Managed-Pressure Cementing: Successful Deepwater Application

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Adverse conditions are often encountered during managed-pressure drilling (MPD). These conditions can include wellbore instability, kicks, and losses that create challenging scenarios for cementing operations, such as a narrow operational window between the pore pressure and the fracture pressure, which can compromise the necessary barrier and require unconventional solutions in deep water. This paper reviews a successful managed-pressure-cementing (MPC) operation and presents findings and lessons learned.

Introduction

The case in this paper is an exploratory well drilled at a water depth of 1700 m in the Caribbean Sea. Well 1 was designed to drill through a potential shallow gas interval directly below a 22‑in. shoe using MPD. A total length of 350 m of 22‑in. open hole was drilled to the 18‑in.-­casing point. An unexpected high-pressure water/gas sand was encountered below the 22-in. shoe with a pore pressure of approximately 9.85 lbm/gal.

The fracture pressure at the 22‑in. shoe was measured to be 9.9 lbm/gal, which leaves a narrow operational window of 0.05 lbm/gal. The strength of the 22‑in. shoe was confirmed by a dynamic ­formation-integrity test.

A conventional cement operation was not feasible because of the required mud-weight (MW) increment and the resulting equivalent circulating density (ECD) with the 18-in. liner at the bottom, which limited the likelihood of success. The use of a high-strength, low-density slurry weighted to 11.5 lbm/gal was considered but was rejected because of the additional time and logistics required for a special cement-slurry dry blend. This resulted in the decision to implement MPC to increase the probability of achieving a barrier in place through the 18-in.-liner shoe and overlap the previous casing shoe.

MPC Overview

Cementing through MPD equipment is known as MPC, where pressure is applied at surface throughout the cementing operation to maintain an overbalanced environment.

During the engineering and planning phases, discussions and reviews were conducted with all companies involved to analyze different scenarios. On the basis of these discussions and reviews, a target ECD for the cementing operation was established. The anchor point used was ­defined at a 22-in.-shoe depth, with a value between 9.85 and 9.9 lbm/gal, which was suitable for avoiding un­desired wellbore-fluid influxes and losses. The anchor point is a defined depth in the open hole at which ECD values are kept steady using the MPD choke to provide overbalanced conditions without exceeding the fracture gradient. At such conditions, the cement was designed with cementing software.

MPC Operation Considerations

Precementing operations were defined to help provide optimal safety and reliability for MPC execution. These operations included the following.

Shoe Strength. Knowing and confirming the strength of the shoe at any openhole depth are important for determining the maximum limit that can be placed on the formation. To obtain these data, a dynamic leakoff test (DLOT) was to be conducted before the cementing operation.

Fluid Management. Because communication was identified as a crucial factor, a fluids-management plan was developed to notify all personnel involved in the operation and to maintain a clear procedure for supplying fluids. Fluid issues included

  • Spacer—The spacer was prepared in a pit tank once the pumpable volume and density required for
  • the operation were confirmed.
  • Mix water for cement slurries—On-the-fly mixing and pumping of cement slurry required a continuous supply of mix water to the mixing tub.
  • Synthetic-based mud (SBM)—The SBM for the cement-operation displacement was supplied by the rig to the cement-unit displacement tanks and was pumped in a controlled manner from the displacement tanks to downhole.

For the operation, the final surface-backpressure (SBP) schedule was fine tuned after the DLOT, which was the primary reference.

MPC Execution

Because of the narrow operational window, the 18-in. liner was run in MPD mode, controlling the tripping speed and adjusting the SBP according to the swab-and-surge estimation calculations provided by the mud company. The 18-in. liner with inner string was run to total depth while recording losses. Autofill float equipment could not be used to avoid losses because a statically underbalanced fluid was being used, which could result in a kick up the liner string while running the casing. Once casing was landed, circulation was established with minor losses, applying SBP to maintain bottomhole pressure above formation pressure. Once the liner reached total depth, a DLOT was conducted before the cementing operation. The intent was to calibrate the loss point and fine tune the maximum pressure before losses occurred.

The test results were then used to calibrate the cement hydraulic simulator to match the SBP recorded as the primary reference for the condition at 200 gal/min, which was the maximum rate planned for the cementing operation.

The cement operation was executed as follows (Fig. 1):

  • Circulate using an MPD surface line at 400 gal/min, with MPD holding back 295 psi.
  • Pump 5 bbl of spacer and pressure test surface lines to 250 and 5,000 psi for 5 minutes each.
  • Circulate using an MPD surface line at 400 gal/min, with MPD holding back 145 psi. Pump 100 bbl of spacer with 14 bbl of returns.
  • Stop to clean tanks. Apply 195-psi SBP.
  • Observe a gain of 5 bbl. Increase SBP to 205 psi.
  • Begin pumping lead slurry. Reduce SBP to 145 psi as per schedule.
  • Because of batch-mixer delivery issues, mix water was supplied for only 40 bbl of the pumped lead slurry weighted to 12.5 lbm/gal, leaving 20 bbl remaining. The necessary SBP to be applied was uncertain because the shutdown of the mix-water supply was not expected. A decision was made to apply 170 psi to control losses, which was then increased to 180 psi.
  • Place pumps back on line, returning 29.8 bbl. Reduce SBP to 145 psi. Switch to a tail slurry weighted to 15.8 lbm/gal, pumping 199 bbl as planned.
  • Shut down pumps to switch over to pumping 15 bbl of 10.1-lbm/gal spacer behind the cement.
  • A total of 15 bbl was returned. Apply an SBP of 145 psi at the beginning of displacing 190 bbl of 9.1-lbm/gal SBM. Maintain SBP at 145 psi until the end of the operation because the returns were not confirmed with volume-out measurements.
  • Check floats. Pressure before bleed off was 640 psi. Floats held, returning 1.5 bbl.
  • Pumps shut down, holding 150‑psi SBP. Increase SBP from 150 to 230 psi. Well was stable (no gain or losses).
  • Turn over to rig to set 18-in. seal assembly.
  • Perform positive and negative tests on the seal assembly.

 

Fig. 1—Actual values from the MPC operation.

MPC Results

The DLOT results show that losses were present once the casing reached total depth. At this point, the MPC operational conditions were focused on avoiding kicks and providing well control.

Limited well conditioning was performed with casing at the bottom because of the seepage reported on the DLOT. When the wear sleeve/setting tool was pulled out of hole after the cementing operation, cuttings were present. Cuttings on the back side are a possible reason for the induced losses because they could increase the annular friction factor and equivalent MW. Under such conditions, the MPC operation was concluded with losses but without influxes, allowing safe operation during and after the MPC operation. Pumping pressure indicated good behavior related to the cement-­column lifting pressure. Post-job analysis showed that, once the cement slurries entered the annulus, the pumping pressure began increasing constantly, matching the hydrostatic increase expected on the basis of the density of the slurry. Once the slurry passed the 22-in. shoe, losses stopped because of the solids in the slurry. This indicated that the losses were at the preceding shoe. The constant lifting pressure recorded until the end of operations indicated that the barrier placement was as expected for this MPC operation.

Data from the actual operation were processed and recalculated using the cementing software, allowing a comparison of the actual pumping pressure with pumping pressure recalculated with the software and confirming that the recorded operation pressure was consistent with the fluid positions. Further operations in the well supported the conclusion that the cement operation was satisfactory.

Conclusions and Recommendations

MPC is recommended for providing zonal isolation for operators drilling in narrow operational windows and experiencing an influx of fluids.

Identified improvement areas include

  • A contingency SBP vs. volume-in measurements should be defined in case of an unplanned shutdown.
  • Manual MPC operations require clear communication during operations to ensure knowledge of returns, fluids in, and applied SBP so that decisions can be made in a timely manner.
  • Using constant flow rates during cementing allows volumes to be controlled and pressures to be changed, making the identification of losses easier. This is particularly important with a narrow operational window.
  • Managed-pressure systems control volumetric parameters by flow in vs. flow out. For MPC, the primary reference is flow out of the well. Therefore, a solution is required that leverages the capabilities of managed-pressure systems. In the current case, the use of mass-flowmeter systems for flow-out measurements disabled the readings for flow in, which required tracking the pumping schedule manually and allowed precise measurements of fluid returns and control to stay on the proposed SBP schedule.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28139, “Managed-Pressure-Cementing Successful Application in Deepwater Exploration: Case Study,” by Alejandro De la Cruz Sasso, Halliburton, and Thiago Pinheiro da Silva and Patrick Brand, SPE, Blade Energy Partners, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Managed-Pressure Cementing: Successful Deepwater Application

01 May 2018

Volume: 70 | Issue: 5

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