The question of how to best complete a water well has perplexed well drillers since the first hand-dug wells were lined with bricks and stone. Well screen selection is nearly always a compromise, balancing the desire to optimize flow rate with the need to control sedimentation within the well. Horizontal remediation well designers must juggle these same variables, while also considering the difficulties of construction to a greater extent than they would those of a vertical well.
Well Performance: Key Factor In Well Design
Well performance is, not surprisingly, the key issue for designers. A successful well design (including the screen, filter pack, if any, and development methodology) should accomplish several tasks simultaneously:
Although materials and techniques have long been established to address these criteria in the construction of vertical wells, they do not always transfer easily to a horizontal configuration. Differences in construction between the two applications result in the need to consider additional factors in horizontal well design. The two most significant differences are the tensile and bending stresses felt by the casing during installation of the well assembly and the constraints of a horizontal configuration on well completion and development methods. Under some circumstances, compressive strength can be a factor as well, such as in single-ended well completions where the casing is pushed into the well bore. Resistance to crushing by overburden soils (ring stiffness) can also be important if poorly consolidated material is being penetrated or the installation is very shallow --particularly if the casing is longitudinally slotted.
Casing Durability: Tensile Strength, Stiffness And Abrasion
Horizontal well casings are subject to higher stresses and greater potential for damage during installation than vertical wells of similar dimensions. The pullback power of horizontal drill rigs, measured in thousands of pounds, can easily exceed the tensile strength of most plastic well casings. This is particularly true where stress is concentrated on one portion of the casing as it moves through a curved section of the well bore. In vertical wells, the tensile strength of the pipe is not generally a factor unless the well is very deep, because the casing must only withstand gravitational forces as it is lowered into the well. Drag along the well bore during horizontal well installation can also damage the casing surface, even if total failure doesn't occur. This may occur as abrasion or breakage within localized areas of the screen section.
Designing for maximum tensile strength is a relatively straightforward exercise, based on material type (e.g., steel is stronger than HDPE),wall thickness (e.g., Schedule 80 pipe is stronger than Schedule 40), and slot type and spacing. Beyond these fundamental choices, the designer must make sure that the screen's other strength parameters are adequate to survive installation in a horizontal well.
Screen designs using pipe-based synthetic materials, e.g., PVC pre-pack screens, must also be constructed and installed very carefully since stress can be concentrated at inherently weak threaded couplings. Pipe breakage in PVC screens has also occurred where baffle assembly screws have been placed too close to the slots during manufacture, causing failure through the slots.
Extremely stiff casing can be installed, but requires that the well bore be drilled with very straight sections and a very smooth and gradual transition from the surface launch angle to the horizontal screen section. Often this transition must be extended in order to active at a workable bend radius for the pipe. It may also require that the well bore be reamed to several times the pipe diameter. In the first instance, longer transitions require more setback and more riser pipe, resulting in higher costs and greater pressure or vacuum loss in sparging or soil vapor extraction systems. In the second case, an oversized well bore increases drilling costs and makes the completed well more difficult to develop.
Surface damage of the screen as a result of drag during installation is avoidable, but difficult to monitor. Damage can result in a breach in the screen, allowing formation materials to enter the well unrestricted, which can cause diminished permeability.
Brittle well screen materials, such as PVC, can have sections of slots broken out relatively easily during installation, without causing complete parting of the screen. This damage may not be noticed until the well sands in or a pump is damaged at a later time. Geotextile filters, wrapped around the pipe and factory banded or fastened with steel banding in the field, can create an effective filter but can tear or be dislodged if snagged during installation, exposing the slotted pipe beneath. These problems have been addressed with the recent introduction of several products to the horizontal well industry, as well as through special installation techniques. Both of these developments are explored in more detail below.
Filter Packs: Natural, Field-Constructed And Integrated
In an ideal well design, vertical or horizontal, there is a well screen situated within a well bore of somewhat larger diameter, with a graded filter bridging the gap to progressively block fines from reaching the screen as fluid travels from formation to well. Figure 2 shows a schematic view of this construction. In a vertical well, this filter is commonly composed of loose sand. The screen slot size and sand gradation are selected to provide filtration on the basis of the grain size distribution of the surrounding soil.
Horizontal wells present different challenges:
1. Unless centralizers are used, the screen will certainly rest on the well bore invert. This thwarts any attempt to create a sand pack completely surrounding the casing.
2. If centralizers are used, they must be sturdy enough to support the weight of the well screen in a horizontal position; most off-the-shelf centralizers are not.
3. Depending on the stiffness of the screen and the straightness of the well bore, centralizers must be closely spaced to adequately support the screen. However, attaching several centralizers to the well screen will cause significant drag during the casing installation.
4. Unlike the placement of sand in a vertical well, the sand pack in a horizontal well must be placed along the screened interval using a sand injection device or as a slurry, using a tremie pipe, since gravity alone won't move the sand laterally through the well bore. With centralizers attached to lift the casing, pushing injection tubes or tremies into the well bore annulus is a high-risk, difficult undertaking.
The following subsections describe some of the considerations and available products for installing horizontal wells with a natural filter pack, field-constructed filters, and manufactured, integrated filter systems.
Since a sand filter pack like that used in vertical wells is impractical to construct in horizontal wells, designers must look at other options. The simplest is no filter at all, a "natural pack" completion.
In practical terms, this means that soil vapor installations are unlikely to suffer from sedimentation within the screen unless the soil is dry, noncohesive, fine-grained, and the entry velocity of the soil vapor is high. Occasionally, pulses of water (e.g., precipitation, irrigation water) that move downward through the vadose zone toward the water table may also mobilize sediments into the screen.
Groundwater extraction wells are more prone to sedimentation than soil vapor wells. Slot or perforation sizes and grain-size distributions must be considered carefully before deciding to construct these wells without some type of filtration. Most groundwater extraction wells benefit from some form of filtration.
Injection wells must be considered on a case-by-case basis. Bioventing installations within the vadose zone rarely require a filter. But air sparge or steam injection wells, unless oper ated continuously, can become silted or sanded in if the injection operations are cycled or temporarily suspended. For them, filter systems help ensure continued operation.
Field Constructed Filters
One method of constructing a functional filter system is to wrap geotextile, usually of the non woven around the outside of the well screen. Although this method which is sometimes called a "filter wrap" provides an added measure of filtration, factory con struction is expensive, and field construction is both expensive and time-consuming. Geotextile is robust, resisting tearing during installation, but it does increase the skin friction and drag of the screened interval during installation. If snags occur, the casing may break or the fabric can be dislodged, leaving gaps through which soil can enter the screen.
Pre-packed well screens are manufactured by several companies and are based on a variety of pipe materials, including PVC, HDPE and stainless steel. Basically a completed-wellon-a-pipe, pre-pack screens may be custom built in nearly any configuration to meet project needs.
This versatility does not come without a cost. Pre-pack screens are expensive per linear foot and are more difficult to install than other solutions. Installation difficulties result primarily from the weight and stiffness of a pre-pack casing section. Unless a pipe-based design is used, as mentioned above, even steel pre-pack may not survive the rigors of installation in a horizontal well.
Innovations in Horizontal Screen Design
Some well screen materials have been developed recently to address many of the strength and performance issues mentioned above. For lack of an industry standard term, they are referred to here generically as "filter screens" because they incorporate a filtration system designed to eliminate the need for an external sand pack. A summary of their strengths and shortfalls is presented here and on Table 1.
EnviroFlex (Manufacturer: Titan Industries, Inc.)
The outer casing is available in a variety of material types (steel, PVC, or HDPE) to permit matching the screen type to installation and chemical resistance requirements. The Enviroflex screen is also economical about midway in price between standard slotted screen and pre-pack screen for a given diameter.
HydroQuest (Manufacturer: TerraFilter)
During construction, this material is susceptible to damage by tearing, bunching or other failure of the external membrane. Direct installation in a well bore is difficult because of the poor bonding of the filter to the base pipe. Therefore, installation using a carrier casing (described below) should be considered.
Schumasol (Manufacturer: Schumacher)
A filter screen constructed of sintered polyethylene resin beads, Schumasol was developed to maximize the open area of the screen while maintaining filtration properties that meet or exceed pre-pack performance. It achieves excellent filtration characteristics, but the porous construction of the screen limits its tensile strength. Installation using a carrier casing is specified by the manufacturer. Schumasol has been successfully installed on several major remediation projects in the United States.
Stratapac (Manufacturer: Pall Well Technology)
Originally designed for the petroleum-drilling industry, the Stratapac filter screens are another composite, with several layers of stainless steel mesh coated with metallic particles bonded between inner and outer perforated metal casings. Test results reported by Pall indicate good hydraulic performance, and the material is available in sizes practical for horizontal installations. The Stratapac screen, with its all-steel construction, is reportedly resistant to damage during installation and does not require specialized installation techniques. Its drawbacks include relatively heavy weight and stiffness, coupled with high cost.
Table 1: Comparison of available integrated filter screens
Construction And Development Techniques
Using a carrier casing is a proven technique for installing more fragile well screens. With this technique, the drilling operations proceed as usual, with the advancement of a pilot bore, followed by reaming. Rather than installing the well screen directly after reaming, a solid casing with an inside diameter larger than the outside diameter of the well screen is installed. The well screen is placed within this carrier casing (generally pulled in a single operation along with the carrier, but sometimes it is inserted after the carrier is in place). The carrier is then pulled out, leaving the well screen in place. Often the carrier casing is flooded with a development solution prior to removal in order to facilitate the breakdown of fluids (mud) used in the drilling process. This method reduces the potential of breakage or surface damage during the installation of screens such as Schumasol or HydroQuest.
The disadvantages of using a carrier casing include: the added expense of procuring the extra casing and drilling a larger diameter borehole, the extra labor involved, and the larger diameter well bore that must be developed. After use, the carrier casing is usually discarded since it has come in contact with contaminated soils, although it could be cut into smaller sections and cleaned. These costs may be offset or absorbed on larger installations where the drilling fluids are recirculated and the carrier may be reused.
Direct-pull techniques are facilitated by strong, slippery, smooth-sided well screens that emulate the installation characteristics of utility conduits. By keeping friction and potential obstructions to a minimum, the probability of a successful installation is increased. This means that the attachment of well components to the exterior of the well screen or casing should be avoided, except for those that are absolutely necessary.
With the increasing popularity of horizontal wells as a delivery or extraction system for remediation, continued innovation is certain to reduce costs and improve performance. The introduction of new, second-generation well screens to the market is one expected development.
Currently, stainless steel is frequently specified for horizontal well installations. This is often a needless cost for two reasons. First, horizontal wells are rarely used as monitoring wells on sites that are already perforated with vertical well networks. Yet, stainless steel is often specified in horizontal wells largely because the monitoring wells were specified as such to prevent sampling errors (false positives). Although one does not want to introduce new contaminants through installation of a remediation system, the chance of this occurring by using a more economical horizontal well screen, such as High Density Polyethylene (HDPE) is relatively low. Since the horizontal wells are not sampled, chances of sample contamination are slight.
Secondly, the efficiency of horizontal remediation systems in removing contaminants is being documented with increasing frequency. Systems often are operated only a few months to achieve target cleanup goals and rarely run for more than three years. Within this short time frame, plastic casings, particularly HDPE, offer a better combination of strength, chemical resistance, and economy than do stainless steel casings. New well screen developments will focus on retaining strength and flexibility for installation while providing filtration properties unavailable in a natural-pack well.Better development techniques are likely to spark further innovation. For instance, many current specifications for jetting require the labor-intensive process of running hundreds of feet of PVC pipe into a well and moving the pipe back and forth by hand over specified intervals of distance and time. Equipment manufacturers will eventually develop tools to mechanize this process and save the arms of countless drill helpers. Continued improvements in drilling fluid formulations will also aid in the development process, keeping the well bore stable during screen installation but facilitating development afterwards.