The petroleum industry is constantly driving to reduce capex and increase economic recoverability while minimizing environmental impact and surface footprint. By combining the three advanced drilling techniques of multilateral drilling, underbalanced drilling (UBD) and directional coiled tubing drilling (CTD), an operator can capture significant value out of known reserves.

The highest well productivity is achieved through maximizing reservoir contact per well/surface slot and minimizing reservoir damage. Multilateral drilling reduces capex through drilling multiple reservoir sections per surface slot while also increasing reservoir contact per surface slot. UBD minimizes reservoir damage, which maximizes the productivity of each lateral. CTD is inherently set up for underbalanced operations (UBCTD), and CTD bottomhole assemblies (BHAs) can achieve high build rates of up to 50 degrees per 30 m (100 ft) to allow multiple targets to be accessed from the mother wellbore.

Selecting a BHA

A directional CTD BHA consists of a coil connector, cablehead, electric or mechanical disconnect, downhole orienter, sensor package, motor or turbine with a bent housing, and a drillbit. Drilling directionally on coiled tubing (CT) is similar to conventional slide-and-rotate drilling on a rotary. As CT cannot be rotated from surface, all the rotation needs to be carried out downhole through the orienter. The rotating orienter allows the toolface to be set from surface or for the motor to be rotated to drill a straight hole.

Service companies also can provide additional BHA modules such as a gyro module for orienting a whipstock and for drilling in the presence of magnetic interference immediately after exiting the casing.

CT drilling faces two fundamental challenges: transferring weight to the bit and length limitations of the lateral sections. If the well trajectory plans for high doglegs, then it can be difficult to transfer weight to the bit. This is accentuated by the inability to rotate the whole drillstring as in conventional drilling. It is essential to have a weight-on-bit (WOB) sensor in the BHA so the driller can see that the weight is actually being transferred to the bit and react accordingly. The length of laterals that can be drilled with CT also are affected by the tortuosity, but this is particularly true in horizontal sections. The more tortuous the wellbore, the shorter the lateral length will be. CTD BHAs that have a continuous rotating orienter prevent this tortuosity from occurring and therefore maximize the available WOB and lateral length (Figure 1).

FIGURE 1. A straight wellbore increases the potential length of a lateral section compared to a wavy wellbore. (Source: AnTech)

Designing a multilateral well

All well designs require a multidisciplinary team to be successful. When designing a multilateral well, an integrated team of subsurface specialists and directional drilling specialists is even more essential to successfully drill the well. The well design and completion strategy is heavily affected by the reservoir characteristics, horizontal and vertical permeability, the geological structure, and geosteering requirements. The first step is to clarify if significant productivity gains can be made from utilizing multilaterals over other techniques. Once established, it is an iterative process between the directional drilling contractor and the operator’s engineering and subsurface teams to find the best way to design the well.

There are a near-infinite number of wellbore paths for multilateral wells. The two most common are stacked laterals and forked laterals (Figure 2). Stacked laterals can access different layers of a laminated reservoir. Forked laterals are all at a similar depth and are most commonly used to increase reservoir contact in a specific formation. Clarifying the objective for the multilaterals early on helps reduce the number of iterations required of the trajectory.

FIGURE 2. Stacked laterals offer access to different layers of a laminated reservoir, while forked laterals are at a similar depth and help increase reservoir contact in a specific formation. (Source: AnTech)

Once the trajectories are drafted, the wells must be modeled to ensure drillability and to specify surface equipment. For the CTD section the main areas for analysis are the available WOB, CT lock-up limit, borehole cleaning and surface pressures. The CT can be specified from these models. Production and geomechanics models also must be run to ensure the separation equipment is suitably specified and the amount of the underbalance applied to the wellbore does not cause wellbore stability issues.

Sidetracking techniques

To create the additional well path from the mother wellbore, a sidetrack must be initiated. There are two main categories of sidetracking a well: cased-hole sidetracks and openhole sidetracks. The cheapest and fastest way to carry out a cased-hole sidetrack is to use a whipstock and a window milled in the casing rather than section milling.

Multiple whipstocks can be set in the mother wellbore and retrieved if required. For openhole sidetracks the drilling BHA is used to create a trough in an inclined section of the wellbore. Once the trough is initiated, the WOB can be increased to carry on the borehole section. An openhole sidetrack also can be initiated off a cement plug with special procedures.

Since CTD BHAs operate on wireline, this allows significant amounts of real-time data to be received from the BHA. This helps to speed up the sidetracking process because rather than relying completely on time drilling, the directional driller can see the WOB and torque-on-bit responses to each operation and optimize on the fly. This is the case with both openhole and cased-hole sidetracks. A special module is required to monitor the casing milling operations since the vibration levels are so high.


A multilateral will not provide a good return on investment if the laterals are not drilled into the target zones. The options for geosteering on UBCTD are relatively limited compared to a conventional LWD service. Gamma ray and resistivity are available on certain CTD BHAs. A biostratigraphy service also can be used to identify changing formations. The UBD package can provide a significant amount of data that can be used for geosteering and reservoir characterization while drilling. The large amount of additional information that can be gathered from the real-time downhole sensors and the UBD package, if used correctly as part of an integrated data acquisition and reservoir evaluation strategy, can remove the need for expensive LWD tools or wireline logs.

Drilling practices

Drilling on CT has been avoided in the past due to concerns over stuck pipe and borehole cleaning issues. When drilling reentry wells using CTD, the borehole size is usually closer to the BHA size than in conventional drilling. In addition, since the pipe is not rotated, a greater focus needs to be placed on good borehole cleaning practices. Every CTD project must be modeled and analyzed to ensure the well can be drilled successfully. When drilling the borehole sections, the real-time drilling parameters must be monitored to identify any indications of borehole problems. Drilling practices also are adapted to ensure the borehole is clean and free of ledges. For example, a short trip must be made at every 46 m (150 ft) to ream the borehole, and at every 91 m to 137 m (300 ft to 450 ft) a long trip back to the casing window must be made. Because the driller is able to see these changes in downhole conditions at surface, there is an opportunity to prevent these issues and optimize the uptime of the operation.