Risk and Uncertainty Flashcards
How are reserves calculated?
Reserves = STOIIP (oil in place) x RF (recovery factor)
How are reserves defined? What are defines as resources?
- Reserves are known to exist, assessed to be economically viable and proven by drilling and defined by strict criteria
- Resources are everything else
Define P90: proven reserves
- Stock market valuation
- Reasonably certain to be commercially recoverable at present
- Producible from existing wells (infill wells from lowest known oil, step-out wells into tested fault blocks)
Define P50: probable reserves
- More likely recoverable than not
- Step out wells into untested adjacent fault blocks
- Reserves one sand thickness below lowest known oil
- Incremental reserves from untested reservoir process
- Additional reserves from more optimistic interpretation of reserves than proven
Define P10: Possible reserves
- Volumes that might be recovered
- Step out wells into untested fault blocks
- Volumes more than one sand thickness below LKO
- Incremental reserves more optimistic than probable
- From speculative reservoir processes
- Includes non-commercial discoveries
- Does not include exploration lead/prospects
Describe the deterministic approach to estimating reserves
Give an advantage and disadvantage of this method
- Estimates for min, most likely and max values for each parameter are input
Pro: good idea of the magnitude of reserves, quick and easy to input new data
Con: Often uncertainty around input values (N:G, porosity etc)
Describe statistical approach to reserve estimation
Why is it done?
- Assesses statistical probability of each parameter and distribution estimated
- Done to better address uncertainty in values
Describe the Monte Carlo method of statistical reserve estimation
- Data input into computer programme randomly iteratively samples the individual parameter distributions and multiplies them to calculate the resource/reserves
What data is required for reserve estimations? (9 things)
- Depth structure map on reservoir top
- Estimated Min/ML/Max closures
- Gross reservoir thickness
- N:G
- Porostiy
- Sw
- FVF
- Fill factor
- Conversion factor
Gross rock volume is the most important parameter due to it being the biggest parameter.
How is it calculated and what is it important to recognise/address?
- Calculated using structure maps from seismic on top reservoir horizon
- Important to recognise and address: Top reservoir Uncertainty Max spill point = true maximum - Depth conversion constant
Water saturation is present in all reservoir and is irreducible water between or on grains.
What are the two main controlling factors? Why?
- Hydrocarbon type and density (heavy vs light O + G): heavy oil has higher saturation than light oil and gas
- Porosity: higher porosity - higher hydrocarbon saturation
What is meant by the fill factor?
How can it be estimated?
- Many structures arent full to mapped spill point, fill factor is expected fill %
- Estimated from analysis of nearby fields with similar trap types, in no other info then assume it is 1
- Where structure mapped is large and source kitchen is small, some allowance for lack of charge may be made in reserve calculation of 0.75 or less
What is meant by the FVF (formation value factor)?
Oil reduced as pressure decreases towards the surface
What is the gas expansion factor (Bg)?
Expands as brought to surface thus given volume there is more gas volume at surface than depth
What are nine factors does the recovery factor depend on?
- Reservoir porosity
- Fluid type
- Reservoir complexity
- Volume of reservoir
- Drive mechanism
- Production method
- Economics (time)
- Market conditions
- Secondary recovery
