Standards you need to know when designing a cement slurry for oil well applications

Have you ever wondered what tests you should (or could) ask the lab to run when you are preparing for a cement job? Have you ever been part of an argument around a lab test result because it was run under a procedure that you think may or may not be representative of the conditions of your current well? In this two-part article series, I will go through the current standards and the testing procedures you need to know to match the heavy-weighters.

All operators around the world refer to lab procedures specified by the American Petroleum Institute (API) standards, most of which are also International Standard Organization (ISO) procedures today.

These standards don’t cover all available test apparatuses widely used today, but they offer a way “to promote standardization and uniformity and to eliminate discrepancies and misunderstandings relative to cement slurry behavior in the laboratory as well as in actual cementing operations."

The norms are not an invention of recent days. They were a result of close collaboration between O&G operators and their service companies with the API organization. They reflect the newest advances in slurries evaluation and the current tendencies as a basis for the design of competent cement systems for well integrity.

In 1937 the American Petroleum Institute established the first committee to study cements. There already existed several cement-testing laboratories equipped with strength-measuring apparatus and stirring devices to determine the fluidity or pumpability of cement slurries at down-hole temperatures. These laboratories orchestrated many new developments in well cements between 1937 and 1950.

During that period, a need arose for standardization of cement testing. The API Committee on Oil-Well Cements issued in 1948 the first edition of the Tentative API Code 32, entitled ‘API Code for Testing Cements Used in Wells.’ This code encompassed a specification governing test procedures, methods, and equipment used to evaluate and define those physical properties of particular importance to the oil and gas industry. 

The code also included field data for wells 5.000 to 14.000 ft deep -all in the U.S. - having average mud circulating pressures and bottom hole temperatures that form the basis for casing-cementing well simulation schedules today. Based on casing type, depth, and temperature gradient, labs around the world use these ‘schedules' as the sources to determine the corresponding bottom hole circulating temperature.

API Code 32 was later re-designated RP 10B in 1956 and advanced to ‘standard’ by the national API Committee. An evolved version of this norm (with the same name) is what the industry uses today for cement slurries lab evaluation: API RP 10B-2 (ISO 10426-2).

Along the years, the API continue to develop standards to cover other aspects of the cementing process, below is a list of the recommended API and ISO practices and standards related to cementing lab testing as they exist today:

• API RP 10B-3 (ISO 10426-3) Recommended Practice on Testing of Deepwater Well Cement Formulations
• API RP 10B-4 (ISO 10426-4) Recommended Practice on Preparation and Testing of Foamed Cement Slurries
• API RP 10B-5 (ISO 10426-5) Recommended Practice on Determination of Shrinkage/Expansion of Well Cements.
• API RP 10B-6 (ISO 10426-6) Recommended Practice on Determining the Static Gel Strength of Cement Formulations.
• API 10A Specification for Cements and Materials for Well Cementing.

Now, are all these standards up for use every time you prepare for a cement job? The short answer is: No. 

Two of these standards aim at certain types of operations:

• Standard 10B-3 is specific to wells in deep-water environments, addressing the cool-down effect caused by low seabed temperatures.
• Standard 10B-4 is specific for wells where foam cement will be used. Foam cement is used to address cementing across low frac gradient formations.

The other two standards are intended to address wells that suffer from two particular kinds of downhole conditions:

• Standard 10B-5 aims to measure and control cement changes in volume after setting. Typically measured in wells where expanding agents were added to the cement slurry (to obtain about 1% radial expansion after setting and sealing micro-annulus) or wells that are susceptible to the creation of micro-annuli due to the cement natural contraction (4 to 6%).
• Standards 10B-6 is intended to evaluate the slurry ability to provide gas migration control. The test assesses the ability of the slurry to control gas percolation during the transition time (change from liquid to gel/solid). During the transition time, the slurry loses its ability to transmit hydrostatic pressure to the formation when gels exceed 100 lbf/100ft2 making it susceptible to gas invasion. Then, when gels reach 500 lbf/100ft2, the slurry regain enough resistance to prevent gas migration. If the transition from one gel strength to the other happens quickly enough (< 45 mins) the risk of gas migration is considered negligible.

In my next article, I will advice you on test procedures that apply to these standards.

Read more: Cement plugs: A routine or a nightmare?