T.K. Akhmedzhanov, I. B. Igembaev, D.K. Baiseit, A. S.

Abd Elmaksoud

DISADVANTAGES AND BENEFITS OF HORIZONTAL WELLS

Abstract

Horizontal well technology was originally developed for use in petroleum production and
underground utility installation, but recently has been adapted for environmental remediation
applications. In the environmental remediation industry, horizontal wells provide unique characteristics
and advantages that can improve the effectiveness of established soil and groundwater cleanup
technologies now using traditional vertical well techniques. The steering capability associated with
some horizontal well drilling techniques allows installation in areas containing underground utilities,
vertical wells, and other subsurface obstructions. Horizontal wells can be installed beneath buildings and
other surface structures, allowing access for treatment to areas generally inaccessible to vertical wells.
The orientation of horizontal wells compared with vertical wells may require fewer wells to achieve
similar remediation goals due to the greater surface area associated with the lengthwise screened area of
these wells. Horizontal screens provide greater surface area in contact with contaminated soil or
groundwater, allowing more effective transfer of materials used for remedial treatment. In this paper the
horizontal well technology and a review of the economic benefits and disadvantages of horizontal wells
are included. Although horizontal wells have been drilled as early as 1927, the major thrust of drilling
horizontal wells started in 1980. Initial wells were short length wells (about 250 ft. long wells). In 1985,
the first medium radius horizontal well was drilled using a down-hole mud motor. Since then, using
horizontal wells has become a common practice. Today, the medium radius drilling technique is the most
commonly used drilling method.

Introduction

Some of the early horizontal well efforts date back to 1930. After World War II, with the advent
of jet perforation, major industry efforts were focused on casing the drilled hole and perforating in the
desired zones. The field implementation of this perforation technique was a great success and at least for
a while horizontal drilling took a back seat. In the late 70s and early 80s, with oil prices around $35 a
barrel, interest in horizontal wells was reignited. The purpose of the horizontal wells was to enhance well
productivity, reduce water and gas coning, intersect natural fractures and to improve well economics. In
the early 80s, Elf Aquitaine, a French company, introduced horizontal wells to the oil industry to produce
a heavy oil carbonate reservoir in the Rospo Mare Field, offshore Italy, in the Adriatic Sea. At the same
time, in the U.S., several companies were using horizontal wells to reduce gas coning in the Abo Reef in
New Mexico. They were also using horizontal wells to intersect fractures in the fractured carbonate
reservoirs in Oklahoma, Kansas and Texas.2 The drilling technique used by Elf Aquaitain was very
different from that used in the U.S. The Elf technology involved drilling long radius (1000 ft. turn radius,
see Fig. 1) and long length (a few thousand ft.) wells. They were also using down-hole motors to turn the
bit and drill wells. To date, this long radius drilling technology remains suitable to develop offshore fields
around the world. In the U.S., initial efforts were with the short radius drilling technique where turn
radius was around 30 ft. The wells were drilled using stabilizers, knuckle joint and flexible collars. A
mushroom type, helical collar joint was used to provide necessary flexibility to the drill pipe to turn from
the vertical to the horizontal direction in a short distance. Well completion was either open-hole or with a
slotted liner. The typical well length was 100 to 300 ft. The major disadvantage of this drilling technology was
its limited completion options and high cost of drilling. In the mid-eighties, the cost of drilling the 30 ft. radius
well was of the order of $2000 to $3000 per ft.
 Fig.1: A Schematic of Different Drilling Techniques

To minimize this drilling cost, and to drill long length wells, a medium radius drilling technology
was developed. Turn radius for the medium radius wells was about 300 ft. to 600 ft. and it utilized down-
hole motors. To date, medium radius technology remains the most common method to drill horizontal
wells. This drilling method provides various completion as well as artificial lift options. It is quite
common to see well lengths varying from 1000 ft. to 5000 ft, short radius technology has also evolved
over time and there has been significant cost reduction. This, however, remains a niche market mostly in
low productivity wells in the U.S. and parts of China. In the U.S., small independents with marginal wells
(production rate less than 10 BOPD) use low cost, short radius technology to enhance well production
(Ali, S., et al, 2002).

Benefits of horizontal wells

Horizontal well remediation systems are usually faster, cheaper, and more effective than the
baseline technology of vertical wells. They provide:
1. Higher rates and reserves as compared to vertical wells. These results in less finding cost and less
operating cost per barrel of oil produced. In the U.S., as shown in the example in this paper, in places where
vertical well operating costs are $7 to $9 per barrel of oil, the horizontal well operating costs are $3 to $4 per
barrel.
2. For many horizontal well projects, the finding (developing) cost, defined as well cost divided by
well reserves, is about $3 to $4/bbl. This is about 25% to 50% lower than the cost of buying proved
producing reserves (ODriscoll, K.P., et al, 2000)
3. To produce the same amount of oil, one needs fewer horizontal wells as compared to vertical
wells. This results in reduced need for surface pipelines, locations, etc.
4. Improved access to contaminants at sites with surface restrictions (e.g., buildings),
5. Improved hydraulic control along leading edge of contaminant plume,
6. Minimal surface disturbance because fewer wellheads may be required,
7. Ability to monitor beneath contaminant sources (e.g., tanks, pits, lagoons),
8. Increased surface-area contact with contaminants,
9. Reduced operating expenses because fewer wells may be required, and
10. Access to off-site contamination to be treated by on-site operations.
Horizontal environmental wells can be used for ground-water or soil-vapor removal for surface treatment;
in situ treatment of ground water and soil; hydraulic control of ground water; and monitoring of soil vapor or
ground water (e.g., beneath contaminant sources). Horizontal wells can be installed by directional drilling or by
trenching and backfilling (if specific site conditions allow it). Trenching and backfilling requires shallow depths
and continuous surface access. Directional drilling can be used to install impermeable or permeable barriers and
can be combined with fracturing technology in low permeability sediments. Horizontal drilling concerns include
reduced permeability of the geologic formation during well installation caused by compaction drilling tools or
due to introduced drilling fluids (same as for vertical drilling); and potential for drilling fluids to foul
uncontaminated areas, damage equipment, and interrupt utility services, or compromise soil stability beneath
pavement and structures. Costs for disposal of contaminated backfill and drilling fluids/cuttings must be assessed
during technology selection. For operators, experience in directional drilling is a must, and experience in drilling
water well and hazardous waste sites is preferred (Kara, D.T., et al, 2001).
Disadvantages of horizontal wells

1. High cost as compared to a vertical well. In the U.S., a new horizontal well drilled from the surface,
costs 1.5 to 2.5 times more than a vertical well. A re-entry horizontal well costs about 0.4 to 1.3 times a vertical
well cost.
2. Generally only one zone at a time can be produced using a horizontal well. If the reservoir has
multiple pay-zones, especially with large differences in vertical depth, or large differences in
permeability, it is not easy to drain all the layers using a single horizontal well.
3. The overall current commercial success rate of horizontal wells in the U.S. appears to be 65%.
(This success ratio improves as more horizontal wells are drilled in the given formation in a particular
area.) This means, initially it is probable that only 2 out of 3 drilled wells will be commercially
successful. This creates extra initial risk for the project (Tribe, I.R., et al, 2003).
4. Hole cleaning. As the drillstring lies on the low side of the hole, beds of cuttings build up around
the bottom of edge of the drillstring. These can be very hard to shift (fig.2).

Fig.2: Shows poor hole cleaning Fig.3: Shows friction force

5. Frictional forces.The power needed to turn the drillstring or to pull it out of the hole are higher
on horizontal well than on a normally deviated or vertical well (fig.3).
6. Accurate navigation in the reservoir. Navigation within the reservoir is relative the reservoir
characteristics and not computed according to inclination and azimuth only (fig.4).

REFERENCES

Ali, S., Dickerson, R.C., Brady, M.E., Panlan, M. and Foxenberg, W.E.:
Technology Advances Boost Horizontal Open-hole Gravel Packing, Oil and
Gas Journal, p. 51, July 8, 2002.
Guntis, Mortis, Complex Well Geometries Boost Orinoco Heavy Oil
Production Rates, Oil and Gas Journal, February 28, 2000.
Kara, D.T., Hearn, D.D., Grant, L.L. and Blount, C.G.: Dynamically
Overbalanced Coiled-tubing Drilling on the North Slope of Alaska, SPE Drilling
& Completion Journal, p. 91, June 2001.
ODriscoll, K.P., Amin, N.M., Tantawi, I.Y.: New Treatment for
Removal of Mud-Polymer Damage in Multi-lateral Wells Drilled Using Starch-
Based Fluids, SPE Drill & Completion Journal, p. 167, September 2000.
Tribe, I.R., Burns, L., Howell, P.D. and Dickson, R.: Precise Well
Placement with Rotory Steerable System and Logging While Drilling
Measurements, SPE Drilling & Completion Journal, p. 42, March, 2003.

Fig.4: Shows navigation in the

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 Summary
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Kazakh national technical university named after