5 Dec, 2017
How Does Horizontal Well Planning Differ from Other Directional Wells?
This tip continues from the August TOTM - Introduction to Directional Drilling
It has been said many times that "failing to plan is the same as planning to fail." In horizontal drilling, this is certainly true because planning is one of the most important steps in drilling a horizontal well.
Planning a horizontal wellbore is different from planning a normal directional well. In a normal directional wellbore, the target is usually described in terms of a departure at a certain true vertical depth (TVD) with tolerances in the horizontal plane (North and East). In a horizontal well, the target is most commonly described by the TVD plus or minus a tolerance. The departure target is usually more flexible than the TVD target. If the horizontal well also has a tight departure target, it is more difficult to drill.
For example, a formation top may be at a true vertical depth of 4,000 feet and the formation is 20 feet thick. The placement of a horizontal well in this formation will require the wellbore to be horizontal at a TVD of 4,010 feet, plus or minus ten feet. There have been some horizontal wells drilled with a TVD target tolerance of plus or minus 1.5 feet. The wellbore must stay within a three foot vertical zone. From this example, we can gather that target tolerances for horizontal wellbores are much smaller than typical directional wells. Consequently, they are a little harder to hit and greater care must be exercised in drilling a horizontal wellbore.
Horizontal well planning is a multi-disciplined project and must include personnel from:
PLANNING A HORIZONTAL WELLBORE
The first step in planning a horizontal wellbore is to gather all the information possible about the well and the formation to be drilled. Available data from offset wells, even vertical wells should be collected. These include well logs, bit records, mud logs, directional data, daily reports and any other data that might be helpful. Even vertical offset wells can provide valuable information for drilling a horizontal well including target depths.
The reason for drilling the horizontal wellbore must be defined. Is the horizontal well being drilled to prevent water or gas coning or to intersect vertical fractures? Many times, the reason for drilling the horizontal well drives the completion which in turn, drives the drilling program. The type of completion must always be considered in horizontal well planning. If the well is being drilled to prevent water coning, then the wellbore will be placed near the top of the producing interval away from the water. Gas coning would require that the well be placed near the bottom of the producing interval. If the well is being drilled to intersect natural fractures, the wellbore may be drilled from the top of the reservoir at the end of the build curve to the bottom of the formation at the end of the horizontal section as shown in Figure 1-1. In unconventional horizontal wells, the well is usually drilled in the direction of the minimum stress so the induced fractures are perpendicular to the horizontal. Is the well going to be completed with the plug and perf method or the using ball and sleeves?
Figure 1-1 – Placement of a horizontal wellbore in a fractured formation
Knowing the exact geology of the target is extremely important. Remember, TVD targets can be very small and bed dip is a major consideration. A bed dip of only two or three degrees can cause the horizontal wellbore to fall outside the target interval in only a short distance, and the geology of a formation can be slightly more complicated than originally expected. Figure 1-2 is an example of what can happen in a horizontal well. The left side of the figure was the planned wellbore path and geology. The right side is the actual wellbore path and geology. The actual conditions in the formation did not match the predicted conditions. As a result, the operator ended up with a poor horizontal well.
Figure 1-2 – Example of a planned versus actual horizontal well where geology is uncertain.
Planning a horizontal wellbore path must take into consideration all the geologic constraints and the determined reason for drilling the horizontal well.
If the geology of the formations is not precisely known it may require that the formation is drilled vertically and logged and/or cored before drilling horizontally. The vertical well or pilot hole defines the target TVD and provides information about the lithology changes within the formation. Then, the wellbore is plugged back, sidetracked and drilled horizontally in a more favorable position. Drilling a well horizontally is expensive and if the geologic data is inadequate, the chances of a commercially viable horizontal wellbore decrease significantly. Most horizontal wells require some amount of geosteering.
Once the target constraints have been defined, the wellbore must be planned. Review the offset data to determine where casing must be set. Decide what bit size will be required to drill the horizontal section. In many horizontal wells, casing is set through the build curve to eliminate any potential problems with formations above the pay zone. However, casing set through the build curve is not a requirement. It depends upon the stability of the formations above the pay zone and the completion method. The horizontal well takes longer to drill than a vertical well. Formations above the pay zone may deteriorate with time. Even though these formations may not be a problem in a vertical well, they may start to be a problem due to the longer drilling time in a horizontal well. Each well must be considered individually.
If the horizontal well is to be completed open hole or with a slotted liner, water producing formations above the pay zone may have to be cased. Casing the section after the horizontal portion has been drilled will require running an external casing packer for isolation and cementing above the packer. In open hole completions, the formation above the pay zone may not be stable over a long period of time.
When planning the build rate you must take a number of considerations into account. First, long radius builds are time consuming, more expensive, and it is more difficult to hit the desired TVD target unless drilled with a steerable system. Short radius build rates make it easier to hit the target, but bending stresses in tubular and restrictions going around a curve limit drilling and completion alternatives. Also, the short radius build will yield the shortest horizontal wellbore length. Medium radius is a compromise between the long and short radius. It uses what is now considered conventional equipment, allows hitting smaller target intervals, is not relatively costly, and allows most completion options. Medium radius is the preferred choice for most horizontal wells that do not have large departure targets.
Radius Build Comparison
Figure 1-3 shows how the TVD of the wellbore will change when the actual build rate is ±10% of the planned build rate.
Figure 1-3 – Comparison of the error produced by a ±10% change in build rate.
The actual build rate is usually based on preference or available kickoff points. Typically, higher build rates are used in smaller diameter holes and lower build rates are used in larger diameter holes. The dogleg severity limit to prevent fatigue of 4½ inch drill pipe is about 18º/100 feet; whereas, the limit for 3½ inch drill pipe is 24º/100 feet. Above these limits, fatigue can be a problem; although, the drill pipe can still be rotated at higher dogleg severities. Also, the tools used to build inclination cannot build as fast in a large diameter hole as a small diameter hole. An 8½ inch hole is limited to about 15 to 18º/100 foot build rates depending upon whose motor configuration is being used. There are intermediate radius tools available which can build up to 50º/100’. A 6 inch hole is limited to about a 25º/100 foot build rate though some short radius tools are now available for higher build rates. The maximum build rate for a 12¼ inch hole will be about 12 to 14º/100 feet. There are always exceptions to the above numbers.
The operator must decide what build rate to use. Generally, the higher build rates will yield less time drilling and, therefore, less cost. The build rate may also be determined by hole problems or casing setting depths. In either case, the operator selects the build rate or kickoff point. If the kickoff point is selected, the build rate is calculated and vice versa.
In most offshore applications, the operator must drill a directional well in order to get to the portion of the reservoir where the horizontal needs to be located. Therefore, most offshore horizontal wells are drilled with long radius or lower build rates. In unconventional pad drilling, the upper part of the wellbore is drilled as a conventional directional well to “nudge” the wellbore away from the other wells and line it up to drill the horizontal. In the nudge portion of the well, the dogleg severities are kept low.
When the target requirements are small, it may be necessary to make some adjustments to the build curve to hit the intended target. Nowadays, the build rate of most motor assemblies is predictable to within ten to fifteen percent. With previous experience in a specific area, the build rates are even more predictable. In areas with little experience drilling horizontal wells, it is not uncommon to plan the well with a tangent section, fractional orientation or soft landing if the build rate is at the upper end of medium radius. At the lower end of medium radius, the build curve is steerable. A tangent section is a short portion of the build curve drilled at a relatively constant inclination as shown in Figure 1-4. For example, the wellbore may build inclination at 12º/100 feet to 45º. Then a 100 foot section is drilled at 45º before continuing to build inclination at 12º/100 feet.
Figure 1-4 - Methods used to adjust TVD
The tangent section allows for differences between planned and actual build rates. If the actual build rate is less than the planned build rate, the well reaches 90º too deep. If it is greater than the planned rate, the wellbore will reach 90º too shallow as shown in Figure 4-16. If the build rate is greater than anticipated, the tangent section can be lengthened to consume more TVD. Conversely, if the build rate is less than anticipated, the tangent section is shortened providing more TVD to work with. At one time, it was very common to plan a tangent section for a medium radius horizontal well, but they are not common anymore. Tangent sections are not needed for wells with large TVD targets. Tangent sections cost money if the drill string has to be tripped to drill the tangent section, and should be avoided if possible. It can be avoided by running a steerable motor assembly capable of building the desired build rates.
Another alternative is to use what is termed a soft landing. Figure 1-5 illustrates a soft landing. A higher build rate is used in the first 70º to 75º. Then a steerable system is run in the hole and the last portion of the well drilled at a lower rate. The soft landing will yield higher departures since departure is significant above 70º.
Figure 1-5 - Soft Landing method of adjusting TVD
Planning a horizontal well includes gathering precise information and the inclusion of a multi-disciplined project team. It differs from a normal directional well due to the fact it requires more precise information and has a higher difficulty of drilling. This difficulty can cost a company more money if the planning is not done correctly the first time. Planning a horizontal wellbore path must take into consideration all the geologic constraints and the determined reason for drilling the horizontal well. Additionally, the appropriate build rate or kickoff point must be selected. This includes deciding how adjustments to the TVD of the build curve will be made (soft landing, steerable, fractional orientation). Today, there are many computer programs that can do three-dimensional planning that goes beyond the scope of manual, and help more accurately plan a horizontal well.
To learn more about this topic, we recommend attending an upcoming session of Directional, Horizontal, and Multilateral Drilling(DHD) or Basic Drilling Technology(BDT).
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