Continuous Innovation Achieves Better Zonal Isolation in Extended-Reach Wells

Fig. 1—Different failure modes of cement sheaths.

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Operators at a UAE field experienced several challenges while cementing production sections. A systematic approach was applied that used standard cementing best practices as a starting point. An advanced cement-placement software was used to model prejob circulation rates, bottomhole circulating temperatures, centralizer placement, and mud removal. To enhance conventional chemistry-based mud cleaning, an engineered fiber-based scrubbing additive was used in spacers with a microemulsion-based surfactant. Finally, real-time monitoring software was used to evaluate cement placement in real time.


Artificial islands are built in the UAE to reduce life-cycle development costs and enable long-term maximum recovery levels by accommodating up to 300 wellheads on a single island. The wells drilled from these islands are complex extended-reach horizontal laterals with measured depths planned at more than 35,000 ft. Cementing these wells is extremely challenging, largely because of long open holes with high deviation, the use of nonaqueous fluids (NAFs) for shale stability, and lost circulation while drilling and cementing.

Additional stringent prerequisites, such as casing pressure tests at high pressures after the cement is set and the need to isolate narrowly separated sublayers of reservoir, increase the complexity of cementing these wells. This paper demonstrates the continuous improvements in the prevailing stepwise approach and the role of the improvements in attaining needed zonal isolation. These improvements can be divided into four main sections:

  • Advanced cement-placement modeling
  • Engineered fiber-based scrubbing agents in spacers
  • Engineered flexible expanding cement systems
  • Real-time cement-job monitoring

Advanced Cement-Placement Modeling

Good casing centralization during cementing operations is key to achieving proper mud displacement and obtaining hydraulic isolation in the annulus. To this end, centralizers are often placed along the casing to position it centrally in the borehole. The optimal number of centralizers and their spacing are determined through software simulations. Until now, the industry typically has used calculation methods derived from American Petroleum Institute Specification 10D to predict casing eccentricity in the wellbore. The calculations are based on an analytical soft-string method, which models an element of casing string between two centralizers as a bifixed beam. However, for extended-reach wells, the use of a numerical stiff-string method to compute torque-and-drag forces is becoming increasingly popular. It accounts for tubular bending stiffness and provides a more-realistic analysis of the stresses and loads acting on the drillstring and the borehole. The stiff-string technique, based on the finite-element method, is, therefore, proposed as an alternative and more-effective solution for computing casing centralization for cementing operations, especially in deviated sections.

The results from the stiff-string centralization model are further used in an advanced annular-displacement simulator capable of providing more-accurate simulations of gravity-induced segregated flow in horizontal and deviated wells. This advanced simulator is capable of simulating the azimuthal flow instabilities and their dramatic effect on mud removal. In horizontal wells, the casing eccentricity is toward the bottom of the hole. Because the path of least resistance is at the top, along the wide part of the annulus, the displacing fluid initially follows this path. After a while, a plume of dense fluid seats on top of the lighter mud. This situation becomes unstable as buoyancy forces exceed the yield stress of the mud. The resulting azimuthal instabilities generate azimuthal flow around the annulus and greatly help in displacing the mud, which otherwise would have remained a static mud channel along the narrow side.

The model is also capable of predicting the effect of casing rotation in the annulus. With rotation, the displacing fluid flows mostly along the wide part of the annulus. By increasing casing rotation, the spread of the fluid can be reduced significantly as the rotating casing drags the fluids in and out of the narrow side. Simulations performed using this advanced simulator often show that casing rotation is an efficient means of removing mud channels. Historical data also suggested that the quality of the cement bond was better when casing was rotated, further validating the results of the advanced simulator.

Engineered Fiber-Based Scrubbing Agents in Spacers

NAFs are valued in the industry for their performance and inhibition characteristics in water-sensitive formations and their low friction factors for drilling long horizontal sections. However, nonaqueous drilling fluids are some of the least-compatible fluids with cement slurries and can be challenging to displace and remove from the wellbore. Commingling of the mud and the cement slurry might result in a buildup of unpredictable viscous sludge at the mud/cement interface. The spacer formulations for the current project were improved by introducing a new solvent external microemulsion. This new microemulsion is a robust water-wetting and demulsifying agent and has the least effect on cement properties.

The spacer systems are improved further by the introduction of a scrubbing mechanism in conjunction with the chemistry-based mud cleaning provided by the new microemulsion. This scrubbing mechanism was achieved by introducing an engineered fiber-based scrubbing agent into the spacer system. These engineered fiber-based scrubbing agents provide mechanically enhanced cleaning of the casing and formation surfaces. The scrubbing and adsorption mechanisms exhibited by the spacer ensure that the surfaces are mechanically wiped and left water-wet, which allows for proper bonding of the cement with the surfaces. Laboratory testing and cement-bond logs indicated that the mud removal was better with the addition of the new scrubbing agent compared with using only conventional chemistry-based spacer design.

Engineered Flexible Expanding Cement Systems

Breaking of the cement sheath would cause problems related to sustained annular pressure and would compromise the zonal isolation. Hence, the mechanical properties (unconfined compressive strength, tensile strength, Young’s modulus, and Poisson’s ratio) for cement systems must be thoroughly designed to withstand the downhole stresses.

Cement-sheath-stress-analysis software can predict induced stresses from changes in pressure and temperature. As an input, the software considers mechanical properties of the rock and set cement and casing-wall thickness along with a planned pressure- and temperature-change cycle. It analyzes the set cement against three modes of failure (Fig. 1 above). A failure in compression or traction occurs if the stresses exceed the compressive strength or tensile strength of the set cement, respectively. A microannulus occurs if the cement sheath loses its hydraulic bond with the casing or the formation.

On the basis of recommendations from the stress-analysis software, a flexible and expandable cement system was developed with the needed mechanical properties. The actual mechanical properties were then validated in a geomechanics laboratory.

The effectiveness of the flexible expandable tail slurry was evaluated with ultrasonic measurements after a high-pressure casing test. The analysis indicated robust cement bond across some vital zones even after a 4,500-psi pressure test. The proposed cement system maintained the needed integrity on both the casing side and the formation side.

Real-Time Cement-Job Monitoring

Being able to predict the results of the cement job even before the cement sets is crucial for planning the drilling operations ahead. Standard cement-job monitoring is often limited to the acquisition of pressure, rate, and density measurements, and comparison with prejob simulations is not possible in real time. Therefore, differences between measured and simulated behaviors cannot be analyzed until the job is finished. Execution of cement jobs could be improved further if the experts onshore can assist those at the offshore field location in determining anomalies during the job before they become critical issues. For this purpose, a new real-time cement-job-monitoring simulator tool has been developed to improve the interpretations and diagnoses of critical job parameters while the cement job is in progress.

This new software tool is fed with two sources of data. The first source is the cementing-design data file that contains all the details about the job design (e.g., well description, fluid properties and volumes, placement design and planned pumping schedule, hydraulic simulation, centralization details, and mud-removal analysis). The second source is the rigsite data acquisition; these data are transmitted remotely to a wellsite-information-transfer-standard-markup-language server. The computer then combines data from the cement-job design with acquisition data from the cement unit and rig to provide a detailed picture of the operation by comparing acquired values with predictions computed in real time.

Data acquired during the cement placement are processed by a hydraulic simulator incorporated into the software to provide key information about fluid position in the annulus, comparative trends of acquired vs. simulated surface pressures, density quality assurance and quality control, and real-time visualization of dynamic well security. On the basis of the real-time estimation of fluid position in the annulus and other key parameters measured during the job execution, a contingency plan can be followed, thus avoiding the need to wait on a detailed post-job analysis of the raw acquisition data.

The real-time cement-job-monitoring tool was used successfully and enhanced decision making while cementing.


After establishing field-specific guidelines during the past 2.5 years, continuous success was replicated in more than 20 wells for all the rigs operating in this UAE field. Advanced ultrasonic cement-bond-logging tools, along with advanced processing and interpretation techniques, facilitated reliable, conclusive, and representative evaluations and ­zonal-isolation decisions. The cement-bond logs for the cement jobs showed significant improvement, and high-integrity wells were delivered successfully.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 188295, “Success Through Continuous Innovation To Achieve Better Zonal Isolation in Extended-Reach Wells in a UAE Field,” by Abdelkerim Doutoum Mahamat Habib, SPE, Yousif Saleh Al Katheeri, Sheldon Seales, SPE, Rayaz Evans Ramdeen, Romulo Francisco Bermudez, SPE, and Luis Eduardo Navas, SPE, ZADCO; and Saurabh Kapoor, SPE, Surya Pallapothu, Azza El Hassan, and Bipin Jain, SPE, Schlumberger, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed.

Continuous Innovation Achieves Better Zonal Isolation in Extended-Reach Wells

01 May 2018

Volume: 70 | Issue: 5


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