Overview

Lightweight cements are used in many wells. However, they are increasingly used in deepwater wells and are the system of choice for high stress environments and shallow flow conditions such as those prevalent in the Gulf of Mexico. Currently, there is little information regarding the stability of lightweight cement systems under wellbore conditions. Operators and regulators do not have the information to predict or understand the properties of lightweight cement as it is placed in the well and at bottom hole conditions. Knowledge about the integrity and longevity of wellbore cements over decadal is poor, with little or no data or tools available for researching the situation. As wells are increasingly repurposed for secondary and tertiary purposes, ensuring that cement remains a barrier to subsurface flow is key. Research in this project addressed these challenges by improving the science base for wellbore integrity as it relates to the near-term and long-term efficacy of lightweight cements. Because cement stability is critical to ensure that gas will not break out of the slurry, cement must be tested under conditions that simulate placement in the well and situations that exist in post placement. This project determined lightweight cement stability at various depths in the well and correlated those test results with current of atmospheric testing.

Outcomes

NETL researched the physical and chemical behavior of typical wellbore cements to better understand how various cement formulations perform, with emphasis on potential failure pathways and remediation technologies. Researchers performed laboratory characterization studies of commonly used industry standard formulations of foam cements and have obtained the first computed tomography (CT) images of foamed cement systems. The CT characterization of the samples allowed quantitative analysis of physical properties and structures within the cement. The researchers also developed a reliable methodology to probe the microstructure of foamed cements under in-situ conditions. The team used this methodology to determine stability of foamed cement systems at various depths in the subsurface and correlate those properties with the current method of atmospheric testing. Initial results from this study’s atmospheric foamed cement experiments are summarized in Computed Tomography and Statistical Analysis of Bubble Size Distributions in Atmospheric-Generated Foamed Cement.

The use of CT imaging and statistical analysis is an effective method of characterizing the microstructure of foamed cement. Commonly held assumptions that foamed cements with higher foam qualities (entrained-nitrogen fractions) have higher bubble connectivity, higher permeability, lower foam stability, and lower compressive strength were confirmed and published.

The atmospheric foamed cement generation method [outlined in API RP 10B-4 (2015)] and the current foamed generation methods used in industrial-scale foamed cementing equipment do not provide similar bubble size distributions (BSD) in foamed cements of similar foam qualities. When comparing samples of a similar foam qualities, the atmospheric foamed cement generation method in API RP 10B-4 (2015) creates an average bubble size considerably larger than field foamed cementing equipment. Foam cement samples generated with field equipment display higher foamed stability and lower permeability compared to samples generated at atmospheric conditions in the laboratory.

Results of the analytical data generated with atmospherically generated foamed cement samples confirmed the current industry guideline of limiting the in-situ target design of foamed cement nitrogen fraction to 30 to 35 percent. Above this level, atmospherically generated foamed cements display significantly increased permeability and decreased foamed stability. The measured mechanical properties of both field-generated and atmospherically-generated foamed cements designed within current industry guidelines of a maximum 30 to 35 percent nitrogen fraction are suitable for applications to isolate subsurface formations in oil and gas well cementing operations.

The primary outcome of phase 1 of this project was in-situ characterization foam cement. Other key outcomes included:

• The first high resolution x-ray CT three dimensional images of foamed cement across a range of foam qualities and pressures to provide a better understanding of foamed cement.
• The finding that the foam is changing while scanned due to buoyancy caused by bubble migration. A series of scans were conducted to obtain a time lapse of this and gain insight into the behavior.
• An Assessment of Research Needs Related to Improving Primary Cement Isolation of Formations in Deep Offshore Wells identifies research needs in the cementing of offshore wells.
• Signed a non-disclosure agreement with Schlumberger to partner in generating foamed cement at a variety of conditions and directly collect the cement at in-situ pressures for real-time imaging in the CT scanners.

This project is continuing as Phase 2 Well Cement Behavior and Gas Migration.

Research Products

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