Planning a Horizontal Well

A Horizontal Well used for contaminated site cleanup, also known as a Horizontal Remediation Well, can be a powerful tool to achieve site closure. The following article discusses some of the most important issues, concerns and site characteristics that require consideration during the horizontal well placement and horizontal well screen design processes to provide an effective horizontal remediation well system.

Key factors in planning a Horizontal Remediation Well are:

1. Hydrogeologic characteristics that limit the effectiveness of the wells
2. Subsurface obstacles that restrict well placement
3. Site logistical factors that limit the location of directional drilling equipment and constrain drilling operations
4. Remediation system equipment that puts boundaries on the range of operational parameters of the horizontal well system
5. Remedial objectives and horizontal well success criteria that impose minimum requirements on the placement and operation of the horizontal well system
6. Cost and other resource restrictions on capital outlays and operation and maintenance of the project

Geotechnical and Hydrogeologic Characteristics

Horizontal remediation wells either deliver fluids (liquid or gas) into porous soil or rock
formations or extract fluids from the formations. Hydrogeologic layering, groundwater head distribution, and hydraulic or pneumatic conductivity of target formations dictate the maximum spacing and depth horizon of horizontal well screens.

Geotechnical data useful when planning a horizontal well project includes grain-size
distribution and blow counts from vertical drilling operations. Grain-size distribution dictates screen slot size and helps predict the amount of time required to achieve complete development of the horizontal well after its operation has begun. Blow counts predict the response of the soil or rock formation to the directional drill bit, and help in the selection of appropriate drilling equipment.

Hydrogeologic data comes in various forms, but boring logs, cross sections and potentiometric surface or water table maps are the most common ways of conveying the information needed to plan a horizontal remediation well. Boring logs are essential for: 1) understanding the lithologic characteristics the directional drill bit will encounter; and 2) predicting what drilling fluid characteristics will be needed to maintain a stable wellbore during the directional drilling. The best way to optimize well screen depths and locations is using hydrogeologic cross sections, since target depth is often defined by lithologic contacts that change in elevation across a site. Maps of groundwater head elevations are needed to avoid injection or extraction with insufficient or excessive hydraulic head. In addition, awareness of surface water locations will help prevent short-circuiting, channeling or hydraulic head control that may impede drilling fluid circulation during directional drilling and the efficient delivery or recovery of fluids during horizontal well remediation system operation.

Subsurface Obstacles

Horizontal well paths must avoid utility lines, building foundations and vertical wells. Horizontal well riser sections sometimes pass within feet of water and sewer lines, which can act as preferential pathways for fluids. In addition, if the backfill of such utility lines is more conductive than the native formation, as it is with many old utility installations, it can interfere with the drilling process by diverting drilling fluids away from the wellbore. Electric and communication utility lines can limit the accuracy of electronic surveying systems used to ensure that the horizontal well is completed in the predetermined location and depth. Reinforced concrete may also interfere with the accuracy of walkover locating sonde instruments used to determine wellbore locations and depths. Often, numerous vertical groundwater monitoring and/or remediation wells are installed at a site prior to horizontal well installation. These, too, can act as drilling fluid preferential pathways and complicate the drilling process, sometimes necessitating rerouting of the horizontal well path.

All three types of subsurface obstacles—utility lines, building foundations, and vertical wells can potentially create problems during horizontal well operation by channeling fluids that are being injected or extracted. This can impede fluid delivery or hamper the development of a consistent zone of influence around the horizontal well. This highlights why it is important to correctly identify subsurface obstacle locations and design the wellbore to minimize their potential influence.

Site Logistics

Surface features such as buildings, roads, property boundaries and overhead utility lines limit the possible locations of horizontal wellheads. This is in addition to the required minimum distance between a wellhead and the horizontal screen section. Wellheads commonly must be accessible to equipment used in possible rehabilitation efforts later in the life of the wells. Underground vaults or posts must enable vacuum trucks and tanks to be placed within workable distances from a wellhead. Vault covers must have the requisite vehicular weight rating to accommodate site activities. Overhead utility lines can restrict high-mast vertical drilling rig locations, thereby limiting the possible locations of completed wellheads. This is generally much less of a concern for directional drilling rigs due to the low-profile machinery design.

Normal business operations should be minimally affected by a directional drilling operation. The ability to continue conducting business at a site during drilling is one of the many advantages of installing horizontal wells. The drilling plan should account for site traffic patterns and work schedules to ensure that the drilling operation does not interrupt business operations at the site.

Operational Parameters

The surficial equipment used to inject or extract fluid -pumps, blowers, or compressors-impose boundaries on the operational parameters of the delivery or recovery system. Any remediation system, be it a groundwater, non-aqueous phase or multiple-phase extraction system, chemical oxidant or bioamendment delivery system, an air sparge or biosparge
system, or a soil vapor extraction system, will operate within a range of pressures and flow rates. Pressures or vacuum in various systems can range from a few pounds per square inch (psi) of a few inches of water or mercury, to tens of psi or several feet of water or mercury. Flow rates can range from tenths of a standard cubic foot per minute (scfm) per foot of screen, to tens of scfm. The horizontal well screen design must accommodate the range of pressures and flow rates in a way that provides even flow distribution along the entire well screen. By continuously and consistently delivering or recovering fluids along the entire well screen, within the remediation system’s operational parameters, a horizontal well is able to create a large and effective zone of influence in the target formation. By contrast, vertical wells have very limited zones of influence in the target formation.

Success Criteria

Horizontal wells can be completed using various well materials and a wide range of well diameters. The choice from among available well materials—high density polyethylene (HDPE), polyvinyl chloride (PVC), fiberglass reinforced epoxy (FRE), stainless steel, carbon steel, or black-iron pipe—depends on chemical compatibility with subsurface contaminants or the fluid to be injected, tensile and hoop stresses imposed during the well installation process and the anticipated horizontal well system lifecycle. Common horizontal well diameters range from one or two inches to eight inches or more, depending on the: 1) flow rate and physical properties of the fluid to be injected or extracted; and 2) pressure or vacuum where the injection or extraction will occur. The operational parameters can reflect an aggressive, fast-paced delivery or recovery system that is expected to reach remedial goals over a short period measured in months, or a gradual, measured system that will operate for a long period, measured in years.

Resource Restrictions

Resource restrictions may limit horizontal well specifications as much as success criteria can. Cost considerations occasionally result in specification of FRE pipe instead of stainless steel. While six- or eight-inch well diameter allows the flexibility to insert large pumps or perform extensive well rehabilitation work in the future, well diameter larger than four inches is rarely needed to prevent unacceptable head losses, greater than 10 percent. Cost limitations often constrain design goals to mechanical performance goals defined by flow rate and head loss, rather than expanding the potential functionality of the well. Some large projects are divided into a limited, initial pilot scale system, which is usually operated from 6 months to a year in order to optimize full-scale remedial system requirements and identify potential cost-saving modifications.

Directional Technologies, Inc. is the nation’s expert horizontal remediation well drilling contractor. We design and install horizontal remediation wells. Directional Technologies’ experience in the oil field, environmental remediation and utility industries allows us to complete projects that were previously considered unfeasible. Our company personnel have been successfully directional drilling horizontal wells since 1984!