RESERVOIR DEVELOPMENT PLAN

When the decision is made that sufficient information is available for acceptable
definition of the reservoir, and that reservoir development and production should proceed,
a development plan for the reservoir is prepared. This plan is designed to optimize
recovery of the hydrocarbon within anticipated economic and resource development
limits. This development plan will determine the reservoir production history and is
extremely elaborate and specific. A part of that plan will be the development well
spacing.
Development Wells
The function of development wells is to “effectively and efficiently recover maximum
hydrocarbon from the reservoir in a reasonable production lifetime, maximizing
economic return and resource recovery within necessary environmental limits.” These
development wells not only include producing oil and gas wells, but may also include gas
injection, water injections, and other types of service wells, to optimize the development
of the reservoir. Some wells which are to be used as producing wells for the first several
years may converted in injections wells later in the life of the reservoir, according to this
development plan.
Producing Wells
These are the wells specified in the development plans, for production of the hydrocarbon
to the surface. They may be oil production wells or gas production wells. The spacing of
these wells will be selected based on reservoir properties and economics. A common
spacing for oil reservoirs for onshore operations has been the 40 acre spacing. 1 mile =
5,280 ft. and 1 mile2 is equal to 640 acres of area. 1 acre = 43,560 ft2.
Consider the 1 mile2 area. If that 1 mile2 is divided into quarters, each quarter = 160
acres. If those quarters are further divided into quarters, the result will be 16 square area
of acres each (16 times 40 acres = 640 acres).
A 40 acre spacing for the drilling of development wells implies that one well will be
drilled in each 40 acres. The result will be 16 wells per 1 mile2. Each well, therefore, will
be ¼ mile or 1,320 ft from its offset wells and will have 4 offsets (north, south, east and
west). In the ideal production plan, over a reasonable lifetime of production, each well is
expected to drain a rock cylinder 660 ft in radius and of thickness equal to the
hydrocarbon reservoir rock thickness.Many other spacings are also used. For gas reservoirs, a common spacing has been a 160
acre spacing, or 4 producing gas wells per 1 mile2. It is desirable, if practical, that the
wildcat well and the appraisal wells be among the best producing wells.
Injection Wells
Injection wells are drilled to serve various functions, such as injection of external fluids
into the reservoir, including hydrocarbon (natural) gas, water, nitrogen, CO2, or others, to
enhance the recovery of the original hydrocarbons or to maintain reservoir fluid pressures
during the production life of the reservoir.
Injection wells may previously have been utilized as producing wells, but, in the
development plan for the reservoir, there was included the conversion of some producing
wells into injection wells at a particular time in the production life of the reservoir.
Injection wells may also be drilled to dispose of undesirable fluids, such as salt water,
that are produced to the surface along with the hydrocarbons. These would be considered
as salt water disposal wells, and the salt may be injected into reservoirs other than
hydrocarbon reservoirs. Excess solution gas, for which there is no market, may also be
injected into reservoirs other than those from which it was produced, to store that gas for
future production.
Reservoir Pressure Control
For those reservoirs which initially have reservoir fluid pressures greater than the bubble
point pressure of the hydrocarbons, it is usually desirable to maintain the flowing
bottomhole pressures of the producing wells above the bubble point pressure for a
considerable portion of the production life of the reservoir. It may be possible initially to
maintain this condition by proper selection of the choke size in the wellhead.
If the reservoir fluid pressure is sufficiently higher than the bubble point pressure of the
reservoir hydrocarbons for the well depth and hydrocarbon density, then, within fluid
property limits, the flowing bottomhole pressures can be maintained above the bubble
point pressure by manipulating the production choke size in the wellhead. This indicates,
therefore, that the reservoir pressures in the producing region surrounding the wellbore
will also be maintained above the bubble point pressure, that there will only be liquid
hydrocarbons in the reservoir and that only liquids will be produced into the wellbore at
flowing bottomhole conditions. This is normally desirable in the early production history
of a reservoir.As produced fluid returns to the surface, however, it may reach its bubble point pressure,
so that both gas and liquid may exist at the wellhead. As the natural reservoir fluid
pressure reduces as hydrocarbons are produced, it may be necessary to inject external
fluids into the reservoir to maintain reservoir pressure. Oil production is a volume
displacement process. Idealistically, basing volumes on reservoir conditions, if, for each
reservoir barrel of oil produced, a reservoir barrel of water is injected beneath the oil zone
into the water zone, reservoir fluid pressure should maintained.
As the reservoir nears the end of its productive life, however, it will finally be desirable
to lower the flowing bottomhole pressure, through a controlled procedure, to se low a
pressure value as is feasible, to recover the maximum volumes of remaining oil and gas
(including solution gas) from the reservoir before it is depleted, as determined by
economics, and therefore abandoned,
Gas injection into a natural gas cap, which might exist above the oil zone, could also be
used for pressure maintenance. If the initial reservoir fluid pressure is greater than the
bubble point pressure of the reservoir hydrocarbons, a gas cap might created by gas
injections, even though one did not exist under original natural conditions within the
reservoir. For example, for reservoir where increased water saturations have a significant
adverse effect on permeability to the flow of oil, this gas injection process for pressure
maintenance could be initiated very early, or at the beginning of the productive life of the
reservoir.
Observation Wells
Wells may also be drilled for the purpose of monitoring the reservoir development plan
during the productive life of the reservoir. The wells are equipped with pressure
monitoring systems, to determine the extent of propagation of the pressure transient from
the producing wells into the reservoir. The observation wells may also be used to monitor
encroachment of the gas-oil interface or the oil-water contact into the oil zone, as well as
progress of injected fluids such as the water front, during enhanced oil recovery by
waterflood. These wells may also be converted for functions other than observation, such
as production or injection later in the productive life of the reservoir.