Page 168 - Introduction to Petroleum Engineering

TYPES OF WELLS 155

Answer
Radius of casing = r and radius of pipe = r :
c p
V = π rL −π rL
2
2
c p
π r − ) L
2
r
2
p
= ( c
(
.
.
t
.
= 3 14159 0 292 ft 2 − 0 188 ft 2 ) ×1500 ft ≈ 235 2 ft 3
.

8.4 TYPeS OF WeLLS
Wells are drilled for exploration or development. Exploration wells include wildcat
wells that are used to test a geologic trap that has never been produced, test a new
reservoir in a known field, or extend known limits of a producing reservoir. A dis-
covery well is a wildcat that discovers a new field. Wells for estimating field size
include appraisal wells, delineation wells, and step‐out wells. Development wells are
drilled in the known extent of a producing field. An infill well is drilled between
producing wells in an established field.
Wells may be categorized also by their function. A production well can produce
oil, gas, and water or thermal energy as in geothermal wells. An injection well is used
to inject water or gas and includes wells for water disposal and sequestration. It can
also be used to inject steam for flooding of heavy oil reservoirs.
8.4.1 Well Spacing and Infill Drilling

The distance between wells and the area drained by a well correlate to well spacing
(Figure 8.11). In the United States, a typical spacing is expressed in terms of acres.
For example, the term “40 acre spacing” refers to vertical wells that can drain an area
of 40 acres. A square mile contains 640 acres, so there are 16 areas with 40 acres each
in a square mile. Therefore, the number of wells that can be drilled in a square mile
with 40 acre spacing is 16.
Vertical oil wells tend to be drilled closer to one another than vertical gas wells, because
gas viscosity (usually near 0.02 cp) is much less than oil viscosity. For example, vertical
gas wells may be drilled on 160 acre spacing, which means that only four wells must be
drilled to drain a square mile. If additional wells are needed to adequately drain a reservoir,
more wells can be drilled in the space between existing wells. This is called infill drilling.

example 8.6 Well Spacing

How many vertical production wells are needed to drain a field that covers
20 mi ? Assume 40 acre spacing.
2
Answer
16 wells are needed to drain 1 mi (1mi = 640acres) on 40 acre spacing. Therefore,
2
2
the number of vertical production wells needed is 20mi × 16wells/mi = 320 wells.
2
2