Question 1 of 7: Reservoir Engineering Terminology

Question 1: Reservoir Engineering Terminology (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Find. A brief (one-to-two sentence) engineering definition of each of the ten terms (1)–(10).

Approach. Each term is answered directly from its standard reservoir-engineering definition; no calculation is required.

1) Irreducible water saturation

The minimum water saturation that cannot be reduced further by increasing the displacing (oil or gas) pressure; the remaining water is held immobile by capillary forces in the smallest pores and as a film on grain surfaces.

2) Effective porosity

The fraction of bulk rock volume occupied by interconnected pore space that actually contributes to fluid storage and flow, excluding isolated or dead-end pores counted in the total (absolute) porosity.

3) Residual oil saturation

The oil saturation left trapped and immobile after a displacing fluid (water or gas) has swept a zone as completely as the prevailing forces allow; below this saturation the oil is disconnected and cannot flow regardless of further flooding.

4) Original oil in place

The total volume of oil, expressed in stock-tank barrels (STB), originally contained in a reservoir at the time of discovery, before any production has occurred.

5) Imbibition

The process by which the wetting phase (typically water) increases its saturation by displacing the non-wetting phase (oil or gas), such as water advancing into an oil zone under a water drive or waterflood.

6) Water drive

A reservoir drive mechanism in which an aquifer hydraulically connected to the hydrocarbon zone expands as reservoir pressure declines and encroaches into the reservoir, supporting pressure and displacing oil or gas toward the producing wells.

7) Oil-water contact

The depth in the reservoir at which the oil saturation transitions to 100% water saturation (the practical base of the movable oil column), as distinct from the free water level defined purely by zero capillary pressure.

8) Retrograde gas reservoir

A gas-condensate reservoir in which, as pressure falls below the dew-point pressure during isothermal depletion, liquid hydrocarbon condenses out of the gas phase — the reverse of normal vaporization behaviour, caused by the shape of the two-phase envelope near the critical point.

9) Solution gas-oil ratio

The volume of gas, in standard cubic feet, dissolved in one stock-tank barrel of oil at reservoir pressure and temperature; it is constant above the bubble-point pressure and decreases as pressure falls below it.

10) Under-saturated oil reservoir

A reservoir in which the initial pressure exceeds the oil's bubble-point pressure, so the oil exists as a single liquid phase with no free gas cap and could still dissolve additional gas if pressure were reduced to the bubble point.