Reconsidering horizontal to vertical well ratios for site clean-up

Directional drilling has been used for a variety of purposes; utilities, dewatering, and remedial activities. Cutting costs for remedial activities it is always of consideration. What the costs are to clean-up a site via traditional vertical extraction wells versus using a horizontal well system needs to be reconsidered based upon todays remedial challenges. Traditionally, it has been stated that a single horizontal well is worth 4-11 vertical wells. This seems arbitrary in that both depth of the system and hydrogeological conditions play a large role that does not seem to be considered. A better way to evaluate the cost benefit of one system over another is to base it on the number of vertical wells per linear foot of directional well and base it more on zone of influence of both designs.

BACKGROUND AND HISTORY

Horizontal or directional drilling (HDD) can be defined as any drilling activity that deviates from the vertical. Typically, this deviation is greater than 80 degrees from the vertical.

Directional or horizontal drilling is not new. The technique has been around for over 2,000 years. Some of the earliest examples are from Iran and north Africa (H. Mukherjee and M. Economides, 1991). The technique has been used throughout Europe for tunnels, water supply and drainage. The oil industry began using directional drilling in the early 1940’s. However, it was not until the 1980’s that horizontal drilling began to take off in North America for both the oil and environmental industry (Wilson and Kaback, 1993).

Improvements in locating and directing the drilling bit in loose unconsolidated soils improved greatly in the 1990’s. It is then that the utility companies began to utilize the technique almost exclusively. This also coincided with the onset of fiber-optic cable and two new rig manufacturing companies; Vermeer and Ditch Witch (Farr, 2012). As the technology became more accepted in the 1990’s for the environmental industry, more effort was put into understanding its potential in the evolution of site monitoring and clean-up.

There are several types of boreholes associated with HDD. Continuous boreholes, which have a starting point on the surface and end by returning to the surface. Whereas, blind boreholes enter from the surface but end in the subsurface. There are several different orientations associated with blind boreholes, these are dual, stacked, trilateral or herringbone configurations.

Within the realm of the environmental and industrial industries (mining, river crossing, utilities, etc.), HDD has been used for monitoring, extraction and injection. Wastewater has been disposed of via horizontal wells and water supply and dewatering have been utilized. Using horizontal wells for monitoring is relatively infrequent, but has been used for water and vapor pressures, water and vapor quality within the subsurface.

The use of HDD for the purpose of remediation or site clean-up has been used at many sites throughout the US and overseas (Concurrent Technologies Corp, 2002). HDD has been used in pump and treat, bio-venting and vapor extraction systems. They have been used to deliver chemicals to the subsurface both in the vadose zone and beneath the water table or used to simply air sparge a shallow groundwater system to aid in the removal of hydrocarbons. HDD wells also work well as barriers or hydraulic control systems.