Managed-Pressure Cementing Provides Zonal Isolation in Deepwater Gulf of Mexico
This paper presents the design considerations, methodology, and results of two deepwater MPC operations conducted to cement production casing strings within a target operating window of approximately three-tenths of a pound.
Managed-pressure cementing (MPC) is an important technique for primary cementing operations in wells with narrow pressure margins between the pore and fracture gradients. This paper presents the design considerations, methodology, and results of two deepwater MPC operations conducted to cement production casing strings within a target operating window of approximately three-tenths of a pound. Modern transient hydraulic modeling software permits the calculation of adequate surface pressure levels to control the annular pressure profile during the different stages of a cementing operation. On the basis of a predetermined annular pressure target, different variables can be designed to produce surface and downhole pressures within existing limits of a particular operation. This capability, combined with modern managed-pressure-drilling (MPD) systems, enables accurate control of the annular pressure profile during cementing and makes it possible to obtain near-constant bottomhole pressure (BHP) throughout the cement placement operation while using statically underbalanced mud columns.
Preventing fluid losses during slurry placement generally is necessary in achieving the goals of a cementing operation. Effective zonal isolation behind the casing or liner wall is related intimately to preventing losses toward, and flow from, the formation during and after the cement circulation. When cementing a casing or liner string within limited openhole pressure margins, maintaining the bottomhole circulating pressure below the fracture gradient is not feasible in most cases when using conventional methods.
Cementing fluid dynamics are generally more complex than the fluid dynamics observed during the drilling phase. Additionally, multiple fluids with diverse properties and with generally higher viscosities are pumped at different rates. All of these factors, combined with high cement-slurry densities, contribute to equivalent circulating density (ECD) levels at the well bottom during cementing that are not observed while drilling. In many cases, these ECD levels lead to losses or even fracturing of the formation. The inability to measure downhole pressures during the operation adds to the complexity of the cementing job.
In recent years, MPD systems, with their ability to control precisely the well pressure profile, have emerged as an alternative solution to the problem of cementing efficiently within tight margins in deepwater wells. Used together with advanced hydraulic modeling software that can predict fluid properties accurately downhole under dynamic and static conditions, the latest MPD systems are enabling drilling engineers to manipulate variables that in the past were out of reach during the drilling and cementing-job planning.
The two wells discussed in the complete paper were drilled in the Gulf of Mexico (GOM) in approximately 8,500 ft of water from rig kelly bushing to mudline. The paper provides definitions related to the MPC subject, an overview of the applications, a summary of the engineering approach and calculations, and the results obtained during these cementing operations.
Wells A and B were the second and third drilled by the operator during this campaign. The main objective of these wells was to provide additional drainage infrastructure for reservoir development in the leased area. They were drilled from an ultradeepwater drillship. The rotating control device was a below-tension-ring type, complemented with an applied backpressure MPD system at surface, which included an automated drilling choke.
Both wells were designed and drilled with horizontal directional profiles. Mechanical configuration of Well A consisted of casing strings of 36 in., 13⅝ in., and 9⅝×10⅛ in. preceding the 8½‑in. production interval. Well B consisted of casing strings of 38 and 22 in. at surface and 16-, 13⅜-, and 9⅝‑in. liner strings preceding the 8½-in. production section. The narrow margin predicted between the pore and fracture limits in the lower intervals of both wells demanded the use of statically underbalanced mud weights (SUBMWs) and surface backpressure (SBP) control to maintain BHP within the operating window while drilling to prevent kicks or losses.
Given the limited margins available while drilling, and on the basis of wellbore-stability criteria, both peak ECD and final equivalent static density profiles for both wells exceeded the predicted fracture limit, making conventional cementing for the lower strings unfeasible. To alleviate the high ECD levels during cementing with a structured engineering process, pump schedules were created and complemented with SBP to permit using SUBMWs with the objective of maintaining annular pressures within the available margins throughout the cement job.
The primary problem faced by conventional cementing within a limited margin is the fact that the densities, fluid viscosities, and annular space configuration involved generally lead to peak ECDs and final equivalent mud weight (EMW) columns that exceed the fracture limit at the final stages of the cement job. To address the issue using the MPD methodology, the BHP plan for the cement job was based on the analysis, modeling, and manipulation of the terms comprising the general BHP equation, provided in the complete paper.
MPC Engineering Process
The process implemented to plan the MPC operations on subject Wells A and B consisted of three main stages: data collection, MPC engineering, and program elaboration.
In summary, the objective of the MPC calculation during the planning phase is to find the right set of conditions, and the required SBP levels, that will lead to annular pressures within the available operational window throughout the entire cementing operation. Fig. 1 depicts the iteration process used to calculate the MPC SBP schedules to cement the casing string on Well A and the liner on Well B.
As the first step before conducting the SBP schedule calculation, a base-case cementing model was created using all available information, and the initial conventional cementing simulations were performed to calculate the peak ECDs and final EMW. With the results from this initial model, validation was obtained for the need to use MPC for the casing strings.
The operational overview section of the complete paper includes time-based plots used to track the performance of the operation. The goals of preventing losses and achieving proper cement placement in the annular space were achieved using the MPC approach, and the benefits of this technology were demonstrated in the application.
- The results of the cementing operation on Wells A and B demonstrate the benefits of MPC in tight operational drilling windows. A structured engineering process and efficient execution using MPD closed-loop systems increase the chances of better results than when implementing the conventional cementing approach.
- The results of these MPC operations exceeded the operator’s expectations. The strings were isolated successfully without incidents, losses, or flow from the formation during the jobs.
- This study demonstrates the advantages of using modern MPD systems over the conventional approach with regard to primary cementing within narrow downhole pressure windows often encountered during deepwater drilling operations.
- An effective way to reduce the peak ECD observed before shutting down the pump at the end of the cement displacement is to reduce the mud column density and supplement with SBP as needed. This practice can be carried out safely only if the proper risk assessments and operating and contingency procedures are in place, and experienced personnel are involved during the planning and execution of MPC operations.
- This technique is recommended for consideration in tight-pressure-margin wells, even if the MPD system is not used during drilling. Successful cementing operations that achieve effective zonal isolation are of paramount importance in any well-construction operation, and MPC is an excellent technique for achieving this goal.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 194536, “Managed Pressure Cementing (MPC) Within a Narrow Pressure Window, Deepwater Gulf of Mexico Application,” by Michael Teoh, Sharief Moghazy, SPE, Keith Smelker, and Roger Van Noort, SPE, Shell, and Juan C. Valecillos, Julian Hernandez, and Maurizio Arnone, Weatherford, prepared for the 2019 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Amsterdam, 9–10 April. The paper has not been peer reviewed.