Horizontal wells

Horizontal wells

By oilfield convention, a horizontal well is defined as a well with an inclination angle of 90
degrees from the vertical. A vertical well is one with zero inclination angle.
There is a misconception in some quarters within the industry that horizontal wells were
introduced during the mid to late 1980’s. In fact as the list below shows horizontal drilling
dates back to the 1950’s.

1- MILESTONES IN THE DEVELOPMENT OF HORIZONTAL DRILLING

1950s Russians drilled 43 horizontals
1978 Esso, modern horizontal, Alberta
1979 Arco drilled to overcome high GOR’s and gas coning
1979-83 Elf test 3 onshore horizontals; Elf and Agip drill first
Offshore horizontal (Ropso Mane, Adriatic)
1986 50 horizontals worldwide. Cost 1.5-2 times greater than vertical wells
1987-88 Horizontal well test theory and productivity assessment Guidelines. Number
of horizontals increased dramatically
1989 265 horizontals drilled worldwide.
1990 1000 horizontals drilled.
1991 First Australian horizontal drilled
1992 Over 2,500 horizontals drilled worldwide - 75% in North America (mostly
low permeability, gas/water coning regions)
Developments in hydraulic fracturing, perforating, computer models and
screen completion failures in unconsolidated formations
1993-2000 Horizontal wells are drilled world-wide and become routine wells

2- TYPES OF HORIZONTAL WELLS

There are three types of horizontal wells:
1. Short radius
2. Medium radius; and
3. Long radius

This table gives a summary of the features and tools for each type.

1-SHORT RADIUS WELLS (SRW)

The main features of this type is the very high build-up rate of 60 – 150 degrees /100 ft with
a radius range of 40-100 ft. This type requires specialised articulated motors to affect the
high build angles.

Advantages
1. Enables sharp turns into thin reservoirs
2. Both motor driven and drill pipe driven
3. Laterals can be completed and tied back using special liners
Disadvantages
1. Limited extension possible - Record = 1200’
2. Poor directional control, must snake left then right to
3. Special tools and equipment required

2- MEDIUM RADIUS WELLS (MRW)

The build-up rate for this type is usually 8-30 degrees/100ft with a radius range of 200 to 700
ft. The horizontal drain is usually between 1000 – 3500 ft, see Figure .
A typical well profile consists of build-tangent section and a build-horizontal section. Two
different BHA’s will therefore be required for this type of well.
The second build-up section should ideally start at the top of the "marker zone" and should
reach a maximum of 85-100 degrees on entry into the reservoir. An angle hold assembly
should be used to drill the horizontal section.

3- LONG RADIUS WELLS (LRW)

This is the most common type of horizontal wells especially offshore. The build-up rate is
usually from 2 to 6 degrees/100ft. The most common BHA used is a steerable system containing
a single bent sub with a downhole motor, see Figure .

Two profiles are in common use:
• A single build-up section terminating in the horizontal section
• A build-tangent and then a higher build-lateral section.