Drilling advances

The development of in-situ post-placement integrity monitoring, and additional research into thermal and pressure stress cycles, are but two of the technology gaps pinpointed in a U.S.-orchestrated review of the current state of primary cement isolation of deep offshore formations.

Under the direction of the Department of Energy’s (DOE) National Energy Technology Laboratory (NETL), the six-month study focused primarily on deep and ultra-deepwater wells, many of which are being drilled to total depths exceeding 20,000 ft. Principal investigator Dr. Barbara Kutchko of the NETL Pittsburgh, Pa., Office of Research and Development presented results of the study, entitled “Assessment of Research Needs Related to Improving Primary Cement Isolation of Formations in Deep Offshore Wells,” at a recent Drilling Engineering Association (DEA) quarterly technology forum in Houston.

The evaluation used input from cementing experts, as well as data from the scientific and technical literature, and feedback from API, IADC, DEA and other industry associations. It uncovered five “significant knowledge gaps” that must be addressed “in order to provide sound science for both industry and regulators to improve safety and reduce near- and long-term risks associated with offshore cementing jobs.”

Specifically, the study singled out the necessity for additional research to better assess the quality and reliability of cement before it is placed in the wellbore, followed by the sorely needed development of technology to provide long-term stability monitoring post-placement. In addition, NETL said the findings suggest a disconnect between lab tests and the conditions the cement is subjected to in the field, which become even more pronounced, as operators target the Lower Tertiary and similarly deeper horizons.

Gap analysis. Generally, the study categorized cement failure mechanisms into three distinct modes: at the cement-casing interface; at the cement-wellbore interface; and within the cement body, itself. Further, the investigation classified failure as occurring within three time-frames, beginning with placement and continuing to post-placement, and over the long-term and well after the cement has hydrated. Mostly cement failures were found to be rooted variously in the cement slurry design and mixing, cement setting behavior, downhole conditions, and cyclic stress loading and unloading on the cement bond.

The study identifies, by priority, limitations that warrant additional R&D, with potential solutions falling under four primary disciplines, comprising process improvements, overcoming data quality limitations, improving predictive tools and developing additional data.

By far, the predominant technical gap identified is the inability to monitor cement placement, as well as its integrity over the long haul. According to the investigators, the study revealed general agreement that little understanding exists of not only how effectively cement sets across zones, but more importantly, how well it holds together over time. Consequently, research requires a tracer that could be pumped with the cement and allow measurement after it has set, as well as tools to efficiently and accurately survey the cement bond logging, and in-situ sensors to evaluate integrity.

The monitoring chasm goes hand-in-hand with a fundamental requirement to better understand cement stability under field conditions, both during placement and well after the cement has set. The study, therefore, calls for additional research into cement setting and stability parameters under field conditions that could be used to help pin down key elements for cement design that API could incorporate in either new standards or for updating existing best practices. In particular, the NETL evaluation pointed to the increased use of foamed cement systems in high-stress environments as a focal point for increased examination.

Moreover, the third most pressing issue cited was inadequate quality control between the time the cement is tested in the lab, and when it is mixed and pumped on-site. Accordingly, research is needed to quantify the potential impact of poor quality control and improve best practices to ensure that the cement being pumped remains identical to that tested in the lab. With the frequency of lab testing requirements increasing appreciably, NETL said it is critical that new laboratory protocol and testing equipment, including automated technologies, be engineered to improve and simplify analysis of cement designs.

The authors also recommend more priority be given to designing cements to cope with frequent stress loading and unloading events, post-placement. As such, NETL suggests looking at alternative cements and well isolation technologies for potential applications in high-risk and high-stress cycle applications, designed for thermal and pressure stress cycles in ultra-deep wellbores. The federal investigators suggested a focus on thermal modeling to better understand temperature uncertainties.

Those providing technical input to the federal study also pointed to a dearth of uniform standards, industry-wide, insofar as calculations to determine set cement characteristics and properties are concerned. While tests are available, they are not consistent throughout the industry and, thus, are not considered analytical standards, particularly with respect to critical parameters, such as static gel strength and gas migration. The industry, likewise, lacks a standard stress calculation for set cement, with models restricted to calculations of induced stress values in compression and tensile components, while most of the higher-temperature tests of the mechanical properties of cement include only flexural strength measurements. The complete version of the study is available at http://netl.doe.gov/onsite_research.