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Underbalanced drilling (UBD)
In underbalanced drilling (UBD), the hydrostatic head of the drilling fluid is intentionally designed to be lower than the pressure of the formations that are being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure, or it can be induced by adding different substances to the liquid phase of the drilling fluid, such as:
- Natural gas
Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids that must be circulated from the well, and controlled at surface.
Characteristics of UBD
The effective downhole circulating pressure of the drilling fluid is equal to the hydrostatic pressure of the fluid column, plus associated friction pressures, plus any pressure applied on surface.
Conventionally, wells are drilled overbalanced. In these wells, a column of fluid of a certain density in the hole provides the primary well-control mechanism. The pressure on the bottom of the well will always be designed to be higher than the pressure in the formation (Fig. 1a). In underbalanced drilled wells, a lighter fluid replaces the fluid column, and the pressure on the bottom of the well is designed intentionally to be lower than the pressure in the formation (Fig. 1b).
Because the fluid no longer acts as the primary well-control mechanism, the primary well control in UBD arises from three different mechanisms:
- Hydrostatic pressure (passive) of materials in the wellbore because of the density of the fluid used (mud) and the density contribution of any drilled cuttings.
- Friction pressure (dynamic) from fluid movement because of circulating friction of the fluid used.
- Choke pressure (confining or active), which arises because of the pipe being sealed at surface, resulting in a positive pressure at surface.
Flow from any porous and permeable zones is likely to result when drilling underbalanced. This inflow of formation fluids must be controlled, and any hydrocarbon fluids must be handled safely at surface.
The lower hydrostatic head avoids the buildup of filter cake on the formation as well as the invasion of mud and drilling solids into the formation. This helps to improve productivity of the well and reduce related drilling problems.
UBD produces an influx of formation fluids that must be controlled to avoid well-control problems. This is one of the main differences from conventional drilling. In conventional drilling, pressure control is the main well control principle, while in UBD, flow control is the main well-control principle. In UBD, the fluids from the well are returned to a closed system at surface to control the well. With the well flowing, the blowout preventer (BOP) system is kept closed while drilling, whereas, in conventional overbalanced operations, drilling fluids are returned to an open system with the BOPs open to atmosphere (Fig. 2). Secondary well control is still provided by the BOPs, as is the case with conventional drilling operations.
Lowhead drilling is drilling with the hydrostatic head of the drilling fluid reduced to a pressure marginally higher than the pressure of the formations being drilled. The hydrostatic head of the fluid is maintained above the formation pressure, and reservoir inflow is avoided. Lowhead drilling may be undertaken in formations that would produce H2S, or would cause other issues, if hydrocarbons were produced to surface.
Reasons to consider underbalanced drilling
The reasons for UBD can be broken down into two main categories:
- Maximizing hydrocarbon recovery.
- Minimizing pressure-related drilling problems.
There are also specific advantages and disadvantages of performing a drilling operation underbalanced. These are summarized in Table 1.
Maximizing hydrocarbon recovery
There is no invasion of solids or mud filtrate into the reservoir formation. This often eliminates the requirement for any well cleanup after drilling is completed.
The well is producing as soon as the reservoir is penetrated with a bit. This could also be a disadvantage if hydrocarbon production cannot be handled or stored on site, or if the required export lines are not available.
Because there is no filtrate or solids invasion in an underbalanced drilled reservoir, the need for reservoir stimulation, such as acid washing or massive hydraulic fracture stimulation, is eliminated.
Because of the increased productivity of an underbalanced drilled well combined with the ability to drill infill wells in depleted fields, the recovery of bypassed hydrocarbons is possible. This can significantly extend the life of a field. The improved productivity of the wells also leads to a lower drawdown, which, in turn, can reduce water coning.
Increased reservoir knowledge
During an underbalanced drilling operation, reservoir productivity and the produced fluids can be measured and analyzed while drilling. This allows a well to be drilled longer or shorter, depending on production requirements. An operator is also able to determine the most productive zones in a reservoir in real time, and obtain well test results while drilling.
Skin factors on most underbalanced drilled wells are negative, just as they are in wells drilled and stimulated.
The absence of an overburden on the formation combined with the lack of any filter cake serves to prevent the drillstring from becoming differentially stuck. This is especially useful when drilling with coiled tubing, because coiled tubing lacks tool joint connections that increase the standoff in the borehole and then helps minimize sticking of conventional drillpipe.
In general, a reduction of the hydrostatic pressure in the annulus reduces the fluid losses into a reservoir formation. In UBD, the hydrostatic pressure is reduced to a level at which losses do not occur. This is especially important in the protection of fractures in a reservoir.
Improved penetration rate
The lowering of the wellbore pressure relative to the formation pressure has a significant effect on penetration rate. The reduction in the “chip holddown effect” also has a positive impact on bit life. The increased penetration rate combined with the effective cuttings removal from the face of the bit leads to a significant increase in bit life. In underbalanced drilled wells, sections have been drilled with only one bit where an overbalanced drilled well might need anywhere from three to five bits. It is normally assumed that penetration rates double when drilling underbalanced.
Classification system for underbalanced drilling
A classification system developed by the Intl. Assn. of Drilling Contractors (IADC) is helping establish the risks associated with underbalanced drilled wells (Table 2).
The matrix given easily classifies the majority of known underbalanced applications. This system combines the risk management categories (Levels 0 to 5) with a subclassifier to indicate either “underbalanced” or “low head” drilling using underbalanced technology. To provide a complete method of classifying the type of technology used for one or more sections of a well, or multiple wells in a particular project, a third component of the classification system addresses the underbalanced technique used, as shown in Table 3.
Example of classification system use
A horizontal section of a well is drilled in a known geologic area using a drilling fluid lightened with nitrogen gas to achieve an underbalanced condition through the reservoir section. The maximum predicted bottomhole pressure (BHP) is 3,000 psi with a potential surface shut-in pressure of 2,500 psi. This is classified as a 4-B-4 well indicating classification level 4 risk and UBD drilling with a gasified liquid. All wells classified as a Level 4 or Level 5 underbalanced well require significant planning to ensure safe underbalanced drilling.
Selecting the right candidate for UBD
Most reservoirs can be drilled underbalanced, but some cannot, because of geological issues associated with rock stability. For some reservoirs, it might not be possible to drill underbalanced with the current technology, because they are either prolific producers, or pressures are so high that safety and environmental concerns prevent safe underbalanced drilling. These may include high-pressure or sour wells (although both types have been drilled underbalanced, but with significant engineering considerations and planning).
Candidate selection for UBD must focus not only on the benefits of UBD, but also on additional considerations. It is important that the right reservoir is selected for a UBD operation. Table 4 shows reservoir types that will and will not benefit from UBD. Of course, not only the reservoir has to be evaluated, but also the well design, the possible damage mechanisms, and the economic reasons for UBD. All issues must be considered carefully when choosing whether or not to drill underbalanced.
Reservoir selection issues
Appropriate reservoir screening is essential for the correct selection of a suitable reservoir application for vertical or horizontal UBD. A systematic approach, outlined in the following section, identifies the major areas of study to ascertain if sufficient information is available to initiate the design work for a viable UBD process.
Once this information is gathered and reviewed, and if data show that an UBD operation is the best method for recovering hydrocarbons in an economically and technically successful manner, it is time to mobilize the team to design and execute the UBD operation. Steps in a typical UBD evaluation process are outlined in Table 5. Fig. 3 shows this UBD evaluation process as a flow chart.
It is important not to forget the business driver behind the technology. If benefits cannot be achieved, the project must be reviewed. The improvements from UBD—increased penetration rate, increased production rate, and minimization of impairment—must offset the additional cost of undertaking a UBD project.
This is often the most difficult limitation of UBD to overcome. If the reservoir/production engineers are not convinced that there is a sound reason for drilling underbalanced for productivity reasons, most underbalanced projects will never get past the feasibility stage.
To drill a well underbalanced, extra equipment and people are required, and this adds to the drilling cost of a well. The operators must show a return for their shareholders, so they will want to know if this extra investment is worthwhile before embarking on a UBD project.
Costs associated with underbalanced drilling
The following factors contribute to the cost increases for an underbalanced drilled well in comparison to a conventionally drilled well:
- Pre-engineering studies.
- Rotating diverter system.
- Surface separation and well-control package.
- Snubbing system to deal with pipe light.
- Data acquisition system.
- Extra downhole equipment [nonreturn valves and pressure while drilling (PWD)].
- Special drillstring connections (high-torque gas that is tight with special hardbanding).
- Additional personnel training.
- Additional operational wellsite personnel.
- Additional safety case update consistent with planned UBD operations.
- Extra time required to drill underbalanced.
From industry experience to date, we can state that underbalanced drilled wells are 20 to 30% more expensive than overbalanced drilled wells. This applies to both offshore and onshore operations in a similar area.
Cost alone, however, is not a good measure for the evaluation of UBD. The value of the well must also be recognized. The average three-fold increase in productivity of an underbalanced drilled well can add considerable value to a field development plan or a field rehabilitation program. If we add a potential increased recovery from a field to the value of an underbalanced well, even an increase as small as 1% in total hydrocarbon recovery may have a large impact on field economics.
Prior to a UBD operation, some reservoir engineering work should be carried out. Not only is an accurate reservoir pressure needed, but the damage mechanism of the reservoir must be understood to ensure that the benefits of UBD can be obtained. Some wells or reservoirs are suitable for underbalanced operations, and result in an enhanced recovery. Other formations or fields may not be viable for a variety of reasons. If formation damage is the main driver for UBD, it is important that the reservoir and petroleum engineers understand the damage mechanisms resulting from overbalanced drilling (OBD). We must remember that even underbalanced drilled wells can cause formation damage.
Coreflush testing may be required to establish compatibility between the proposed drilling fluid and the produced reservoir fluids. This is critical if oil reservoirs are to be drilled underbalanced. The potential for scale and emulsion forming must also be reviewed prior to starting operations. We must ascertain the stability of the zone of interest to determine if the proposed well path is structurally capable of being drilled with the anticipated formation drawdown.
Expected productivity with the proposed drawdown must be reviewed. The objective of UBD is to clean the reservoir, and not to produce the well to its maximum capacity. If the reservoir is likely to produce any water, we must take this into account because water influx can have significant effects on the underbalanced process. It is important that expected productivity be analyzed with the reservoir engineers to obtain an accurate indicator as to whether UBD would be beneficial.
Once reservoir issues are fully understood, advantages to drilling underbalanced are proven, and the proposed well profile can be achieved, we can undertake the selection of the surface equipment.
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