by Michael Mims, President/CEO of K&M Technology Group

This is the second article in a short series that will address cementing issues, from a well construction engineer’s perspective, for extended reach and complex well designs. The previous article addressed primary cementing concerns from a mechanical standpoint. This article will continue on the subject of Primary Cementing, however, we will look at cement slurry designs and advanced placement techniques for these slurries to maximize their effectiveness in these difficult wellbore environments.

A key aspect to successfully designing a primary cementing program for high angle wellbores is to fully understand the impact of the internal and external flow regimes. Inside of the pipe, the classic “bullet” flow profile and varying fluid viscosities can act to pump the middle out of fluid sections if they are not mechanically separated. In the annulus, the casing will not be concentric and fluid flow is going to take the path of least resistance. This will create areas of little-to-no flow around the casing and present major challenges during both the hole clean-up and cement placement operations. These problems become more severe as the hole sizes and casing sizes increase, making surface and intermediate casing strings the most challenging.

Further complicating these operations has been the introduction of mandrel casing hangers to the industry. The inherent risk of sticking the pipe off of bottom, along with the operational difficulties associated with emergency slip systems makes the likelihood of using pipe movement during the cementing operation very small (even if you can move the pipe). Therefore, the use of spacer trains (as discussed in the previous article), a properly engineered cement slurry and the use of an appropriate placement technique is critical to the success of the operation.

Cement Slurry Design

Our team has worked on design teams for most of the major operators over the past 10 years in various parts of the world. It has been a rare occasion where the cementing designs have received the amount of engineering attention that is warranted in these difficult wells. In fact, in recent years, the service companies have been given more and more of the responsibility for technical support and in many cases they have been ill-prepared to deal with this added responsibility. The result has almost always been a cementing program that simply looks at pipe centralization, thickening times and water loss and then the program is sent to the field.

Slurry design has several key elements that must be considered in detail when designing a cementing program for ER or complex wells:

• Bottom Hole Temperature is commonly overestimated by using either production data or data from MWD tools while the hole is being drilled. During the cementing operation, cold fluid is being pumped downhole ahead of and during the cementing operation. This creates slurries with thickening times that are much longer than necessary.
• Cement Quality – Class G cement is an extremely small portion (~1%) of the cement that is manufactured in the world. In order to make Class G cement, the plant must clean-out its construction grade cement and run a production of Class G. We’ve found many instances when cement has not met API specifications. Transport method and storage time will also have an effect of cement quality.
• Ability of additives to work in the expected temperature environment – Many of the products being used in the industry today is extremely temperature sensitive. Sodium silicate extenders are a good example of this phenomenon. When slurries are designed with overestimated BHTs, the effect of these products on fluid properties and thickening times can be misrepresented. It is relatively simple for tests to be run over an expected range of temperatures to ensure the applicability of the products being used.
• Size of the Slurry – ER wells often run strings of casing that require all of the cement on-board to be mixed and pumped. These very large slurries are often designed with the same properties even though their pump times may be hours from start to finish. A properly designed cement job will set from the bottom, up to ensure that hydrostatic pressure is maintained on the lowermost slurry as it develops gel strength. This may not be the case if a very large slurry is being pumped with the same design properties.

Walking Squeeze Technique

The primary goal of most surface and intermediate casing cement jobs is to get a competent shoe. This has proven very challenging in large casing/hole sizes and/or when pump rates are restricted due to rig capability.

With the pipe being pulled to the low side of the hole by gravity, most of the fluid flow will occur on the high side of the annulus. This makes the removal of cuttings and sludge on the low side of the hole very difficult and can create a low-side channel. The high angle of ER wellbores also makes getting complete displacement to the top of the annulus difficult which can create a high-side channel.

The “Walking Squeeze” is a technique that was developed in order to combat the problems with high and low-side channeling. This technique requires that the tail slurry be designed with a realistic thickening time equal to the pump time of the slurry. The cement is mixed, pumped and displaced using standard techniques until approximately 20 bbls remains in the displacement. The pumps are then slowed to 1 bpm and the displacement is completed at this very slow rate.

The concept is that the tail cement that is already around the shoe when the displacement is slowed it will begin to develop gel strength. The remaining cement will be displaced into the path of least resistance, which should be the high and low-side channels in the annulus. Walking squeezes are now a common practice in our client wells and they have virtually eliminated remedial shoe jobs.

The final article in this series will address liner running and cementing, remedial cementing and cement plugs as they apply to ER and complex wells.

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(as published in the Chevron newsletter)