Page 86 - Principles of Catalyst Development

Part I: Reservoir Engineering Primer 67

The sum of the drive indices equals one, thus

+ 1, + I* = 1 (8,2)
I v + I gc + I w
Equation (8.2) can be derived by rearranging Eq. (2.3). A comparison of the
magnitudes of the drive indices indicates which drive is dominating the perform-
ance of the reservoir.
Although the above discussion referred to oil reservoirs, similar comments
apply to gas reservoirs. Water drive and gas expansion with reservoir pressure
depletion are the most common drives for gas reservoirs. Gas reservoir recovery
can be as high as 70% to 90% of original gas in place (OGIP) because of the
relatively high mobility of gas.
Gas storage reservoirs have a different life cycle than gas reservoirs that
are being depleted. Gas storage reservoirs are used as a warehouse for gas. If
the gas is used to as a fuel for power plants, it will also need to be periodically
produced and replenished. The performance attributes of a gas storage reservoir
are [Tek, 1996, pg. 4]:
* Verification of inventory
* Assurance of deliverability
* Containment against migration
The gas inventory consists of working gas and cushion gas. Gas deliverability
must be sufficient to account for swings in demand. Demand swings arise from
such factors as seasonal variations. Gas containment is needed to conserve the
amount of stored gas. For more discussion of natural gas storage in reservoirs,
see references such as Tek [1996], Smith [1990], and Katz and Lee [1990].

Secondary Production
Primary depletion is usually not sufficient to optimize recovery from an
oil reservoir. Oil recovery can be doubled or tripled by supplemental natural
reservoir energy. The supplemental energy is provided using an external energy
source, such as water injection or gas injection. The injection of water or natural
gas may be referred to as pressure maintenance or secondary production. The
latter term arose because injection usually followed a period of primary pressure
depletion, and was therefore the second production method used in a field. Many