According to the MMS, about 11,000 of the 14,000 producing wells in the Gulf of Mexico exhibit sustained annular casing pressure. Thus, a significant number of offshore wells may not have adequate long-term zonal isolation.
In 2000, the MMS issued a safety alert, "Annular Gas Migration as a Result of Poor Cement Jobs," to notify offshore drillers, operators and service companies of the problem. To solve the problem, MMS is sponsoring a joint industry project (JIP), titled "Long-term Integrity of Deepwater Cement Systems Under Stress/Compaction Conditions," to conduct research, determine the properties that affect the cement's capability to seal fluids, and develop correlations between cement properties and sealing performance under downhole conditions. The JIP will evaluate a variety of the cementing systems used offshore and their capability to seal off zones where subsidence, compaction and thermal or hydraulic stresses threaten cement integrity during the life of the well.
The JIP was started in November 2001, under the MMS Technology Assessment & Research program for Operational Safety and Engineering Research. Seven major operators have joined, with a representative of each sponsor serving on the steering committee. The steering committee's multinational nature underscores the importance of cement sealing performance not only in the Gulf of Mexico, but also in the North Sea and offshore African and South American fields.
Causes of the problem
Annular pressure can be the result of many factors, some related to cement composition and others to the downhole environment. Stress exerted on the cement column during the life of the well can be thermally or hydraulically induced during well intervention operations, or it can result from compaction or subsidence. Stress gradients can be large enough to cause mechanical failure of
the cement.
Shallow water flows
Shallow formations that are penetrated while drilling deepwater wells often require extraordinary zonal isolation procedures to prevent shallow water flows. Severe operational, economic and environmental consequences caused by the immediate flow of water from shallow formations up to the sea floor require adequate sealing of the surface casings penetrating these zones. Significant effort has been devoted to development of cement compositions to alleviate shallow water flows; however, the long-term integrity of the seal provided by such special compositions has not been evaluated.
Additionally, the lithology of deeper strata may increase the potential of subsidence at any depth as pore pressure is reduced. Cement compositions used during the construction of these wells should withstand stresses exerted by subsidence while providing an annular seal.
Defining cement failure
MMS annular casing pressure statistics indicate performance is one measure of seal failure. Annular seal failure is the only mechanism that would allow pressure buildup on the annulus at the wellhead. In addition to performance criteria, numerical models can predict mechanical failure of a cement seal under a certain stress state, but this failure may or may not result in fluids flowing to cause a pressure buildup. Thus, although mechanical failure may have occurred in the annulus, the seal's performance still can be acceptable. The JIP's experiments will measure the performance criterion of seal integrity, rather than focus on numerically simulated mechanical failure.
Scope of work
Investigating a cement's capability to seal under downhole stress and compaction conditions is a challenge. Developing a correlation between conventional cement tests and rock properties tests using realistic annular seal models is one objective of the JIP research project. This correlation will allow prediction of various cement systems' capability to seal under downhole stress conditions. A series of cement seal evaluation tests will be conducted in devices that approximate the various stresses applied to the cemented annulus throughout the well's operating life. One such device, an annular seal device, is being considered for two applications. In one application, it measures the cement system's capability to provide shear bond to the pipe from the cement (Figure 1). In the other application, it measures the cement system's bulk permeability to gas, thereby indicating its capability to seal against water or gas flows in realistic geometries and in-situ conditions (Figure 2). The test parameters that will be evaluated include:
• cement compositions of varying densities from conventional normal weights to foamed cements;
• thermal cycling-induced stress;
• pressure cycling-induced stress;
• multiple cycles;
• compaction conditions, from no compaction to soft formations with significant compaction; and
• mechanical properties of the cements.
Planning the program
First, the JIP steering committee will prioritize the possible cement compositions to be tested, based on volume of use, critical nature of the seal in the area of use, stress potential in the area of use and degree of performance knowledge. Next, the steering committee will specify the maximum number of compositions that can be evaluated within the existing budget and begin testing. With additional funding, additional cement systems will be investigated.
Standard performance properties of the selected cement compositions will be measured in the laboratory and will include thickening time, rheology, free water, compressive strength, fluid loss and intrinsic permeability.
Additionally, standard mechanical property measurements will be performed on each composition at selected intervals after the composition has cured under realistic temperature conditions. Initial measurements will include shear bond strength, tensile strength, Young's modulus, Poisson's ratio and others as determined by the steering committee.
With the laboratory data, these properties will be used to predict the failure mode under in-situ stress conditions and the potential for annular seal loss. Young's modulus will be a significant parameter for estimating cement capability in failure mode as well as sealing mode after failure. The procedures used to measure Young's modulus will vary to include ultrasonic measurements, physical measurements and pore-fluid flow measurement through the bulk permeability after the initial failure. The latter procedure helps determine cement set failure and fluid flow after failure.
Annular seal testing
Annular seal testing will focus on the cement composition's sealing capabilities in a realistic annular configuration. The basic test apparatus is an annulus filled with the specific slurry and exposed to the testing conditions. At various times, gas is applied to the bottom of the model and the flow of gas through the model measured. This flow of gas indicates the cement system's capability to seal under downhole conditions. Two variations of the same test apparatus will be used initially in this investigation.
Figure 1 shows the annular seal device used to measure the shear bond strength. Figure 2 shows the device as it is used to measure the loss of annular seal under high-stress environments (by measurement of a bulk permeability to gas). This model has geometries that simulate downhole conditions. The data from this model can be used to compare various cement systems' capabilities to withstand cycling stress caused by pressure and temperature. Temperature and pressure gradients will be applied to simulate stresses induced from production or well intervention; then they will be cycled according to a realistic testing plan. The cement will be exposed to simulated soft and hard formations. Stress cycles and seal measurements will be performed at predetermined time intervals.
In the future, additional tests and models will be developed to simulate compaction and formation subsidence.
Technology transfer
Technical reports will be compiled throughout the project. In addition to a detailed final report that will be presented in spring 2003, monthly one-page summaries will be provided to the participants.
The final report will include:
• bulk permeabilities of various cement compositions in an annular configuration under compaction and induced stress conditions;
• mechanical properties of cements correlated with seal performance;
• detailed analysis of the long-term durability of various cement compositions; and
• recommendations of cement properties that provide long-term durability in a well scenario.
Joining the JIP
Improving cementing quality is a safety, environmental and economic objective each operator should address using the best available methods.
"We are encouraging additional operators to join this project," said Paul Martin, chief of the Engineering Research branch of the MMS. "By working together, we can all find some solutions to this very important issue of casing pressures in the Gulf of Mexico."
For more information, contact JIP project manager Fred Sabins at f.sabins@ cementingsolutions.com.

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