Abstract
Horizontal wells and boreholes are used in many industries including oil and gas, coal degassification, ground water remediation, and the pipeline industry. Other industries have potential applications such as sulfur Frasch mining and waste water disposal. Significant development of horizontal well technology occurred in the 1980's in the oil and gas industry. This paper reviews development of the @geology and documents uses in groundwater remediation, and pipeline placement and proposed use in sulfur Frasch mining. Finally horizontal well cost issues are reviewed.
Horizontal Well overview
Horizontal well applications within the oil and gas industry may be divided into four primary types:
a) increased formation exposure
b) restricted site access or drilling from a central location
c) coning of reservoir fluids
d) connecting of heterogeneities
Although these applications are important in the oil and gas industry, the concept is relevant in many circumstances where subsurface fluid flow or injection is a primary objective. [Within this paper, a "horizontal" well or borehole is taken to mean a borehole drilled at 80 degrees to 110 degrees angle, with a vertical defined as 0 degrees].
Increased formation exposure - Horizontal wells are advantageous compared to vertical wells in thin formations where maximizing exposure of the borehole to the formation is important. Borehole exposure to the formation in a vertical well is equal to formation thickness (assuming horizontal bedding planes). In horizontal wells, horizontal well length is the factor which limits borehole exposure to the formation, not formation thickness. Horizontal wells thus provide the greatest advantage over vertical wells in thinner formations. For example, in a formation of 25 feet thickness, maximum vertical well exposure is 25 feet. A 1000 ft. horizontal well in the same formation contacts 1000 feet, or 40 times that of a vertical well. However, a 1000 ft. horizontal well in a formation of 200 ft. thickness contacts only 5 times the formation that a vertical well contacts.
The increased productivity of a horizontal well compared to a vertical well in a given formation is easily calculated with analytic equational. In a formation of 25 ft thickness, a 1000 ft. horizontal well produces at about four times the rate of a vertical well (Figure 1). The productivity advantage diminishes in a 200 ft formation, with the horizontal well producing only 2.5 times that of a vertical well. The theoretical maximum productivity of a horizontal well is about 7 to 8 times that of a vertical well, depending on vertical well length, formation, and fluid properties.
Drilling from a central location - Offshore, arctic, urban, and mountainous locations may restrict surface site access for drilling and well operations. Horizontal drilling, with its long lateral reach, places wellbores in formation targets which are otherwise difficult or impossible to reach with conventional wells and drilling techniques.
Coning from fluid contacts - Horizontal wells minimize coning or withdrawal of undesirable fluids where fluids of different densities are present, such as gas, oil, and water. In vertical wells the conical pressure profile around the wellbore attracts undesirable fluids (gas and water) to the wellbore with the desirable fluid (oil). A horizontal well minimizes the effect by distributing the pressure drawdown of production more uniformly over a long distance, allowing water-free or gas-free production for a longer period.
Connecting Heterogeneities - In many formations with heterogeneous characteristics such as fractures, stream channels, Aeolian dunes, etc., horizontal wells enhance fluid flow by connecting the heterogeneities to the producing wellbore. The heterogeneities, particularly fractures with high vertical permeability, enhance flow by providing vertical conduits to supplement horizontal flow to the horizontal well. In contrast, a vertical well has only a small chance of intersecting a linear feature of limited extent such as fracture, stream channel, or Aeolian dune. The fractured Austin Chalk (US) and Aeolian Rotliegendes formation (UK) are examples of successes using horizontal wells to connect heterogeneities.
Horizontal wells progressed from an experimental technique in the 1980's to a standard tool for off and gas development in the 1990's. Progress has been seen in many major areas of horizontal well technology, including drilling, logging and completions.
First developed was the long radius method, which is an extension of directional drilling techniques developed over the last 30 years (Figure 2). Radius of curvature is typically 1000 ft or more, with maximum horizontal lengths ( as of 1993) of 8200 ft2.
The medium radius method was developed in the early 1980's with a 400 to 900 ft. radius of curvature. This method has the twin advantages of having the capability to drill long horizontal sections of 3000 to 4000 ft. and yet being able to intersect the target reservoir within short distances (less than a 1000 feet horizontally) of the vertical well section, which may be located on a seismic shot point. The majority of horizontal wells worldwide in the oil and gas industry today are drilled using this method.
Also developed in the 1980's was the short radius method, with a radius of curvature of 30 to 60 ft. One advantage of this method is intersection of the formation within a very short distance of the kickoff point. Short radius wells cost less than other methods because well curved sections between horizontal and vertical require considerably less hole length, and therefore less drilling time. However, constraints of this method are maximum length and well diameter. The relatively high rate of curvature means drilling tools are typically less than 4.5 inches diameter, restricting completion options and constraining future well remedial treatments and workover operations. Maximum lengths of short radius wells are currently about 800 ft to 1000 ft.
The ultra-short radius method employs flexible armored hose, drilling with very high pressure water. The jetting action physically removes the rock material. Turning radius is about 1 ft, with maximum lengths of about 100 ft.
Drill pipe metallurgy, drilling mud, survey technology and mud motors are all key elements in successful horizontal wells. These technologies have developed to the point where nearly any well configuration is possible. As recently as 1985, the most basic configuration of a vertical section followed by a 2000 ft. horizontal section was an engineering challenge. Within the last eight years, drilling technology improved to the point where many well configurations are possible, including dual laterals, quad laterals with branches of more than 2000 feet lateral length each3, and multiple laterals in the horizontal plane (Figure 3).
Logging technology - Virtually any type of log available in conventional wells can be run in a horizontal well using drill pipe or coiled tubing conveyance to push and pull the logs into and out of the horizontal section. Real time log measurements are now available from Measurement-while-drilling (MWD) and Logging-while-drilling (LWD) techniques. Sensors located from 2 to 60 feet behind the drill bit send data to the surface in real time using pressure pulses in the drilling mud column or other means of transmission. MWD data include real time directional survey measurements, with gamma ray and short normal resistivity. LWD is a more recent development providing real time density and neutron porosity, caliper, and temperature data, as well as the data provided by MWD4.
LVM and MWD data have many applications. In some fields, cost is saved by running MWD or LWD in place of wireline logs in non-veritical wells. Also, the data can be interpreted in real time to determine the hole trajectory relative to the well plan. When deviations from the well plan occur, the data collected give critical information for real time trajectory corrections5.
Completion technology - Many types of horizontal well completions are being developed. The purpose of the completion is primarily to hold the horizontal hole section open and enhance fluid flow. Completion possibilities include slotted liners, gravel packs, pre-packed well screens, and external casing packers. For very competent formations, the hole may be left open with no completion equipment inserted.
Horizontal wells have found many actual or proposed uses in other industries such as groundwater remediation, solution mining, pipeline crossings, water production or disposal, and lateral exploration. Environmental, pipeline crossings, and solution mining are discussed below.
Ground water remediation - The linear shape of the horizontal well lends itself to the typically ovate shape of a ground water contamination plume. As mentioned above, the horizontal well gives greatest advantage compared to a vertical well in @ formations. A similar advantage occurs in the case of @ contamination plumes. Vertical well exposure to a 10 ft thick contamination plume is 10 feet only, where as a horizontal well of 500 ft length may lie entirely in the plume, providing 50 times the exposure compared to a vertical well. A single horizontal well may therefore replace many vertical wells in ground water cleanup applications (Figure 4).
Another potential application is that of plume containment. A strategically place horizontal well across the path of a migrating or expanding plume may restrict the plume from further movement (Figure 5). Ground water sparging has also been proposed6. Parallel horizontal wells are drilled, one above the other. Air is injected in the lower well, drilled in or below the contamination plume. A contaminated air/water mixture is collected in the upper well and pumped out (Figure 6).
The horizontal well's directional capability is particularly suited to cleaning up con tamination under developed or inaccessible areas such as a factory, farm or urban area. The drilling rig may be set up at a convenient location within reach of the horizontal well target (Figure 7). The well is then drilled under the developed area with a minimum of disturbance, saving considerable cost compared to traditional trench and drain methods of clean up.
In July 1992, two horizontal wells were drilled to clean up groundwater con tamination under the BASF Corporation Geismar Site in Ascension Parish, Louisiana7. Manufacture of herbicides at this facility contaminated ground water with ethyldichloride (EDC) and monochlorobenzene (MCB).
The original remediation plan called for traditional techniques such as vertical wells and french drains. Disadvantages of this method included limited site access for heavy equipment, chemical exposure risks to workers, installation of a large number of vertical wells, possible damage to building superstructure by soil subsidence, and generation of large volumes of contaminated soil requiring expensive offsite treatment and disposal. These disadvantages and their associated high cost gave impetus to seeking alternative remediation methods.
Horizontal wells, a relatively new technique for groundwater remediation at that time, were evaluated and shown to have many advantages over the original plan. Advantages included less chemical exposure to workers, less risk of building damage, reduced requirement for heavy equipment access, and elimination of up to 20 vertical wells. Overall cost savings using the horizontal wells was estimated at $3.1 million.
The first well, H-50, was drilled in 17 days, with a completed horizontal section of 363 ft. Precise wellbore control was require to meet design objectives of 12 ft vertical depth of the horizontal section, and the requirement to stay between a row of 60 ft deep building pilings located 10 feet either side of the wellbore path. To achieve these objectives, the wellbore path was given a tolerance of 2 degrees horizontal deviation and 1 ft. vertical deviation. Using experience gained in the first well, the second well, H-51 was completed in only 6 days, with a completed horizontal section of 400 ft. Wells were completed with stainless steel pre-packed screens.
Initial data indicated the horizontal wells were successfully recovering the contaminants. During the first several months of pumping, the wells recovered 361 lb. of MCB and EDC.
Pipeline crossing - Pipeline crossing of rivers is another application of horizontal borehole technology. One such example comes from the Sacramento River in California8. A 42" gas pipeline was planned to traverse the Sacramento Delta area of California. The traditional trenching method of pipe laying was considered to endanger the fragile river levees. Horizontal borehole technology reduced risk to the levees by drilling under the levees (Figure 8).
First a 13 3/8" hole was drilled under the river, from one river bank to the other. A hole opening tool increased hole diameter to 60 inches. Finally the 42 inch pipe was pulled and pushed through the 60 inch hole to complete the process. In this manner, three 42 inch pipelines were laid under three sections of the river with lengths between 2850 ft. and 3860 ft. During the process, levees were not damaged. The entire process took about two years, one year for design, and one for implementation.
Sulfur Frasch mining - The horizontal well may also find use in Frasch mining of sulfur. Normally Frasch mining is accomplished by steam injection and liquefied sulfur production from vertical wells. Sulfur deposits mined in this way require minimum thicknesses, concentrations, and permeability of the sulfur-bearing rock. Low permeability or thin deposits cannot be economically mined with the traditional vertical well steam injection\production technique. Horizontal wells, having the advantage of increased formation exposure in thin formations, have been proposed to increase steam infectivity and improve sulfur recovery. Possible designs using horizontal wells include an injector/producer pair, one drilled above the other (Figure 9).
Three aspects of horizontal well cost are discussed: the learning curve, reuse of existing wells, and costs of environmental wells.
Learning curve - Numerous horizontal wells drilled in a given field may exhibit a cost "learning curve." The basis of comparison is the vertical well cost, as most of the wells in the field are usually vertical wells prior to drilling the first horizontal well. Typically the first horizontal well costs 2 to 3 times the cost of the average vertical well. As drilling methods, efficiency, and materials improve with experience, horizontal well costs steadily decrease to a ratio of 1.2 to 1.5 times the vertical well cost (Figure 10).
This final cost ratio of 1.2 to 1.5 represents a minimum cost plateau below which the cost cannot be reduced without a technological "leap" which increases drilling efficiency or decreases cost. Such a leap was the introduction mud motors and Measurement-While-Drilling technology in the mid-1980's. Horizontal well costs dropped to a lower cost plateau due, in part, to those new technologies.
Sidetracking - Emphasis on cost saving encourages re-use of existing vertical wellbores for horizontal wells which saves redrilling most of the vertical portion of the hole. At an appropriate depth above the target zone, a 1. window" is milled in the vertical well's casing. The hole is then "kicked off', creating a curved section followed by a horizontal hole section.
Re-use of the vertical well may achieve cost savings of 20% to 50% compared to a new horizontal well, depending on the depth of the well. In oil and gas wells, sidetracking of the vertical well should be considered between vertical depths of about 2000 ft. to 10000 ft. With targets of less than 2000 ft, the cost of drilling a new vertical section is small compared to the cost of the horizontal section; therefore cost saving by re-use of an existing vertical well is minimal. Below 10000 ft. mechanical risk may increase significantly.
Environmental costs - Costs of horizontal wells for environmental uses should consider both installation costs and operating costs9. Vertical wells are relatively low cost, in the range of $5000 to $25000 each. Horizontal well initial cost may be up to 10 times the cost of a vertical well. However, other cost considerations may more than offset low initial vertical well costs. Each well, vertical or horizontal, will require a pump, surface piping to a treatment facility, trenching of surface pipes, chemical analysis, pump testing, periodic water sampling and testing, periodic pump tests, maintenance, and electricity for pumping. If one horizontal well replaces 10 vertical wells, expenditure on these items will be reduced by a similar factor. A 10-year projection of remediation costs suggests horizontal wells may reduce costs if both initial and operating costs are considered (Table 1).
1.Many industries, such as coal degassification, ground water remediation, the pipeline industry, and oil and gas, sulfur Frasch mining, and waste water disposal have applications for horizontal wells.
2. Considerable technology development has occurred since the mid 1980's in long, medium and short radius wells. Many configurations of horizontal wells are now possible, from single lateral wells to multilateral wells in different horizons.
3. Horizontal boreholes provide an excellent means of lateral investigation of the formation by conventional logging techniques, or real-time measurements of Measurement-While-Drilling (MWD) and Logging While Drilling (LWD).
4. A project with many horizontal wells may exhibit a cost learning curve. -Cost reduction below the cost plateau may only be possible with the introduction of new technologies.
5. Environmental well costs should consider both initial well costs and operating costs over the life of the project.
1. Joshi, S. D., Horizontal Well Technology, published by Pennwell Books, Tulsa, Oklahoma, 1991.
2. Cade, R. W.; and Joshi, S. D.; Extended Reach and IOR Applications: North Sea Experience, presented at IOR Conference sponsored by UK Dept. of Trade and Industry, 10 November 1993.
3. Geometric vs. Geologic Steering, Supplement to Petroleum Engineer International, mid 1993.
4. Evolution of MWD Technology Accelerates, Petroleum Engineer International, May 1993, pp. 25-35.
5. Califf, B.; and Kerr, D.; UPRC Completes First Quad-Lateral Well, Petroleum Engineer International, September 1993, pp. 44-48.
6. Hall, George P., Environmental Applications of Horizontal Wells, prepared for Trenchless Tedmology: An Advanced Technical Seminar, Holiday Inn, Vicksburg, Mississippi, 26-30 January, 1993.
7. Conger, Robert M.; and Trichel, Keith; A Groundwater Pumping Application for Reinediation of a Chlorinated Hydrocarbon Plume with Horizontal Well Technology, Article provided by Eastman Cherrington Environ.mental Systems, Houston, Tx.
8. Cherrington, M.; Weeks, K. R.; and Anderson, H. V.; Directional Drillingfor Gas Line Sets Two Records, Oil and Gas Joumal, 6 Sept. 1993, pp. 45-56.
9. Karlsson, H.; Losonsky, G.; and Jacques, G. E.; Horizontal Wellbore Completions for Aquifer Restoration and their Economics, Article provided by Eastinan Chenington Environrnental Systerns, Houston, Tx.
