Seeking higher productivity, prolongation of a reservoir’s commercial life, reduction of drilling costs… etc. have always symbolized the ideal targets for any oil or natural gas operator trying to achieve. Horizontal drilling has become one of the most valuable technologies ever introduced into the business, which leads to the attainment of these desired targets.
Compared to vertical wells, the horizontal ones enjoy a higher productivity and pay contact per well, thereby reducing the number of wells required to drain the reservoir. Also, they allow operators to take advantage of highly heterogeneous or layered reservoirs, reservoirs with fractures, or water or gas coning problems.
Focusing first on its objectives, the horizontal drilling aims at significantly exposing more reservoir rock to the well bore surface than can be achieved via drilling of a conventional vertical well. As for its technical objectives, they include the need to intersect multiple fracture systems within a reservoir and the need to avoid unnecessarily premature water or gas intrusion that would interfere with oil production. Economically, they increase productivity and contribute to the prolongation of the reservoir’s commercial life.
Unlike a directional well that is drilled to position a reservoir entry point, a horizontal well is commonly defined as any well in which the lower part of the well bore parallels the oil zone, said eng. El-Sayed Orabi, the Egyptian General Petroleum Corporation (EGPC). The angle of inclination used to drill the well does not have to reach 90° for the well to be considered a horizontal well. Applications for horizontal wells include the exploitation of thin oil-rim reservoirs, avoidance of drawdown-related problems such as water/gas coning, and extension of wells by means of multiple drain holes, further added Orabi.
Horizontal wells are characterized by being a controlled, directional completion technique for exploiting unrecovered mobile hydrocarbons in existing fields. These reserves usually remain since the reservoir’s heterogeneity has prevented efficient development using vertical wells. The primary screening tool is recovery efficiency as measured by the percentage recovered of original oil in-place (OOIP) or original gas-in-place (OGIP). Screening is carried out by reservoir characterization, followed by reservoir simulation.
Scenarios of horizontal usage
Horizontal drilling can be used in many situations where conventional drilling is either impossible or cost prohibitive. This type of drilling can be used to efficiently extract soil vapor, to identify the causes of decreased well performance, to place leak detection sensors beneath solid or hazardous waste landfills, to install gas collection systems at landfills or similar waste dumps, to stabilizing hillsides for mine waste dumps or other unstable granular soil masses, to convey fluids between vertical wells and treatment facilities…etc.
The Pros and Cons
Eng. Ahmed Hassan, Chief Reservoir Engineer, Pico International Petroleum highlighted the various privileges of drilling horizontally; among which the higher productivities. Relative to unstimulated vertical wells, the horizontal wells generally have higher productivities. Rule of thumb, the productivity improvement by horizontal wells over vertical wells in the same reservoirs is in the range of 1-10 with mean value of 2.0. Moreover, the horizontal wells have wider drainage areas and consequently higher recovery factors. Hassan added that horizontal wells are to outperform hydraulic fractured wells in certain conditions, which are the following:
– The reservoir section is thin
– The reservoir has high vertical permeability
– The completed lateral length is much greater than the effective fracture half length
– The hydraulic fracture has low conductivity
As a matter of fact, the horizontal wells have been considered by many experts as a preferred method of recovering oil and gas from reservoirs in which these fluids occupy strata that are horizontal, or nearly so, since they offer greater contact area with the productive layer than vertical wells.
Economically, the cost factor for a horizontal well may be as much as two or three times that of a vertical well, yet the production factor can be enhanced as much as 15 or 20 times, making it very attractive.
According to studies conducted by the U.S Department of Energy, this type of drilling can result in an increase in reserves in place by nearly 2% of the original oil in place. The production ratio for horizontal wells versus vertical wells is 3.2 to 1, while the cost ratio of horizontal versus vertical wells is only 2 to 1. These findings reflect the effectiveness of horizontal drilling.
Listing the privileges should not ignore the obstacles facing this technique. The horizontal drilling is associated with higher drilling costs and greater mechanical risks. For instance, the economic success rate in the US is about 60%, with failure usually occurring as a result of high drilling/completion costs, formation damage, reservoir heterogeneity, or insufficient geological characterization.
Hassan stated liquid unloading problems along the lateral section, water blockage in the heel of the well and uneven production distribution along the lateral section represent the major disadvantages of the horizontal drilling.
Factors of Success
A drilling process can take few months, while to set its program, it may require more than a year. Thus, despite the time it takes, a horizontal drilling program should first take into consideration the following factors, as Hassan mentioned, to ensure a successful drilling implementation, which are:
Accurate depth conversion of 3D seismic data using well control
Identification and correlation of major geologic markers throughout the field
Knowledge of expected net pay and its areal distribution from an updated depositional facies model
Real time adjustments of well paths using LWD resistivity and G.R measurements compared to the geological model
The Egyptian Implementation of Horizontal Drilling
Fortunately, Egypt has reserved its place as one of the countries implementing this advanced technique. Currently, there are approximately 20 horizontal wells; all are 90-degree direction in addition to more than 90-extended reach wells; less than 90-degree direction. Most of these wells are mainly drilled in the areas of Gulf of Suez fields and Eastern Desert, while two or three lie in the Western Desert, highlighted Orabi.
Despite the attempts to increase the implementation of horizontal drilling in Egypt, there are some obstacles hindering its expansion, among which are high costs, lack of Experience in such technique and operation problems, stated Orabi.
By Yomna Bassiouni