Single-Gradient Subsea-Mud-Lift-Drilling Technology in Deepwater Gulf of Mexico
As with other MPD systems, SMD technology offers early detection of influxes (kicks) and minimizes downhole losses to weak subsurface formations.
Subsea-mud-lift-drilling (SMD) technology is a form of managed-pressure drilling (MPD). As with other MPD systems, SMD technology offers early detection of influxes (kicks) and minimizes downhole losses to weak subsurface formations. However, significant differences are built into the SMD system. This paper will highlight the benefits of single-gradient SMD (SMD-S) technology, the execution of the most recent deployment, and test results that represent the final steps in moving toward continued MPD operations in the deepwater Gulf of Mexico (GOM).
SMD is a sophisticated subsea MPD technology development and commercialization project that has just completed its fourth offshore deployment. With every deployment, the learnings have been captured and addressed successfully. The most recent deployment in the deepwater GOM has demonstrated that the technology works as designed by successfully accomplishing nearly all test objectives. The confidence is now very high that the industry will see the successful commercial delivery of a drilling technology that offers potential for a more-efficient drilling operation. In addition, this technology will lead ultimately to enhanced production and recovery from deepwater assets. SMD has two operational modes, SMD-S and dual-gradient SMD (SMD-D), as illustrated in Fig. 1 (above).
SMD-D. A fluid with a density equivalent to seawater is used in the riser and a heavier mud weight is used below the mudline. This combination of gradients results in an annular-pressure profile that more closely follows the natural pore-pressure and fracture-gradient trends. This significantly improves the ability to stay within the pressure window much longer without changing mud weights. Dual-gradient drilling helps to eliminate casing points that are required in conventional drilling and offers significant production benefits in the deepwater environment.
In order to maintain constant bottomhole pressure (BHP), this technique allows annulus pressure to be trapped below a subsea rotating control device (SRD) during connections by increasing the maximum-lift-pump (MLP) inlet pressure to help manage ballooning or control wellbore stability.
SMD-S. The same mud density is placed in the riser and below the mudline, so the well is effectively in single-gradient mode. Static and dynamic BHPs for an SMD-S system are established with equations provided in the complete paper.
Pressure can be pumped off during circulation or trapped during connections with the MLP by reducing or adding pressure below the SRD to offset the effect of annular frictional pressure (AFD) as the rig pumps are ramped up to the drilling flow rate. This effectively allows the system to maintain a dynamic BHP in the well that is approximately equivalent to the static BHP, thereby providing greatly improved stability through tight pressure windows such as those commonly seen in the deepwater GOM.
Note that a unique attribute of the SMD-S system is the ability to pump off AFP from under the bearing-latch assembly (BLA) in the SRD. In this configuration, the well is always hydrostatically overbalanced, even in the event of an equipment failure or unplanned loss of power to the system.
To safely and reliably perform SMD-S drilling, the SMD-system hardware and operating procedures were required to satisfy a number of basic requirements. The system needed to demonstrate defined performance criteria of all routine drilling operations, safely manage nonroutine drilling operations (e.g., well-control and lost-circulation events), and maintain the well in a safe condition in the event of equipment failure.
Overall, cased-hole testing results were very positive and the technology concept was validated successfully along with the mechanical and operational viability of the MLP and the SRD to perform single-gradient MPD operations. Forty-four test objectives were attempted during the testing sequence, and 43 objectives were met.
The main focus of the testing program for SMD-S was to determine the stability of the MLP inlet pressure and the ability to manage the wellbore-pressure profile within a controlled and predictable range. Routine drilling procedures were tested in cased hole, with special emphasis on the start- and stop-circulation procedures while controlling AFP. Further, an assessment was made of the MLP-inlet-pressure control while tripping pipe through the BLA installed on the SRD, along with verifying the ability of the pump to maintain a close-to-constant bottomhole pressure during these operations. Within the nonroutine operations, the ability to detect a kick and respond to a potential equipment failure was also tested.
Test Results and Key Findings
MLP-Inlet-Pressure-Control Test. The MLP pressure showed stability at different conditions. Flow rates were varied from 300 to 800 gal/min, combined with different MLP inlet set points from 400 to 1,000 psi.
The MLP demonstrated control of setting the inlet pressure while following a schedule automatically from 750 to 1,000 psi as rig-pump flow rate was reduced from 800 to 0 gal/min, followed by manual control of the ramp-up schedule. Very good MLP-inlet-pressure stability was observed while pumping at different flow rates once steady-state flow was achieved.
The MLP managed small increments of pressure (25-psi steps) during AFP management while controlling the setting manually or automatically. During normal-drilling-related tests, the system was able to maintain BHP within a 10‑psi band.
The SMD-S test for dynamic-flow-check procedure was performed by reducing the MLP inlet pressure from approximately 1,000 psi to approximately 400 psi while maintaining both rig-pump and MLP rates at a constant 800 gal/min; equivalent pressure reduction was observed on the BHP, which was verified with the real-time measurement data from the annular-pressure-while-drilling (APWD) tool.
AFP Management During Connection. The SMD-S connection procedure was tested and verified successfully. The AFP algorithm works by automatically adjusting the MLP inlet pressure against a predefined flow rate vs. pressure table input by the system operators. While the AFP algorithm worked reasonably well for the MLP ramp-down sequence, logic errors were noticed during MLP ramp up, which needed the manual intervention of the system operators to correct.
The BHP tracked the MLP inlet pressure during the connection-procedure test.
This test demonstrated that the bottomhole static density and bottomhole circulating density can be controlled precisely by use of the SMD-S technique.
Kick Detection. The purpose of this test was to verify the kick-detection mechanism with the SMD system and test the ability to shut in and line up to circulate out a kick conventionally. During the test, the drill crew was able to identify the influx in less than a 2-bbl gain in the system, even with the low influx rate of only 25 gal/min. The kick-detection capability of the pump worked as designed.
BLA. The BLA successfully latched into the SRD joint and sheared from the running tool, so the setting procedure was executed as designed. After setting, the BLA-sealing element was tested successfully in holding a pressure differential across the seal. The test simulated tripping 70 tool joints through the element while varying tripping speed, rotary speed, and MLP inlet pressure; the maximum differential pressure tested was 750 psi. Rotary speed was varied from 50 to 100 rev/min.
The BLA sealing element was inspected at the provider’s shop after retrieval, and it showed no signs of abnormal wear.
Benefits of SMD-S System
Wells currently in the deepwater-GOM portfolio will benefit from the application of the technology in the following areas:
- SMD-S will improve the likelihood of accessing the top of the Wilcox formation with larger drift by eliminating the contingency casing strings that are set as a result of dealing with losses and kicks encountered while drilling through tight pressure windows.
- By adjusting the MLP inlet pressure, SMD-S will allow better management of the BHP to mitigate or eliminate losses when drilling through tight margins.
- Ballooning can be mitigated by maintaining near-constant BHP during drilling.
SMD Forward Plan
Testing results clearly demonstrated that the design fundamentally works and that the SMD system can offer tremendous value to deepwater wells through MPD. The system demonstrated control and sensitivity to wellbore pressures well within the design parameters. In the near term, the operator plans to use and mature SMD-S technology in drilling deepwater-GOM wells.
Once the SMD-S technology is implemented successfully, testing of SMD-D will commence. It is notable that the equipment used for SMD-D is unchanged from that of SMD-S, so testing of the dual-gradient capabilities is effectively an operational procedure test.
SMD-D remains the ultimate objective because this is the only technology available today that offers the ability to eliminate casing strings in a deepwater well while maintaining all conventional-drilling margins. While eliminating casing strings does save drilling time and reduces mechanical risk in deepwater wells from tight mechanical clearances, the real value-adding benefit is that it offers the ability to reach a reservoir with larger casings than with conventional single-gradient MPD systems.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 174881, “Successful Testing of Single-Gradient Subsea-Mud-Lift-Drilling Technology in Deepwater Gulf of Mexico,” by Sharifur Rahman, Calvin Holt, David Dowell, Danilo Morales, and Siri Davis, Chevron, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28–30 September. The paper has not been peer reviewed.