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

Extended Reach “primary” operations are already difficult enough without stewing “remedial” operations into these wells. That statement has never been more correct than for our cementing operations. Obtaining a good primary cement job on a long string of casing in an extended reach wellbore is a considerable challenge, especially if zonal isolation is critical to the well’s success. The same is true for setting plugs in extended reach wells.

Add oil based mud (or one of the many synthetics) to the equation and the degree of difficulty has now more than doubled.

This will be the first in a space-available number of articles, which addresses cementing issues such as primary cementing, remedial cementing and setting cement plugs in extended reach wells. These articles will address issues such as:

Preparing the well to run casing

• hole cleanup measures
• the use of backreaming
• thinning the mud down prior to POH to PU casing

Running the casing

• ECD’s while running pipe (critical if pipe is floated)
• maximum running speeds
• centralization strategies and new technologies
• casing jewelry and float equipment

Preparing the hole for cement

• the importance of circulation time and rate
• the use of spacer trains to clean up oil based mud, filter cake and sludge
• maintaining the integrity of components that are pumped downhole
• maximizing the effectiveness of hydraulic and chemical forces downhole

Cementing

• design the slurries to set from the bottom up
• getting a good handle on BHT’s and design realistic thickening times
• look at ECD’s and lost circulation. Design options to overcome ECD problems.
• use of ultra-lite scavenger lead slurry as final prep for primary slurries
• designing long slurry intervals in parts (varying set times)
• design tail slurry with a set time equal to pump time
• use of “walking squeeze” technique to ensure a good shoe job
• Design options and post job considerations for dealing with gas isolation

Keep in mind that this series is being written by well construction engineers, not by cementing engineers. No theory will be presented, simply a representation of field results from various clients around the world.

Primary Cementing

The success of a primary cement job on a long string of casing that requires zonal isolation in an extended reach well depends on a number of factors:

• hole condition and cleanliness
• formation integrity
• drilling fluid type and density
• plans to drill below that casing shoe (pipe size)
• pressure differentials between zones
• whether gas also has to be isolated along with oil and water
• rig mixing, storage and pumping capacity
• quality of the cement
• ability of the cementing unit to handle complex cementing operations

Preparing the hole to run casing and then the subsequent preparation of the hole to accept the cement job are two critical elements contributing to the success of the final job. In extended reach wells, pipe is often set off of bottom due to a plug of cuttings being pushed ahead of the casing. This has proven especially true for wells >70 degrees where pump rates at TD are heavily compromised. In these cases, backreaming out of the hole (with great care) has been an effective means of eliminating cuttings beds problems during both the casing run and the subsequent cement job. This is a time consuming and potentially hazardous operation and rig personnel should be given strict procedures to ensure that stuck pipe doesn’t result during this operation.
If Equivalent Circulating Densities (ECD’s) are expected to be a problem during the casing run and/or the cement job, then a trip back to bottom after backreaming may be warranted to thin down the mud. This will help the mud to flow by the casing easier and make it easier to pump once the casing is on bottom, thus reducing the mud’s contribution to the cement job’s ECD’s.

Once the casing string is on bottom, the one certainty in most cases is that pipe movement will not be possible. Therefore, more detailed engineering will be required to obtain a successful cement job. The first important point to determine is what forces are available to aid in the cementing process? Assuming that the pipe cannot be moved, mechanical forces are out. That leaves, hydraulic forces, chemical forces and pressure as the remaining means of affecting the cement job.

The second step to a successful cement job will be to establish the goals of that job. In this case, they will likely be:

• clean the wellbore of all cuttings
• remove the oil based mud from the pipe and wellbore
• mix and place an engineered slurry that is fit for purpose
• maintain a favorable environment in the wellbore while the cement is curing

This author will not debate the need for proper centralization across critical intervals for the cement job. Centralizer type and quantity should be chosen carefully because the addition of centralizers does stiffen the casing string and make it more difficult to get into the ground. Our client’s most recent experiences with roller centralizers seems to suggest that they may be “drag neutral”, meaning that the increase in pipe stiffness is almost exactly offset by their reduced drag feature. If this result continues to repeat itself, then their use will provide positive centralization to the casing string without adding any additional drag. In any event, their use should be restricted to those areas of the string where cement quality is critical. With the pipe properly centralized we will be able to maximize the effectiveness of our hydraulic and chemical forces downhole.

In order to effectively utilize the hydraulic forces that we have available to us, we must take advantage of:

• turbulent flow
• plug flow
• fluid properties

The flow properties will be controlled by high pump rates and varying fluid viscosities. The fluid properties that will vary will include the already mentioned variable viscosities in addition to density, chemical make-up and gelling characteristics.

In order to effectively combine the hydraulic and chemical forces, spacer trains have been utilized prior to cementing operations. These trains act to clean the hole through the use of turbulent flow with low viscosity surfactant spacers combined with weighted and sometimes fibrous spacers in plug flow placed in the well in an alternating fashion. The goal of the chemical forces is to achieve maximum contact time across the critical areas of the wellbore in order to dissolve the sludge and filter cake. The goal of the hydraulic forces is to physically remove the oil based mud, filter cake and sludge from the wellbore.

The spacer train is generally mixed and pumped just ahead of the primary cement job and is often separated by multiple bottom plugs (maximum that this author has seen used is 9 bottom plugs).

Subsequent articles will address cement slurry designs, advanced placement techniques, cementing plug designs and placement and well/casing design options to maximize operational efficiency.

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(as published in the Chevron newsletter)