Common chance

Prospects, or segments of a prospect, have common chance if the outcome of one of them (success or failure) alters the chance of success in the other. Some or all of the elements which make up overall geological chance of success (trap, seal, reservoir, charge, etc.) may be common.

Consolidation

The addition of volumes

Dependency

Variables are dependent when the value of one of them depends on the value of the other. For example, when porosity increases it is often observed that water saturation decreases.

Risks are dependent when the outcome of one prospect (success or failure) alters the chance of success of another.

Deterministic calculation

A calculation of prospect or field volumes using one value for each input. People used to do this in the old days - incredible but true.

EPOS, or Chance of Economic Success

The chance that a prospect will have an economically successful result. Economic success is judged by economic hurdles or cut-offs, which are minimum quantities of recoverable oil and/or gas.

Formation volume factor

The hydrocarbon volume in the reservoir divided by the hydrocarbon volume at surface ("standard conditions").

For oil, the FVF (known as Bo) is a number greater than 1, because oil shrinks as gas is released on its way to the surface.

For gas, REP uses the inverse of the FVF ((1/Bg) so the number is again greater than 1. Also known as the gas expansion factor.

Note that "standard conditions" may be defined differently in different countries, though the differences are usually trivial, at least in a REP context.

Gas expansion factor

See Formation volume factor above.

GOR (Gas Oil Ratio)

The amount of gas which is produced along with oil, from an oil reservoir. This gas has 2 sources: 1. solution gas, dissolved in the oil and released when the pressure is reduced; 2. free gas produced from a gas cap.

In REP, you need to enter the solution gas-oil ratio, since any gas cap is handled separately.

GPOS, or Chance of Geological Success

The chance that a prospect will discover producible hydrocarbons. The geological chance of success is a combination of underlying chances, including reservoir, trap, seal and charge. You may consider the chances of success at a regional (play) or at a prospect specific level.

Different companies have different ways of risking prospects, and REP has several different risking schemes. Do not mix them up!

Gross reservoir volume

The volume of rock in the trap above the hydrocarbon-water contact.

Gross rock volume (GRV)

The volume of rock in the trap above a certain depth, usually the spill point or hydrocarbon-water contact.

Hydrocarbon-in-place.

The amount of hydrocarbon (oil or gas) existing in a reservoir. By convention, in-place volumes are quoted as equivalent volumes at surface conditions. This is done using formation volume factors. The volume actually occupied by hydrocarbon in the reservoir is known as the hydrocarbon pore volume.

Inert gas

Non-hydrocarbon gas in the reservoir. Common examples are nitrogen and carbon dioxide.

Iteration

In the Monte-Carlo method, the program chooses random values for each input variable (spill point, porosity, etc.) to calculate a result. It does this many times. Each time it does it is called an iteration or realization.

Layer

A vertical subdivision of a single reservoir interval. In a REP context, layers have distinct thickness, net-to-gross ratio, porosity, water saturation and recovery factor. Layers have the same structural shape (defined by the area/depth table of the top layer), hydrocarbon contact or spill point and hydrocarbon type.

Lead

An unproven accumulation of hydrocarbons, and an early stage of technical appreciation. When the work has been done, the lead may become a prospect.

Model

A particular interpretation or possible outcome of a prospect. A prospect can have any number of models, but only one (at most) can be "correct".

NGL

Natural gas liquids "drop out" of gas when the pressure is reduced. REP considers three basic reservoir hydrocarbon types: oil, gas and gas condensate. NGL may be produced from gas and gas condensate reservoirs.

Phase

A hydrocarbon for which volumes are calculated; for example, oil-in-place, recoverable gas.

Phase chance

The chance that a particular hydrocarbon type will be discovered. REP normally reports phase chance as a relative probability - which can be explained as the chance that oil (for example) is discovered if any hydrocarbons are discovered at all.

Prospect

An unproven accumulation of hydrocarbons. See also Lead above.

Realization

See Iteration above.

Recovery factor

The proportion of any hydrocarbon existing in a reservoir ("in-place") to the amount that is expected to be produced ("recoverable").

Reserves

"Those quantities of petroleum which are anticipated to be commercially recovered from known accumulations from a given date forward" (SPE)

Note the vital elements: "commercially recovered" and "known accumulation". You cannot have reserves in prospects. You cannot have sub-commercial reserves. You cannot have reserves-in-place.

Hydrocarbon that can be economically produced.

Segment

A bit of a prospect. Segments can be fault blocks, layers or any other sub-division which has geological identity.

Shrinkage factor

A factor converting recoverable gas volumes to sales gas volumes. The difference between the two includes shrinkage due to further liquids extraction, fuel gas usage etc.. Use of shrinkage is option in REP.