The Potential For Horizontal Wells For Petroleum Production
Authors: Butler Roger M. 
Published: March 2010

Horizontal wells drilled as deviated wells from the surface can improve the efficiency and economy of oil recovery operations. Rapid development in this area is being spurred by improved drilling technology which allows more favourable economics. The improvement offered by horizontal wells results from their more extensive contact with the reservoir. This can provide higher production rates or it can allow lower fluid velocities at the well bore while still providing total flows which are economic. The advantage over conventional wells can be exploited in several ways. Specific applications where horizontal wells can have advantages include:Reservoirs where conventional wells have low productivity.Reservoirs with vertical fractures.Oil reservoirs where recovery is limited by water or gas coning.Thick continuous heavy oil and bitumen containing sands where steam-assisted gravity drainage is practical.

Horizontal wells are usually unsuitable for thick reservoirs having high horizontal permeabilities but low vertical permeability. This paper reviews the drilling and utilization of horizontal wells for the recovery of conventional crude oils and gas and also for the recovery of bitumen and heavy oils.

Introduction

The rate of production of oil or other fluids from a reservoir is controlled by pressure gradients. With conventional wells it is broadly recognized that most of the pressure decline occurs close to the well because of the convergent flow.

In order to achieve greater production rates, it is necessary to decrease the resistance to flow within the reservoir, particularly in the near well-bore region. Much of petroleum engineering is concerned with means for accomplishing this.

The commonest approach is to use multiple wells and to locate these throughout the producing reservoir. This approach reduces the near well-bore resistance by increasing the total availability of contact area between the reservoir and the producing wellbores. Also, by distributing the wells over the reservoir area, the average distance that fluids have to flow before production is smaller and this too reduces the over-all resistance to flow.

The over-all resistance to fluid flow can also be decreased by employing means for reducing the flow resistance around individual wells; practical techniques include well stimulation using, for example, acids to enhance the near well-bore permeability or fracture treatments for the same purpose.

The construction of wells which penetrate the reservoir in a horizontal rather than a vertical or near-vertical direction provides an alternative means for improving contact with the reservoir. Such wells are usually drilled from the surface by means of deviated drilling techniques, although there are alternatives such as the construction of wells from underground mine workings(1). Well stimulation, fracturing, in-fill drilling and horizontal wells are, in many respects, alternate methods of achieving essentially the same objective.

The decreased resistance to flow around an extended horizontal well as compared to that around a vertical well means thatewer wells are necessary to achieve the same reservoir drainage. The improved contact with the reservoir achieved by means of a horizontal well has a dual nature-the near well-bore flow resistance is reduced because of the extended length and resulting lower velocity but also, particularly with very long wells, the length carries the well through a much larger area of the reservoir.