Deployment of a new azimuthal resistivity tool for coiled tubing drilling not only marks the first time it is being used on coiled tubing but also highlights just how effective it could be in helping operators realize real efficiencies on their projects.
A recent case study conducted on the North Slope of Alaska showcases an operator's innovative approach to enhancing geosteering sidetracks through coiled tubing drilling (CTD) to tap into previously unswept oil reserves. Collaborating with a seasoned engineering and service company that is new to the region, the operator has initiated a transformative change in the geosteering process to improve well placement. The newly implemented geosteering tools offer azimuthal formation data and facilitate continuous rotation. This deployment represents a groundbreaking application of this technology for CTD, marking its inaugural use globally.
Geosteering technologies enable operators to steer and remain within productive zones, thereby increasing production rates and reducing carbon emissions linked to drilling by minimizing the overall drilling time. By utilizing real-time data and sophisticated 3-D visualization tools, these technologies enhance precision during the drilling process. The introduction of geosteering has transformed wellbore placement, resulting in improved reservoir contact, enhanced well productivity, and increased efficiency and sustainability. These tools dynamically adjust to subsurface challenges, facilitating the drilling of more efficient and high-performing wells.
DIRECTIONAL COILED TUBING DRILLING
Coiled tubing drilling (CTD) utilises a continuous length of steel tubing that is wound around a large reel, from which it is injected into the existing wellbore and deployed downhole. CTD has a number of proven benefits over traditional drilling, the most important being its ability to maintain full control of downhole and surface pressures continuously whilst drilling. In industry terms, CTD has the greatest capabilities for managed pressure drilling (MPD) and underbalanced drilling (UBD).
MPD is the precise control of the wellbore pressure profile by utilising pressure control equipment at surface to control downhole pressures in addition to the drilling fluid. UBD is a subset of MPD, where the pressure in the wellbore is maintained below that of the formation. The use of continuous pipe eliminates the need for making up joints. CTD facilitates continuous drilling and pumping, allowing pressures to be accurately maintained and ensuring underbalanced conditions throughout the entire drilling operation. This application is vital in wells with low pressure, in wells where there are wellbore stability risks integrity, or in wells where the formation productivity can be damaged easily by the drilling fluid. It also can enable the well to continue to produce whilst drilling and may enhance the rate of penetration (ROP) in certain scenarios.
A CTD rig is generally more compact and has fewer components, which decreases both the time and expenses associated with rig relocations, making it especially advantageous for operations in remote locations. On re-entry operations, another time-and-cost advantage comes from not needing to remove the production tubing from the well prior to drilling the new lateral.
CTD is extremely versatile and can be used in a variety of ways, from re-entry drilling to slimhole grass-roots drilling. Typical applications include: depleted wells, unconventional gas shales, underground coal gasification and coalbed methane.
TRADITIONAL GEOSTEERING METHODS VERSUS AZIMUTHAL RESISTIVITY (AziRes)
Conventional resistivity sensors currently on the market for CTD can only measure the bulk resistivity of the formation being drilled. In contrast, an azimuthal resistivity sensor is a geosteering tool that can also predict the distance and the direction of the resistivity boundaries, Fig. 1. Resistivity boundaries can be bed boundaries, if the formations have a different resistivity signature or oil-water/gas-water saturation contacts.
Positioning the resistivity sensor as close to the bit as possible enables early detection of resistivity variations, facilitating timely directional adjustments. In conventional drilling operations, the pipe is rotated from surface and, therefore, an azimuthal measurement can be taken by rotating a fixed resistivity sensor downhole and plotting the data points. However, in CTD, it is not possible to rotate the pipe from surface. The technology applied by the new AziRes technology is the ability to rotate the measurement electronically, so azimuthal measurements can be taken, regardless of whether the tool is rotating or not.
Real-time logs and imaging are generated to visualise the resistivity landscape used to detect formation boundaries and the presence of hydrocarbon or water saturation zones ahead of time. This allows the directional driller to optimally place the wellbore within the zone of interest, saving time and cost to the operator. This technology is capable of operating in sliding mode, which makes it especially appropriate for CTD applications, due to its electrically rotated scanning feature.
COMBINING DIRECTIONAL CTD BHA TECHNOLOGY WITH AZIRES TECHNOLOGY
CTD bottomhole assemblies (BHAs) already have the advantage of a significantly higher telemetry transmission speed; however, as part of the geosteering upgrade, the telemetry was pushed even further. A new monoconductor telemetry system was built to allow for transmission speeds of 50 kilobytes per second (kbps), with the potential of speeds of 250 kbps in the future. This, alone, opens up a broad range of potential technologies and solutions to improve geosteering and drilling efficiency in general.
Unique technology in one CTD BHA on the market, a continuously rotating orienter (CRO), is key to the precise directional capability downhole. This particular CTD BHA is purpose-built for drilling in underbalanced conditions, with a full range of sensors giving it drill-by-wire capability and the ability to detect at-bit porosity changes. The BHA is small in diameter and targets hole sizes between 3-5/8 in. and 4-3/4 in. This makes it particularly suited to thru-tubing or small casing re-entries, Fig. 2.
Built for precise drilling, the CRO of the BHA lets the directional driller incrementally adjust trajectory while in motion. The build section and a perfectly straight lateral can be drilled in one run. This avoids the wavy wellbores that limit reach and weight-on-bit typically drilled with more conventional orienters.
WHY IS CTD USED ON THE NORTH SLOPE
CTD was pioneered in Alaska in the early 1990s, when operators focused on attempting to slow the state's production decline through significant re-entry campaigns. Having a significant pool of suitable donor wells for sidetracks contributed to this success and further development of the technology. MPD using CTD was introduced to reduce the pressure cycles on some of the more active shale formations, greatly reducing the risk for getting stuck, or worse, losing the wellbore. MPD has now become the standard for CTD in Alaska.
Objectives & challenges: Well No. 1. The objective of the first well was to geosteer a 3,000-ft lateral producer within one zone. Drilling challenges included at least one fault with a potential displacement of 15-20 ft, and within that continue to drill within the 20-30-ft thickness. Prolonged exposure to a lower shale layer below could lead to the need for plugbacks, and drilling in the overlying section was also identified as undesirable. The drilled hole was to be completed with a solid cemented and perforated liner.
A hybrid rig was used throughout the project, as it has the capability of running continuous pipe (coiled tubing) and jointed pipe.
Operational summary: Well No. 1. The casing exit was carried out on coiled tubing, using the directional CTD BHA with the incorporated casing exit equipment to set the whipstock and mill the window. The full LWD/MWD capabilities of the BHA allowed for constant monitoring of the downhole conditions and drilling parameters during the casing exit operations. One run was necessary to set the whipstock, and an additional run was required to mill the window exit through the casing.
After the casing exit was complete, azimuthal resistivity technology was incorporated for the first time in a directional CTD BHA with full MWD/LWD capabilities. It was used in conjunction with gamma measurements to enhance geosteering and optimise the wellbore placement within the thin reservoir. A total of 2,400 ft of lateral was drilled. Unforeseen casing integrity issues led to the drilling BHA becoming temporarily detained. The electrical release mechanism was successfully activated, and the BHA was subsequently retrieved with fishing tools. To avoid the compromised section of casing, an additional whipstock was set higher up in the well, and a new sidetrack was initiated from that point with a total length of 3,000 ft of lateral drilled.
Using an orienter with the ability to continuously rotate had a positive impact on the ROP and the achievable Weight on Bit (WOB), thus drilling a straighter trajectory. This is the benefit of continuous rotation of the BHA while drilling horizontally and reduces the need of extended reach tools to reach the targeted total depth.
The liner was run immediately after the drilling phase with the hybrid rig. The solid liner was set at total depth and cemented in place.
Objectives & challenges: Well No. 2. The key objective of the second well was to drill a sidetrack with a total length of approximately 3,000 ft along a northward fault, to achieve the production target. The enhanced lateral reach and the thin reservoir were the main challenges associated with this well. Enhanced geosteering techniques, such as combining azimuthal resistivity and gamma measurements, were adopted to deliver an optimally placed wellbore.
Operational summary: Well No. 2. The casing exit was once more executed, using coiled tubing. Subsequently, two sidetracks were drilled. The initial lateral was drilled to ascertain the formation tops. The second lateral was drilled utilizing the insights obtained from the first sidetrack and was initiated from the first lateral through an open hole sidetrack technique, employing an aluminium billet and an open-hole anchor. A solid liner was then installed to total depth in a single run within the second lateral.
The two sidetracks were successfully executed with a resulting total drilled footage of 4,910 ft. The lateral reach exceeded the initial requirement, and ROPs exceeding 100 ft/hr were observed in the lateral, way beyond existing technology. Further experience was gained with the new geosteering technology to deliver a well that is optimally placed within the thin reservoir.
ENVIRONMENTAL BENEFITS OF IMPROVED GEOSTEERING AND CTD TOOLS
It is also pertinent, at a time when the sector is looking for new methods to become more sustainable in a drive toward net-zero targets, to describe some of the environmental advantages of implementing geosteering tools, such as the AziRes tool and CTD.
Reduced footprint and land disturbance. CTD allows operators to work on a smaller scale. Conventional drilling takes up a large amount of space and often in certain environments involves the clearance of vegetation and the levelling of land. This, in turn, can impact things like soil erosion. CTD's smaller footprint allows for a much-reduced impact upon the local surroundings.
Lower noise pollution and emissions. With smaller rigs, there is a reduction in the amount of emissions emanating from machinery when CTD is introduced. It also means that there is lower fuel consumption, which in turn means a lower amount of emissions.
Use of existing wells. CTD is designed to re-enter existing wells and improve their production. It can be used to access bypassed reserves, as well as stimulating existing wells, meaning there is less need to drill new wells with the associated, increased environmental impact. Ensuring that existing wells are fully exploited means that operators are also getting the most from their investment.
Sidetrack drilling. Where new wells need to be drilled, CTD can also mean that new wells can be created by branching off from an existing well. This means that there is no need for a new well pad and the associated land disturbance, as described above.
CONCLUSIONS
The deployment of a new azimuthal resistivity tool for Coiled Tubing Drilling not only marks the first time the tools are being used on Coiled Tubing but also highlights just how effective these tools could be in helping operators realize real efficiencies on their projects.
These AziRes tools can measure azimuthal resistivity, allowing for the ability to measure the resistivity in different directions/angles from the tool while sliding, or in other words, when the tool is in a fixed position.
The CTD BHA continuously rotating orienter has a substantial impact on the weight-on-bit and ROP, as the directional driller is able to drill a straight wellbore. Therefore, it increases the length of lateral that can be drilled and subsequently increases the number of targets for drilling from the existing well stock. Additionally, reduced drilling time reduces the well cost.
The introduction of azimuthal resistivity for Coiled Tubing Drilling in Alaska is the first implementation of its kind. As the technology is further utilized on the North Slope, increase in performance will continue to occur, bringing huge advantages in wellbore placement accuracy to operators worldwide, realizing the technology's full potential in this sector.
ADAM MISZEWSKI'S role as Global Operations is to deliver AnTech's services safely and efficiently. He is committed to working with customers and service partners to ensure that promised value is realised. Before joining AnTech, Mr. Miszewski worked as a drilling engineer for BP in Aberdeen, UK, and demonstrated his ability to bring projects in on time and on budget without incident. He is chairman of the SPE Dorset Section. Prior to that, he worked for a brief period with Halliburton. Mr. Miszewski graduated from Imperial College, London, with a first-class master's degree in mechanical engineering.
UDO CASSEE is the Regional Operations manager for AnTech Oilfield Service Inc. in Anchorage, Alaska. Until the end of 2023, he held the position of general manager for a drilling contractor, a position he assumed in 2020 after being in the role of operations manager since 2018. Prior to this, Mr. Cassee spent 11 years of his oilfield career with a major service company, where he worked through different management and engineering positions in the Netherlands, Norway, United Kingdom and the U.S. (Alaska). He has extensive experience in cementing; downhole tools and coiled tubing applications; drilling; sidetrack and workover with jointed pipe or coiled tubing; overbalanced and underbalanced drilling with coiled tubing -- offshore, land and Arctic operations. Mr. Cassee holds a BS degree in oil and gas technology from the University Noorderhaaks in Den Helder, the Netherlands.