Vertical vs. Horizontal Wells

Wellbore Geometry as a Constraint on Flow Capacity: Conventional vs Unconventional Reservoirs

Traditional conventional wells are drilled vertically, contacting only a limited thickness of the reservoir, typically tens to hundreds of feet. As a result, the area through which hydrocarbons can flow into the well is relatively small. However, in high permeability reservoirs, this is not an obstacle to economic field development. On the other hand, if a vertical well is uneconomic because of limited permeability, an operator might stimulate the well, and hydraulic fracturing is the ultimate stimulation method. The primary benefits of fracturing a vertical well are two-fold: the flow geometry to the wellbore is altered from radial flow to more effieicnt linear flow, and the surface area of the wellbore is significantly increased.

• With regard to flow geometry, in radial flow with a vertical wellbore, the flowlines around the wellbore are like spokes on a wheel, and hydrocarbon molecules are crowded and compressed as they converge on the wellbore, causing increased pressure drop that diminishes flow. With a hydraulic fracture as a part of the well, the flowlines are transformed into parallel lines perpendicular to the fracture plane (there is no more convergence of flow), and the hydrocarbon molecules are able to approach the fracture without having to compete for space with their neighbors, enhancing flow compared to the radial case.
• When examining contact area to flow, a fracture significantly enhances the surface area of the wellbore. For a ½ ft diameter (D) vertical well fully penetrating a 250 ft thick reservoir (H), the wellbore surface area would be 1,600 ft² (A=pi*D*H). A typical hydraulic fracture height might be the thickness of the reservoir (250 ft in this case) and up to 2,000 ft in total length (L). The fracture has 2 sides that take in flow, so the total surface area is on the order of a million square ft (A=2*H*L). This kind of stimulation can extend economic hydrocarbon development using vertical wells into permeabilities on the order of the tenths of millidarcies or smaller for oil wells.

In unconventional reservoirs, horizontal wells are standard, affording much more contact length with the reservoir than vertical wells, on the order of miles (Figure 1.3.1). Although a horizontal wellbore has much more contact area with the formation than a vertical wellbore, miles instead of 100’s of feet, a horizontal wellbore’s stimulation effect is not competitive with even just one fracture from a vertical wellbore. The major breakthrough for unconventionals was pairing horizontal wells with hydraulic fracturing, first proven during the development of the Barnett Shale in Texas. The horizontal wellbore provides the initiation points for many hydraulic fractures all attached to a single flow conduit that only costs slightly more to drill than a vertical well.

The Power of Using Horizontal Wells with Multiple Hydraulic Fractures

A quick surface area calculation demonstrates the power of this wellbore/fracture geometry. As mentioned earlier, a typical hydraulic fracture alone might have a million square feet of surface area. Along a mile-long horizontal well, the fracture spacing might be 50 ft, meaning the well could have 100 fractures or more, resulting in a total surface area of 100 million square feet. This provides as much as 10,000 times more surface area to flow than an unfractured vertical well, or 100 times more than a fractured vertical well. Production data from the Barnett Shale (available online from the Texas Railroad Commission, www.trrc.gov) shows the impact of these numbers on real production results. A multi-frac horizontal well drilled in the Barnet in 2011 (Figure 1.3.3, Brown 2H well) produced 2 Bcf in only 6 months, whereas a single frac vertical well from 1996 (Rudd, WL #8-A) took 170 months to produce the same amount. These kind of productivity enhancements, enabled by multi-frac horizontal well technology, are what created an oil and gas boom that propelled the United States to production levels unrivalled even by the most prolific Middle East oil producing countries.