Fundamentación Conceptual

According to the Energy Information Administration (EIA) Office of Oil and Gas, US De-partment of Energy, a “unified” definition of horizontal drilling does not exist. Based on dif-ferent sources, the Department of Energy, defines horizontal drilling as “the process of drilling and completing, for production, a well that begins as a vertical or inclined linear bore which

extends from the surface to a subsurface location just above the target oil or gas reservoir called the kickoff point, then bears off on an arc to intersect the reservoir at the entry point, and, thereafter, continues at a near-horizontal attitude tangent to the arc, to substantially or en-tirely remain within the reservoir until the desired bottom hole location is reached.” Shelkhole-siami et al., (1991) define a horizontal well as the result of a drilling and completion technique in which the wellbore remains in a high-angle trajectory roughly parallel to the formation, thereby exposing significantly more pay to production than would be exposed by a vertical wellbore.

Horizontal wells came to play an important role in enhancing the productivity of the wells in the reservoir and subsequently the recovery factor. Usually, horizontal wells will reach ar-eas not contacted by verticals and will solve problems associated with thin zones, fractured reservoirs, water and gas coning, gas reservoirs, waterflooding, heavy oil production, thermal processes and CO2 flooding.

4.4.1 History of Development

Horizontal drilling activities have been documented as early as 1927; however, the first recorded true horizontal oil well was completed in 1929, near Texon, Texas. Later, a horizon-tal well of 500 ft was drilled in the heavy oil field of Franklin, Pennsylvania. After World War II, horizontal drilling benefited from jet perforating, casing the drilled hole, and the perfora-tion or targeted intervals(EIA, 1993).

By the early 1980s with oil prices around $35 and improvements in downhole drilling mo-tors and downhole telemetry equipment, horizontal drilling was commercially viable. Litera-ture review suggests (EIA, 1993; Joshi, 2003) three different horizontal drilling stages de-pending on both technology and prices: early 1980s, late 1980s-early 1990s, late 1990s-today.

During the early 1980s, the development stage of horizontal drilling, many test wells were drilled in Europe and the US. In Europe, Elf Aquitaine tested the technique to produce heavy

oil from a carbonate reservoir in the Rospo Mare oilfield, located offshore Italy in the Medite-rranean Sea. Also, Elf drilled other wells in the Lacq Superieur and Castera Lou oil fields in southwestern France. Experiments in the US at the same time were carried out to reduce gas coning in the Abo Reef in New Mexico; to intersect fractures in carbonate reservoirs in Okla-homa, Kansas, and Texas; and to minimize water and gas coning into the Sadlerochit reservoir in Alaska’s Prudhoe Bay field.

In the late 1980s-early 1990s, the growth of horizontal drilling or the acceptance of the technique in the industry was marked by important drilling campaigns worldwide, with North America having the greatest number of drilled wells. Many efforts to reduce costs used me-dium radius technology. In 1990, worldwide, more than 1,000 horizontal wells were drilled and more than 80% of them targeted the Upper Cretaceous Austin Chalk formation in Texas (EIA, 1993). Noticeable impact on the production of crude oil in certain regions was reported, and in the mid-1990s, crude oil production from horizontal wells in Texas had reached more than 70,000 BOPD.

From the late 1990s to today, many new technologies have been developed to improve horizontal drilling practices. Cost reductions, re-entry wells, coiled tubing drilling, improve-ments in drilling monitoring, logging while drilling (LWD), measurement while drilling (MWD), and geo-steering to drill straight horizontal holes, as well as formation damage reduc-tion and under-balanced drilling are examples of recent stages of horizontal drilling improve-ments.

4.4.2 Well Configurations

The radius of the arc described by the wellbore as it passes from the vertical to the horizon-tal defines the horizonhorizon-tal well classification. In all cases, the classification will be related to both the technology involved and the applicationor the purpose of the well. Some authors (EIA, 1993; Fritz et al., 1991) consider four horizontal methods: short radius, ultrashort radius, medium radius, and long radius. Joshi (2003) added one more configuration, intermediate

short radius, based on the arc of curvature. Table 4.1, after Joshi (2003), constitutes a sum-mary of these five different configurations. In general, the required horizontal displacement, length of the horizontal section, position of the kickoff point, and completion limitations are considered when selecting a radius of curvature.

TABLE 4.1—HORIZONTAL DRILLING TECHNIQUES (after Joshi, 2003)

Type

Radius of curvature,

R (ft)

Build rate

(°/foot drilled) Length (ft) Applications

Ultra-short

radius (a) 1-2 ft 45°-60°/ft

drilled 100-200 ft Commonly used when re-entering exist-ing vertical well (Sidetrack).

Short

radius (b) 20-70 ft 150°-350°/100

ft drilled 100-800 ft

Commonly used when re-entering exist-ing vertical well (Sidetrack). Favorable in small lease blocks.

Intermediate short radius (c)

120-150 ft 1,000 ft Commonly used when re-entering exist-ing vertical well (Sidetrack).

Medium

radius (d) 300-800 ft 6°-30°/100 ft drilled

1,000- 4,000 ft

Favorable for more complex completion methods in leases as small as 20 acres.

Used when re-entering existing vertical well (Sidetrack).

Favorable for leases of more than 160 acres. Usually new well.

4.4.3 Completion Techniques

The appropriate completion scheme will be controlled by taking into account the existing conditions from the drilling to the abandonment of the well to achieve borehole stability and sand control. Joshi (2003) defined four completion schemes for horizontal wells to illustrate those conditions: (1) openhole wells, (2) slotted liner completions, (3) liners with partial iso-lations, and (4) cased-hole cemented completions.

For openhole wells, the formation type represents the major limitation, and stimulation process become very difficult to perform if wells are unstable. In the case of slotted liner com-pletions, the purpose is to control the hole collapse and at the same time insert different tools such as coiled tubing (Fritz et al., 1991). Three types of liners have been used: perforated lin-ers (holes drilled in the liner), slotted linlin-ers (slots of various width and depth are milled along the liner), and prepacked liners. The gravel pack is an option to help to control sand production using slotted liners. The option of liner with partial isolations allows certain types of isolation for stimulation or production control along the well. Finally, cased-hole completions allow cementing and perforation of the liner. Cased-hole completion will be very useful to stimulate wells, such as medium and long radius wells, that have been exposed to drilling fluids for long periods of time and wells that have been drilled in tight formations or low permeability forma-tions.

Short radius wells are limited to openhole or slotted liners. Although in the past the slotted liner completion scheme was a problem to stimulate a well, technological advances such as liquid fracs (acid or water fracs) have become a solution. In fact, today most of the wells in the US are liquid frac. On the other hand, medium to long radius wells have more flexibility since they can support all possible completion types.