Advancing casing drilling to deepwater: Rethinking top hole well construction
A major deficiency in deepwater well design is the shallow setting depth of the initial conductor (or structural) casing in the top hole interval not taking advantage of the increasing overburden and fracture gradient below the seafloor, to deepen the conductor setting depth. The technically limited jetting process is the primary means for installation of the initial conductor in deepwater and responsible for its shallow setting depth.
As shown in Fig. 1, as water depth increases, the rate of overburden increase will decrease. Coupled with the accepted practice of “top-setting” shallow hazards, there may be excessive casings in the top hole interval, especially in the deeper water depths, prematurely slimming well architecture. This early reduction in wellbore geometry may result in narrow operating pressure windows deeper in the well—a prime reason deepwater operators may fail to meet their well construction objectives. Also of note, approximately 70% of all deepwater wells' operating losses exist in the riserless drilling phases, with contracted vessels respudding or experiencing frequent riserless difficulties on the first wells drilled.
Deepwater initial conductor casing is typically installed via the jetting process (Fig. 2), which uses bit and mud motor technology without pipe rotation to “push” the conductor casing into the sediment. This widely accepted practice technically limits the initial conductor setting depth to about 300 ft below mudline and is insufficient for drilling harder formations.
A new approach to top hole well construction is required to leverage the increasing overburden and fracture gradient below the seafloor. This can be accomplished by replacing the jetting process with riserless casing drilling technology. Figure 3 illustrates the benefits of riserless casing drilling.
Casing drilling can deepen the initial conductor casing setting depth to a predetermined fracture gradient (i.e. science) and not have its setting depth limited by a process (i.e. jetting). The overall number of casings can be reduced, resulting in lower well cost and expansion of wellbore geometry in the well. Building on the proven benefits of casing and liner drilling technology, mitigation of shallow hazards is accomplished in a single trip, with cementing operations commencing immediately upon reaching casing setting depth.
Shallow hazard mitigation. Shallow hazards, as defined by IADC, “are adverse drilling subsurface conditions that may be encountered prior to the setting of the first pressure containment string and the emplacement of the BOP upon the well.” These hazards are varied and may include shallow gas, shallow water flows, disassociating gas hydrates, mud volcanoes, faulting, boulders and wellbore instability. Shallow hazards are identified by exploration seismic surveys, pilot hole drilling, stratigraphic modeling and offset data, Fig. 4.
Pre-planning and prevention are keys to risk mitigation. If avoidance of a known shallow hazard area is not feasible, pilot hole drilling is an industry practice for physically identifying shallow hazards in a “managed environment.” The pilot hole is a smaller hole drilled and subsequently plugged prior to initiating primary drilling operations, before BOP installation. The smaller annular area of the pilot hole bottomhole assembly (BHA) and the hole drilled allows for early detection of a shallow hazard and offers better control of hazards, such as water or gas flows, compared to conventional drilling means.
The annular profile created by casing drilling is much narrower, relative to conventional drilling methods and approaches that are seen in a pilot hole drilling program. Casing drilling is advantageous to pilot hole drilling, as pilot hole BHAs require tripping after plugging the pilot hole, whereas in casing drilling, the casing is cemented in place, eliminating the need for tripping and related swabbing effects. Figure 5 is a comparative analysis of pilot holes and casing drilling.
The slimmer casing to hole geometry in casing drilling, coupled with the larger internal diameter in casing, facilitates a “natural dynamic kill” system. Casing drilling, with its larger internal diameter as compared to pilot hole BHA’s, allows a higher bottomhole pressure (BHP) to be generated for controlling and killing a potential influx, especially when the casing-to-hole size ratio exceeds 0.8, Fig. 6.
Surge and swab pressures, created by pipe tripping, are almost non-existent in casing drilling, as the well is being cased as it is drilled. It is estimated that about 13% of well control incidents in U.S. deepwater Gulf of Mexico and Norwegian exploratory operations occur while tripping, due to swab effects. Conductor casing can be cemented in place upon reaching total depth—without tripping—leading to better cement jobs. Cement bond logs have shown better cement integrity in casing drilling applications than conventionally drilled wells.
Wellbore instability. The aforementioned shallow hazards all contribute to wellbore instability issues. Wellbore instability is an undesirable condition, in which structural integrity of an open-hole section of a wellbore is altered via mechanical stresses, erosion due to fluid circulation or via chemical interaction between the drilling fluid and formation fluids and minerals. Wellbore instability in the riserless interval is especially problematic for casing installation after retrieving the drillstring, when drilling unconsolidated or tectonically stressed formations.
Unconsolidated formations are loosely packed, with little or no bonding among the particles or pebbles, and these particles can easily fall into the open-hole section while drilling and tripping. When the drillstring is removed and prior to the casing being run, the formation can collapse. Wellbore instability is common in many deepwater shallow formations; therefore, in conventional riserless drilling applications, it is critical that the casing be run and cemented in place as soon as possible after completion of the respective hole. This makes a compelling argument for casing drilling as a mitigant for wellbore instability, because the well will be cased and cemented as soon as section total depth is reached, without needing the time to trip pipe and run casing.
During casing drilling, stressing or strengthening of the wellbore appears to take place, which is widely known as the “smear effect” in casing drilling circles. It is the plastering of cuttings to the wellbore wall, seemingly enhancing wellbore hoop stress and increasing fracture propagation pressure. The smear effect during casing drilling can help to increase the fracture gradient, mitigate lost circulation and reduce non-productive time in drilling operations. The smear effect has been mentioned in many papers and received credit for faster-than-expected casing drilling performance and higher-quality wellbores.
Riserless casing drilling system. A riserless casing drilling tool (RCDT) system is required to drill in the structural casing. Figure 7 is a conceptual design for a 36-in. x 28-in. RCDT system. A 36-in. x 28-in. RCDT system effectively eliminates the requirement for the “drill ahead tool” typically deployed after jetting, to drill the hole for the second conductor casing.
The RCDT system will be similar in design and function to a hydraulically balanced liner running tool, sans liner hanger and liner top packer. The initial conductor drilling operations will be operationally similar to liner drilling operations. A 32-in. diameter drillable casing bit is connected to the bottom joint of the 28-in casing, giving a 0.88 casing-to-hole size ratio (28 in./32 in.). This ratio is optimal for a dynamic well kill scenario, as BHPs created with casing-to-hole ratios > 0.8 will increase significantly per unit pump rate, as shown in Fig. 6. The higher annular velocities generated per unit pump rate will optimize hole cleaning hydraulics. Conventional drilling BHAs are unable to replicate casing-to-hole size ratios > 0.8, which is considered the minimum for smearing effect benefits.
The RCDT will be hydraulically balanced, to eliminate the potential for an unplanned pressure spike that could result in premature release of the RCDT. The low-pressure wellhead housing (LPWHH) assembly will reach up to the structural casing via a connector joint, and will be located above the casing running tool, where it will freely rotate in the open water and be removed from the primary torque path during casing drilling operations.
The subsea casing drilling tool is compatible with existing subsea LPWHHs and high-pressure wellhead housing designs, since the subsea casing drive will engage the casing on a special profile sleeve. A debris barrier, located above the running tool assembly, will protect the RCDT from drill cuttings that could potentially fall inside the conductor casing as the LPWHH nears the mudline while casing drilling. The 36-in x 28-in tapered casing string has reamer blades attached to the lower part of the 36-in casing joints, to allow passage of the 36-in casing.
The conductor casing cementing system is similar in design to conventional deepwater casing and liner cementing systems, with a cement circulation path going through the drill pipe running string, inner cementing string and float collar, exiting through the nozzles of the casing bit. A dart catcher sub, located at the bottom of the inner cementing string, will be a receptacle for the drill pipe wiper dart after pumping cement.
Mechanical strength ratings of the 36-in. and 28-inch structural casing connectors (i.e., axial, compression, bending, burst and collapse) are a minimum 100% of the respective casing mechanical strength ratings to withstand the mechanical loading from a casing drilling operation. Connection cyclic stress fatigue resistance must be considered, as the 36-in. and 28-in. casing could be rotated for thousands of revolutions.
Centralization of the structural casing is a key consideration, as the maximum annular hydraulic bypass (flow) area is of utmost importance, due to the narrow 28-in. x 32-in. and 36-in. x 41-in. casing drilling annuli. The application of centralizer blades directly onto the casing, without altering the metallurgical properties of the casing, is preferred over slip-on centralizer designs, as the slip-on centralizer sleeves encompass much of the flow area, increasing the chances for a pack-off event. Centralizer blades applied directly to the casing will maximize annular flow area.
Summary. Riserless casing drilling technology in the deepwater top hole interval can provide effective mitigation and isolation of deepwater shallow hazards, especially those that are pressure-related, with potential for inducing gas or water flow into the surrounding wellbore environment. The casing and annular hole geometry inherent with casing drilling make it a “natural dynamic kill” system, in the event that an unforeseen drilling hazard is encountered.
Casing drilling is also a proven means for mitigating wellbore instability, especially wellbores that cannot remain open with conventional means prior to casing installation. Deployment of riserless casing drilling in deepwater will require a paradigm shift in the way we approach deepwater well construction design, as the traditional jetting process for initial conductor installation is replaced with casing drilling. This shift in deepwater well construction will allow deepwater riserless casing setting depths based on the prevailing pore pressure and fracture gradients, as opposed to the established strategy of “top setting” shallow hazards, potentially “wasting” casings and slimming the hole geometry prematurely.
This paradigm shift in deepwater well architecture will enable larger diameter casings, with the potential for the HPWHH casing to be set deeper in some regions, reducing the number of casings. Drilling operating pressure windows can be expanded deeper in the well, reducing the loss and gain cycles characteristic of a narrow pressure window environment, enhancing the potential for optimal hole and production casing size at total depth.
- Aird, P., Deepwater Drilling, Page 469, 2019.
- Akers, T. , “Jetting of structural casing in deepwater environments: job design and operational practices,” SPE paper 10237, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, Sept. 24-27, 2006.
- BSEE, Holand, et al, “Loss of well control occurrence and size estimators, Phase I and II,” p. 175-176, 2017.
- Kotow, K., and Pritchard, “Exploiting shallow formation strengths to deepen riserless casing seats,” Drilling and Completion, Volume 35 (03): 428-437, September 2020.
- Rosenberg, S., K. Kotow, Wakefield, et al, 2022. Riserless casing drilling for mitigation of shallow hazards– A paradigm shift in deepwater well construction, SPE paper 208793-MS, presented at the IADC/SPE Drilling Conference and Exhibition, Galveston, Texas, March 8-10, 2022.
- Salehi, S., Mgboji, A. Aladasani, et al, “Numerical and analytical investigation of smear effect in casing drilling technology: Implications for enhancing wellbore integrity and hole cleaning,” SPE paper 163514, presented at the SPE/IADC Drilling Conference and Exhibition, Amsterdam, The Netherlands, March 5-7, 2013.
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