Horizontal drilling is the process of drilling a well from the surface to a subsurface location just above the target oil or gas reservoir called the “kickoff point”, then deviating the well bore from the vertical plane around a curve to intersect the reservoir at the “entry point” with a near-horizontal inclination, and remaining within the reservoir until the desired bottom hole location is reached.
Most oil and gas reservoirs are much more extensive in their horizontal dimensions than in their vertical (thickness) dimension. By drilling a well which intersects such a reservoir parallel to its plane of more extensive dimension, horizontal drilling exposes significantly more reservoir rock to the well bore than would be the case with a conventional vertical well penetrating the reservoir perpendicular to its plane of more extensive dimension.
The achievement of desired technical objectives via horizontal drilling comes at a price. A horizontal well can cost up to 300 percent more to drill and complete for production than a vertical well directed to the same target horizon. Due to its higher cost, horizontal drilling is currently restricted to situations where vertical wells would not be as financially successful. In an oil reservoir which has good matrix permeability in all directions, no gas cap and no water drive, drilling of horizontal wells would likely be financial folly, since a vertical well program could achieve a similar recovery of oil at lower cost. But when low matrix permeability exists in the reservoir rock (especially in the horizontal plane), or when coning of gas or water can be expected to interfere with full recovery, horizontal drilling becomes a financially viable or even preferred option producing 2.5 to 7 times the rate and reserves of vertical wells. The higher producing rate translates financially to a higher rate of return on investment for the horizontal project than would be achieved by a vertical project.