An industry study assesses the ability of lightweight cements to hold up under downhole conditions.

It's hard to tell whether a cement job is good. Tests at the time of cementing may confirm the service company's testimony that the job is solid, but it may or may not hold up under years of downhole stress.
Public information about cement jobs isn't generally available, but the US Minerals Management Service (MMS) says 11,000 of the 14,000 wells in the Gulf of Mexico show pressure between casings at the wellhead, a sure sign that the zone isolation has been compromised and the annulus is leaking. It's also a warning of potential problems from the well that could force a complete shutdown.
If this pressure gets too high, the operator has to bleed off the gas or shut in the well until pressure can be brought back to the safe level.
The cement outside the casing helps support the casing string and prevent fluids from migrating up the wellbore outside the casing string, where it could contaminate groundwater. It also shields the inside of the casing from corrosion. In short, if the cement job fails, the consequences are possibly harmful and certainly expensive.
An industry group led by Cementing Solutions Inc. has embarked on a study along the Gulf of Mexico coast to make sure lightweight cement jobs hold up through the long run. The initial laboratory tests have been completed, and the group is working on its first demonstration field trial. Conoco Inc. offered a 7,000-ft well in South Texas to test cement under typical field conditions.
Cementing Solutions initiated the project when it submitted an application for funding to the US Department of Energy (DOE) National Energy Technology Laboratory. The proposal outlined testing of lightweight cement that uses tiny, hollow, lightweight glass spheres made by 3M.
A DOE steering committee composed of representatives from operators and service companies chose this project for funding based on the serious level of the problem and the project approach.
DOE considered the project important enough that it put up US $675,000 to match $200,000 in industry cost-share funds for the company to use over 2 years.
The object of the exercise was to investigate performance of various unfoamed and foamed cements with weights as low as 9 lb/gal. Water weighs 8.33 lb/gal. The industry uses lightweight cement in areas where sensitive formations won't hold up to 13 lb/gal and higher cement densities, said Fred Sabins, president of Cementing Solutions in Houston, Texas.
Lightweight cement is particularly important for companies that need to cement wells across reservoirs that have been produced to the point downhole pressures are low.
Foamed cement, which is a common lightweight cement, takes some special equipment infrastructure, he said. Gas, usually nitrogen, must be brought in and injected with the cement. The logistics onsite can be complicated.
Pumping companies don't normally have onsite quality control for testing cements that are pumped downhole. He figured a lightweight cement with the hollow glass spheres would stand up as well as foam cement and require less onsite equipment.
The beads, which look like talcum powder when they're poured into a glass jar, come with varying specifications and the ability to withstand as much as 10,000 psi of pressure with minimal change in density, and there's no temperature restriction.
Testing
Since the cement with beads had to be laboratory- and field-tested, the project generated industry support and is testing the capability of various other lightweight cements to do their jobs efficiently. Pumping service companies each have their own slurries and reasons why they work, and the joint industry project will help evaluate those different types of lightweight cements or combinations of products and spheres. All three of the major service companies are members of the industry investigating group.
This project is more than just a conventional testing of cements to show whether they hold or not. This project is looking at such mechanical properties as Young's Modulus and tensile strength as well. "Ultimately," Sabins said, "we want the best cement integrity. The ability of foam and lightweight cement to provide long-term integrity is a question mark."
To turn that question mark into an exclamation point, the company is testing cement combinations in its laboratories with some of the top experts in the industry. Previous tests have been successful, but no one has performed a comprehensive analysis of the lightweight cement systems, Sabins said.
Those experts come from the three major pumping companies, ExxonMobil, Shell, TXI Energy (a cement supplier), Chandler Engineering (a cement instrument manufacturing company) and 3M (the manufacturer of the beads). In addition to the Conoco well, the testing team also probably will conduct tests at the Rocky Mountain Oil Technology Center near Casper, Wyo. Once members have solid field trials with onshore wells, they will move to offshore applications in the Gulf of Mexico.
Prejob testing of the cements is a very important part of a successful cementing job, Sabins said, but it's not the only part. The cement slurries have to be applied in the field using the best techniques and methods available. Various lightweight cements have been used in the field with mixed successes. The industry still has questions about the most appropriate cements to run in critical cases. Since they can't see whether the job is successful, and a failure might take years to appear, operators want to see the research to increase their confidence in the products.
As far as the beads are concerned, "The object here is to prove the technology of the beads and determine the state of the art in lightweight cements," he said. Once that's done, all of the service companies can take the technology to their people, give it their own individual tweaks of refinement and take it to the field for use in the industry toolbox of applications.
The research company is the neutral ground for the tests, he said.
"We will evaluate this product, lightweight cements, what's available, what they're good and not good for," Sabins added.
The service companies involved in the project have helped out by providing more than 5,000 data points of information on cement jobs they have run using cement weights of less than 13 lb/gal in offshore and onshore applications. That information library shows what industry is running and the range of applications and densities. In the end, they joined the joint industry project because they could see the benefits of the independent analysis.
"We've seen what they're running and where the holes (in the products and techniques) are. At densities less than 11 lb/gal, there is room for additional cement slurry along with the foams that may provide improved performance," he said.
Cement, he added, is like the foundation of a house, but it's the foundation of a well. An owner doesn't want problems in 5 or 10 years. It is also very expensive to fix the problem once it manifests itself.
The program is testing cement in a variety of ways that measure performance under real downhole conditions.
For example, a cement might hold up well under a stress test in the laboratory, but if cement is set at 45° downhole in an offshore well and then subjected to long-term temperatures of 180° when it brings fluids up the wellbore, operators want to know how the cement will hold up.
"We want to model and test real conditions and estimate a 10-year cycling stress test," Sabins said.
It is starting onshore, but by the time this study is completed, the research team and its industry partners will have information about cement performances onshore, offshore and in deepwater in hard and soft formations at different temperatures.
The testing must address each of those working conditions separately and measure the ability of cements to perform under all of those conditions.
The initial onshore test didn't consider the economics of the cement slurry with beads. The goal of the investigation was a tight, long-lasting seal. But a good cement job carries with it cost advantages. Or at least it can eliminate the costs of remediation from a job that doesn't work right.
An additional project evolved as "a sister project." Looking only at the offshore part of the cement application, MMS awarded the company a grant for a study titled "Long-term Integrity of Deep-Water Cement Systems Under Stress/Compaction Conditions." It falls under a problem area the organization already had identified, Area of Interest 9: "Improved cementing methods for shallow casing strings during drilling operations and the prevention of sustained casing pressures during production."
This study is a joint industry project that may include companies like ExxonMobil, Brazil's Petrobras, India's Oil and Natural Gas Corp., Saudi Aramco, Conoco, Italy's Agip, Norway's Norsk Hydro, BP and Unocal.
Each of the client companies will put $50,000 to match the MMS $50,000 seed-money grant. The combination has more than $400,000.
Ultimately, the results of this MMS study focused on deepwater applications and the DOE project focused on offshore and onshore wells could have a positive impact on the products used in lightweight cement jobs and the way those cement jobs are applied around the world.

Comments