In long open-hole extended-reach and deepwater wells, the margin between pore and fracture pressure is narrow, and the equivalent circulating density (ECD) transients produced when the mud pumps are stopped and restarted at each pipe connection are a leading cause of losses, pack-off and stuck pipe. This paper describes the FECCS valve-type continuous circulation system developed by Shenzhen Far East Oil Drilling Engineering (FEODE), which maintains uninterrupted drilling-fluid circulation through every connection, holding ECD stable and keeping the hole clean. The engineering basis for ECD prediction (API RP 13D power-law hydraulics, with measured-depth and true-vertical-depth separation for high-angle wells) is summarised, together with a field validation in which the model reproduced a logged ECD to within 0.8 percent. Two field applications are presented: a 9,508 m extended-reach well, the deepest offshore well drilled in China, in which ECD fluctuation was reduced from 17.6–21 percent to under 5 percent across 68.5 percent of the open hole with no losses, collapse or stuck pipe; and a narrow-window extended-reach well in which rate of penetration was raised 2.5 times against an offset well and the production liner was run to bottom in a single pass. Both applications are independently documented in peer-reviewed petroleum journals.
Keywords: continuous circulation; ECD control; extended-reach drilling; deepwater; hole cleaning; managed annular pressure; well decommissioning
Extended-reach drilling (ERD) and deepwater development continue to push wells to greater measured depth, greater horizontal displacement and longer open-hole intervals. As reach increases, two pressures converge: the open hole must be kept stable inside an ever-narrower window between pore and fracture pressure, and the cuttings bed that accumulates along a high-angle hole must be continuously transported. Both depend on circulation.
In conventional rotary drilling, circulation is interrupted every time a stand of pipe is added or removed. When the pumps stop, the annular friction pressure collapses and bottomhole pressure falls toward the static value; when the pumps restart, a surge is imposed on the formation. The amplitude of this transient, expressed as a swing in ECD, grows with measured depth and with mud rheology. In a wide pressure window the swing is tolerable. In the long, narrow-window wells that define modern ERD it is not: a single connection can fracture the formation, induce losses, or allow the hole to pack off around a stationary string.
Continuous circulation removes the interruption. By maintaining drilling-fluid flow through the make-up and break-out of each connection, the annular pressure profile, and therefore ECD, is held essentially constant, and cuttings transport never pauses. This paper describes FEODE's FECCS valve-type continuous circulation system, the engineering basis by which its effect on ECD is predicted and verified, and two field applications that quantify the benefit. The same capability is increasingly relevant to the global well decommissioning and intervention wave, where re-entry and abandonment of ageing high-angle wells place a premium on pressure stability.
FECCS is a valve-type continuous circulation system. During a connection it diverts the drilling-fluid stream so that flow to the drill string is maintained while the connection is broken, the new joint is made up, and full flow is returned to the string, without ever venting the annulus to a static condition. The pressure boundary is preserved throughout, so the operation is transparent to the well: from the formation's perspective, circulation never stopped.
The system has been developed through successive field generations. The current, fifth-generation unit introduces single-action flow-path switching under programmable-logic control, which reduces the manual steps at each connection and shortens the switching sequence, and it is rated to 7,500 psi working pressure. Qualification testing of the coupling and load path has included pull and torsional loading representative of service, so that the unit can be carried as part of the drilling string rather than as a separate surface-only device.
The operational objective is narrow and specific: keep ECD inside the drilling window at every connection, and keep the annulus clean. The value of the system is therefore best expressed not in its mechanism but in the stability it delivers downhole, which is the subject of the remainder of this paper.
The equivalent circulating density experienced by the formation is the static mud density plus the contribution of annular pressure loss expressed as a density over the vertical column:
ECD = ESD + Pannulus / ( g · H )
FEODE computes the annular pressure loss with the API RP 13D power-law hydraulic model: the rheology is reduced to the power-law indices from the dial readings, the flow regime in each annular section is classified, and the laminar and turbulent friction terms are summed along the annulus. Cuttings loading is carried as an effective-density correction, and the connection surge is treated through the gel-strength term. The model is therefore standard, auditable, and tied to API practice rather than to any proprietary correlation.
Two points of engineering discipline matter for the wells of interest here. First, in high-angle and extended-reach wells the friction and hydrostatic terms must be referenced to different depths: annular friction and the connection surge act along the measured depth of the flow path, whereas the hydrostatic head acts over the true vertical depth. Collapsing both onto a single depth, as a vertical-well model does, mis-states ECD in exactly the wells where the margin is smallest. FEODE's implementation separates measured depth from true vertical depth for this reason. Second, the model is only as good as its agreement with the well: in a field comparison the computed ECD was 10.31 ppg against a logged value of 10.39 ppg, a difference of 0.8 percent, which is well within the uncertainty of the input rheology and supports use of the model for go/no-go decisions in a narrow window.
Against this baseline, the effect of continuous circulation is direct. The connection transient that the model would otherwise have to accommodate is removed at source: because flow is never interrupted, the annular friction term does not collapse and rebuild, and the ECD that the formation sees stays within a narrow band around the circulating value.
The first application is an extended-reach well drilled in 2024 in the eastern South China Sea to a measured depth of 9,508 m, with horizontal displacement of approximately 8,150 m and a horizontal-to-vertical ratio of 4.43. At this measured depth it is the deepest offshore well drilled in China, and it set national records for both well depth and horizontal displacement. The well was delivered on four enabling technologies, of which continuous circulation was one; FECCS was run across the 4,594 to 9,508 m interval, about 68.5 percent of the open-hole section. With circulation maintained through every connection, ECD fluctuation was reduced from a conventional 17.6–21 percent to under 5 percent, hole cleaning remained effective at high rate, the longest single continuous-circulation interval reached 1,744 m, and the well was completed with no losses, no hole collapse and no stuck pipe. The application is documented in the peer-reviewed literature [1].
The second application is an extended-reach well in the eastern South China Sea reaching 7,646 m measured depth at a horizontal-to-vertical ratio of 2.76. A fault encountered at 6,840 m narrowed the safe drilling-fluid density window from 1.20–1.81 to 1.20–1.32 g/cm³, and the well was drilled with oil-based mud throughout, leaving very little tolerance for connection transients. Continuous circulation was applied over the 215.9 mm section from 6,122 to 7,148 m. Holding circulation through each connection kept ECD variation under 2.3 percent and limited the connection torque swing to 2.86 kN·m. Against an offset well, rate of penetration rose by a factor of 2.5 (19.30 against 7.66 m/h), and the 177.8 mm production liner was run to bottom in a single pass with no wiper trip required, saving an estimated 41 hours of rig time. The application is documented in the peer-reviewed literature [2].
The two applications bracket the value of continuous circulation. In the first, the binding constraint was the length and instability of a very long open hole; in the second, it was the narrowness of the pressure window. In both, the same mechanism, a stable annular pressure profile maintained through connections, was decisive, and in both the benefit was measurable: fewer trouble events, higher effective rate of penetration, and casing or liner run to bottom in a single pass.
These are also the conditions that increasingly characterise mature-basin and decommissioning work. As fields age, intervention, sidetracks and plug-and-abandonment are carried out in depleted, narrow-window formations and in high-angle wells originally drilled for reach. The pressure discipline demonstrated above transfers directly to that work. FEODE's experience indicates that continuous circulation should be evaluated not only as a drilling-performance option but as a well-integrity measure wherever the drilling window is narrow and the open hole is long.
Stopping and restarting circulation at each connection imposes an ECD transient whose amplitude grows with measured depth and rheology; in long, narrow-window extended-reach wells this transient is a primary cause of losses, pack-off and stuck pipe. The FECCS valve-type continuous circulation system maintains flow through every connection, holding ECD within a narrow band and keeping the annulus clean while preserving the pressure boundary. ECD is predicted with the standard API RP 13D power-law model, with measured depth and true vertical depth separated for high-angle wells, and the model agreed with a logged ECD to within 0.8 percent. In a 9,508 m well, the deepest offshore well in China, ECD fluctuation was held under 5 percent across 68.5 percent of the open hole with no trouble events; in a narrow-window well, rate of penetration rose 2.5 times and the liner was run to bottom in one pass. The same pressure discipline applies to the intervention and abandonment of ageing high-angle wells.
[1] Zhang Weiguo, Deng Chenghui, Yan De, et al. Offshore drilling engineering technologies for ultra-large extended reach wells in China and their application. China Offshore Oil and Gas, Vol. 37, No. 4 (2025). DOI 10.11935/j.issn.1673-1506.2025.04.012.
[2] Zhang Jie, Wang Zhiwei, Ma Ling. Application of continuous circulation valve drilling in extended reach wells of the Panyu Oilfield. Journal of Southwest Petroleum University (Science & Technology Edition), Vol. 40, No. 4 (2018). DOI 10.11885/j.issn.1674-5086.2017.01.03.01.
[3] American Petroleum Institute. API RP 13D, Rheology and Hydraulics of Oil-well Drilling Fluids. American Petroleum Institute, Washington, D.C.
© 2026 Shenzhen Far East Oil Drilling Engineering Ltd. This white paper presents FEODE's own technology and engineering practice. Quantitative field results are attributed to the independent peer-reviewed sources cited above. See also the field case studies and the continuous circulation system overview.