By Hassan Malik and Mohammed J. Aljubran, Saudi Aramco
The petroleum drilling industry drills many well sections using some type of unconventional drilling technique, or UDT. A drilling operation is considered “unconventional” if it does not follow the drilling method done in the majority of cases.
Conventional drilling techniques:
- Utilize a single-phase drilling fluid, which keeps an overbalanced fluid pressure at the sand face
- Discourage reservoir influx during or after drilling
- Considered open-loop operations, where the returned fluid from the well is directed to a flowline that is open to atmosphere
Unconventional drilling techniques:
- Utilize either single- or multiphase drilling fluids
- Requires a suite of equipment that frequently involve closed-loop mud circulation, rendering moot conventional drilling methods
A good example of UDT is air drilling, where atmospheric air is pumped down the drillstring using compressors and a booster on surface. This makes it unique in its application and operational challenges. Most UDTs are employed on conventional rigs—unless coiled-tubing units are used—which makes them easy to incorporate within a conventional drilling operation.
Although the UDT category (Figure 1) consists of several techniques, we’ll focus on two of the most commonly applied and discussed techniques in the industry: underbalanced drilling (UBD) and managed pressure drilling (MPD).
Confusion prevalent among industry peers involves the few similarities of these two techniques, where many believe they are the same with different names because of operational requirements. On the contrary, the techniques are different in their applications, including differences in operational strategies and implementation.
- Underbalanced drilling is an unconventional drilling technique that typically uses a multiphase drilling fluid to drill in depleted rock formations in order to enhance production.
- Managed pressure drilling is an unconventional drilling technique that primarily uses a single-phase drilling fluid to control equivalent circulating density or dynamic mud weight without adding any weighting material to the drilling fluid.
The main purpose of any MPD operation is to work on issues that can be the cause of heavy mud weight (MW) or high equivalent circulating densities (ECDs). By doing so, it improves the overall efficiency of the operation, leading to a significant amount of savings in both time and money.
We have selected 10 differences, out of many, that will allow the reader to distinguish between these two techniques.
- Enhances production from a depleted formation
- Increases the drilling rate of penetration
- Prevents formation damage
Typically, UBD operations have a differential pressure, or drawdown, at the sandface, so there is a negative change in pressure (ΔP). This means the formation’s pressure will be higher than the pressure inside the wellbore.
Managed Pressure Drilling:
- Improves performance of a drilling operation by avoiding issues that normally exist due to heavy MWs or high equivalent circulating densities (ECDs)
- Has an improved rate of penetration (ROP) in an MPD operation, usually considered a bonus but not the primary goal
- Allows more control on ECD without increasing the MW, allowing formations to be drilled that have a narrow margin of error between the pore pressure gradient & fracture pressure gradient.
Drilling with MPD creates a slightly overbalanced wellbore pressure vs. the formation pore pressure, as in the case of conventional drilling.
Single- vs. Multiphase Drilling Fluids
Typically, UBD allows a multiphase flow mixture to lighten the density of the fluid, creating a low ECD at the sandface, resulting in a lower hydrostatic head of drilling fluid below the sandface pressure. A multiphase drilling fluid will usually be a mixture of materials in two or more thermodynamic phases, like a solid & a gas.
Due to a pressure differential, influx is taken while drilling.
Managed Pressure Drilling
All MPD operations are single-phase drilling fluid operations. The traces of gas that are seen in an MPD operation is similar to a conventional drilling operation where the well is filled with background gas.
Once drilling commences, the added friction will slightly overbalance the formation pressure. The surface applied choke pressure (SACP) helps maintain a constant bottomhole pressure while drilling or when drilling is halted for any other operation.
Besides an increase in ROP, the only common theme between UBD and MPD techniques is a lightweight drilling fluid for which the static hydrostatic head will always be lower than the formation pressure. This statement is probably the only link that may have caused confusion among many drilling engineers, due to a light MW condition each contains.
This confirms that, at any given time, flow from the reservoir is imminent for as long as there is an active perm near the wellbore, which is also called near-wellbore permeability. In the case of UBD, production gain from the reservoir is the target. In MPD, this is not the goal.
More vs. Less Tiering
UBD and MPD utilize varying equipment spreads for different unconventional drilling techniques. Since neither UBD nor MPD are limited to a single application, equipment requirements vary with the types of application. This is true even within the same technique. A tier-based system allows the engineer to accurately predict the correct type and amount of equipment needed at the drill site.
Divided into 7 tiers:
- Tier-1: Single-Phase Injection (SPI) low head drilling
- Tier-2: Two-Phase Injection (TPI) DPI with WBM/OBM w/N2/CH4
- Tier-3: TPI DPI with Foam
- Tier-4: TPI CCI system
- Tier-5: SPI N2 Gas
- Tier-6: TPI System (Parasite String) w/N2 or CH4
- Tier-7: TPI with Coiled Tubing (CT) unit
Managed Pressure Drilling
Divided into 3 tiers:
- Tier-1: Gas knock-out system
- Tier-2: Semi auto choke system
- Tier-3: Fully auto choke system, including backpressure pump or any other high-end tool, such as Microflux or Non-Stop Driller, etc.
Difference in Candidate Screenings
Candidate screening is a filtering process to match the particular UDT to the problem faced by the well section. A detailed screening allows the engineer to identify the right technique and—most importantly—the correct spread of equipment. There is a generic screening result could look like below.
- UBD is an excellent candidate for depleted formations, especially when production enhancement is the target
- UBD is and has been applied in formations with the following characteristics:
- Depleted formations for production enhancement
- Formation damage prevention
- Storage or injector wells
- Depleting nuisance zones, such as high-pressure, low-volume gas pockets in top-hole sections
- Real-time formation evaluation (single-point production test or drawing productivity index while drilling)
- Lost circulation zones
- Tombstone rock drilling in intermediate hole section
Managed Pressure Drilling
- Due to its zero influx policy, MPD can be applied in sections that are prone to lost circulation, areas where stuck pipe is an issue, or in HPHT reservoirs to avoid NPT, mainly in the form of high MW or ECDs.
- To date, 70% of MPD wells have needed ECD control to avoid reaching fracture gradient or creating drilling-induced fractures. Therefore, most MPD today revolves around ECD or pseudo-MW control without increasing any solids in the mud to create the same effect of ECD as created by a weighted mud.
- A few cases have also seen a reduction in casing strings by controlling ECD at the sandface. Once the desired TD is reached, the casing is landed to secure the section without adding any additional strings.
- MPD has also seen a great value in HPHT wells, where longer open holes are able to be drilled, which were not possible with the traditional way of increasing MW to control the formation pressure.
Noting the above differences, it is clear that MPD and UBD are different in their applications. The UBD technique is primarily concerned with production enhancement from depleted reservoirs. MPD works with issues that are related to high MWs and ECDs that result in shorter wellbore length, challenging windows between pore and fracture gradient or areas where lost circulation is induced due to dilation of near-wellbore fractures. The bottomhole circulating pressure throughout an MPD operation remains slightly above the pore pressure.
Drilling mud is simply a non-Newtonian fluid that can be either single- or multiphase. Conventional drilling deploys only a single-phase mud system, whereby unconventional drilling can have either single-phase or a mixture of gas and liquid.
- Only Tier-1 UBD deploys a single-phase drilling fluid system.
- All other tiers of underbalanced drilling operations employ dual- or multiphase drilling fluids. The gaseous phase is nitrogen (an inert gas) or methane—or production gas—which works in harmony with both water-based mud (WBM) & oil-based mud.
Managed Pressure Drilling
- MPD works in the same way as conventional drilling, meaning it has only a single-phase fluid, which could be either WBM or oil-based mud (OBM).
- MPD does not employ any gas injection. The only gas that is present during the managed pressure drilling operation is the gas from the formation, the same as background gas in conventional drilling.
Note that the only similarity in terms of drilling fluid between UBD and MPD is a light mud weight such that—at static conditions—the hydrostatic head of the drilling fluid will always be less than the formation pressure.
- Plastic viscosity & yield point is less dominant in UBD than in an MPD operation.
- Hole cleaning in a UBD well is more dependent on velocities than yield point, while the reverse is true for MPD.
Drillstring & Well Construction Design
The well construction consists of both open hole and casing. The construction of a well drilled conventionally versus a well that is subjected to a unconventional drilling technique differs mainly in the form of hole sizes and the number of possible casing strings. Usually, an unconventional well will have fewer casing strings than a conventionally drilled well.
- The drillstring in a UBD well is equipped with additional float subs that are required for string depressurization. This is an additional feature that is not found in any other string design. The depressurization of the string is necessary before connection.
- A conventional mud-pulse telemetry’s signal is doped by the presence of nitrogen gas inside the drillstring, due to a two-phase mixture. Note that conventional mud-pulse tools require a single-phase in the string for the signal to reach the receiver on surface.
- Almost all UBD operations will have the casing on top of the formation that will be subjected to this technique. This is done to ensure well control (WC) integrity & to avoid exposing long open-hole sections, which may pose a threat due to a multiphase fluid mixture at their faces.
Managed Pressure Drilling
- MPD does not have any special requirement for a drillstring. All MPD operations employ the same drillstring configuration like a conventional well.
- One of the applications of MPD is to reduce casing strings by controlling the nuisance of trouble zones. Many operators have utilized the technique to save on casing costs. This does not mean this will always be the case. Depending on the type of application and formation pressure, a drilling engineer decides on the best possible scenario for arranging his/her casing strings.
Footprints on Location
Footprints-on-location is the area that is occupied by the surface equipment. Underbalanced drilling and managed pressure drilling equipment both bypass the conventional mud loop, devising a new route for the well returns through their equipment. In both cases, it is considered a closed-loop circulation until the return fluid is directed to the shale shakers.
A typical UBD setup would require:
- Rotating control device (RCD) with an emergency shutdown valve
- Compressors with AC units
- Nitrogen production units (NPU)
- High-pressure (HP) 2/3 stage boosters
- High-pressure choke manifold (manual or variable chokes)
- 3/4 phase horizontal separation with either a parallel or built-in sample catcher unit
- Flare line (other than the flare line downstream of the poorboy degasser)
Managed Pressure Drilling
A typical MPD setup includes:
- Rotating control device (RCD)
- High-pressure choke manifold (semi- or full auto-chokes). The auto-chokes in an MPD operation are pivotal to allowing a constant bottomhole circulating pressure (BHCP)/equivalent circulating density (ECD) regime at the sandface
- A vertical separator of 125-psi working pressure (min)
- In some cases, the downstream flowline from the choke manifold is tied into the rig’s poorboy degasser
It is clear that the equipment requirement for UBD and MPD is entirely different, even though both techniques are closed loop or pressurized until the auxiliary separation unit, which is part of the package. UBD consists of units that are also responsible for the nitrogen gas generation; this alone raises the footprint onsite. Therefore, the overall UBD operations cost is higher than for MPD. One has to pay a minimal amount for auto chokes, but the overall cost of MPD operations does not exceed UBD costs.
Drilling methodology deals with the way drilling is conducted in a well section. MPD and UBD each serve a different purpose when they are being utilized in any formation. UBD is mainly known for production enhancement from depleted formations, whereas MPD is utilized in areas where mud weight control is the main purpose or where the pore pressure & fracture gradient window is a challenge and cannot be controlled by conventional mud control.
- Drilling fluid is energized using nitrogen gas, which helps in creating a drawdown at the sandface. The multiphase mixture or injection of gas into a single phase liquid (WBM or OBM) helps to reduce the density of the fluid; hence, the resulting equivalent circulating density (ECD). This allows a pressure drawdown at the sandface, leading to a production gain while drilling.
- Due to gaseous mixture, the fluid is taken at surface and separated at the P tank, where gas and liquid phases go into their separate streams.
Managed Pressure Drilling
- Single-phase fluid similar to conventional drilling is used in MPD. The pseudo-MW or the ECD is controlled using an auto-choke system on the surface. The returned fluid from the annulus is trapped at the choke, which creates backpressure, which is felt through the length of the well. The drilling is continued until TD is reached.
- The premise of MPD in terms of BHCP is to keep the BHCP/ ECD slightly above the pore pressure at all times. In other words, a positive drawdown at the sandface will be treated as critical in an MPD operation.
Well Control Strategy
Conventional well control is straightforward, where certain indications must be fulfilled for the well control process to take place. UDT equipment is not designed for well control—many in the industry are unaware of this fact. In the event of any well control situation, the unconventional drilling technique system is bypassed to make way for conventional well control. As a result, the well control procedure in any UDT operation is not different from conventional well control. The design of the well control process and initiation will differ in both MPD and UBD operations.
- The well control (WC) matrix in a UBD operation has high production rates measured in MMscfd, which when encountered on surface, alters the position of the choke and, therefore, the choke or wellhead pressure.
- Due to a high flow rate operation, the changes in the wellhead pressure directly affects the dynamic rating of the RCD. Therefore, the WC matrix in a UBD operation is based on a large volume of gas or liquid compared with an MPD operation.
Managed Pressure Drilling
- High flow rates in an MPD operation negate the purpose of MPD. The WC matrix for an MPD operation relies on very small influxes, and many vendors prefer to have 2 bbl as their baseline. This means that for any pressure/volume/temperature gain that is ≥ 2 bbl, conventional well control will have to be deployed.
- Changes in the SACP are almost negligible due to these small influxes, unlike in a UBD operation.
Annulus Return Flow Measuring Devices
Measuring devices for annulus return flow in any drilling operation work based on whether there is a change in the volume of the flow. This is typically true for the equation of continuity Q = Av where if Q = K, then A1v1 = A2v2, confirming that the pump rate for any given hole section remains constant and volume pumped in V1 equals volume return from the well V2. If V1 ≠ V2, then it is either a gain or loss of fluid from the formation. The measuring devices in UDTs have attracted a lot of attention recently, but they differ in the case of MPD and UBD.
- Throughout a UBD operation, V2 > V1 in order for the operation to be called true UBD. This is mainly true for depleted reservoirs that have been selected for production enhancement.
- Incoming flow rate is critical in a UBD operation due to a complex mix of nitrogen, drilling fluid, reservoir gas or oil or, in some cases, water. Therefore, a separator with a device that can separate out these phases and measure them separately will be key to tracking production rates.
- Generally, all separators are equipped with a Daniel orifice meter that measures the incoming gas rate. The liquid rate is measured using the amount of volume that is shipped from the separator to the tank farm region on an hourly or as per need basis.
Managed Pressure Drilling
- For a true MPD operation, V1 = V2 at all times. This also shows that MPD works to control any influx or any damage near the wellbore that can cause propagation of fractures that can lead to lost circulation.
- Due to MPD being slightly overbalanced or balanced with the pore pressure, a Coriolis flow meter is now part of the package to measure changes in flow rates and density.
- In the event of any change in return volume, the auto-choke has to be adjusted to apply backpressure that would allow the system to turn into a safe mode and would eradicate the problem of any gain or loss.
- Notice in the above, the Coriolis flow meter is not designed to read separate flow rates. Rather, it measures the overall volume or density change.
It is clear from the above comparison that MPD and UBD are two different UDTs. The application of one can neither be called the other, nor merge into the other. The few similarities between these techniques can be seen in the form of a light MW or a RCD (or rotating head) installed above the BOP, an operational barrier during any UDT operation. Knowing the correct application of these techniques would help engineers identify well sections prone to these techniques. The correct identification of these two techniques would also help to select the right equipment that will avoid spending unnecessary cost toward the operation. DC
The authors would like to thank the Upstream Professional Development Center for spending their time in reviewing this article and taking all the necessary steps for its approval.
Hassan (HAS) Malik joined Saudi Aramco in 2013 and is currently a Technical Advisor at Upstream Professional Development & Training Center. HAS joined the O&G industry in 2001 & has an extensive background in drilling and workover engineering and operations mainly with international oil and gas operating and service companies. HAS retains a keen interest in unconventional drilling techniques and has published several papers & has delivered numerous workshops/seminars. HAS has a natural passion towards teaching and training and currently delivers advanced level courses in fluid mechanics, Underbalanced & Managed Pressure Drilling. He is currently working towards his PhD in Petroleum Engineering. HAS takes a keen interest in Computational Fluid Dynamics and has been attending CFD lectures at Cranfield University UK since 2016. In 1999, he received his MEng (Hons) in Petroleum Engineering from the Imperial College of Science Technology & Medicine from London UK.
Mohammad J. Aljubran joined Saudi Aramco in mid-2015 as a Petroleum Engineer with the Drilling Technology Team of Saudi Aramco’s Exploration and Petroleum Engineering Center – Advanced Research Center (EXPECARC). He published and presented several Society of Petroleum Engineers (SPE) technical papers and journals, and filed four patent applications in the area of drilling and completion at a very early stage of his career. Mohammad was a lead member of the University of Oklahoma team that won first place at the 2015 SPE Drillbotics competition in automated rig design and construction. In 2015, he received his B.S. degree in Petroleum Engineering from the University of Oklahoma, Norman, OK.