floor and the rig. The drill string is lowered through the riser. At the sea floor is the blowout preventer. This has a pair of hydraulically-powered clamps that can close off the pipe leading up to the rig in the case of a blowout.

To add stability to the well, engineers use metal casings, much as they do with land-based oil rigs. These casings help keep the well from collapsing in on itself. Each casing is lined with cement walls. Casings get narrower as the well gets deeper. Oil companies use progressively smaller drill bits as the well's depth increases. At each spot where a narrower casing joins with a wider one engineers use a liner hanger O-ring to seal the two sections together.

If the MODU hits oil, engineers must seal the well to prepare it for a production rig. A pair of plugs will seal off the well bore. The bottom plug sits near the oil deposit. Drilling mud or seawater provides the pressure to hold the plug in place while a top plug caps the oil well. The well is then ready for a production rig to take over.


  1. Don't copy other people's photos. Against copyright

    1. Thanks. I've made sure all of my images are from pixabay, and are copyright free.


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