THE VALUE OF USING MICROFINE CEMENT IN ENGINEERING WELLBORE DESIGNS

by Don Getzlaf, Microblend Solutions Inc.
CADE eNews
April 2006

Have you ever experienced gas migration to surface on a large casing string and a bond log confirms your hypothesis that a microannulus exists within your wellbore? You attempt a standard cement squeeze and after numerous attempts the gas pressure still exists.

Have you been drilling a surface casing and encounter a sand stringer and sand and water continues to flow into the wellbore during the drilling operation? You attempt a cement plug, but after drilling out the plug the sand and water continue to flow.

What caused the failures? What engineering principles could I have used to be more successful?

The author struggled with solving these same problems over the years and what technologies were available that gave an environmentally sound engineered approach that assured success on a consistent basis. The search included both the application of models from the grouting industry, as well as their understanding of ground water and the chemical reactions that the ground water has with various products. Next came how these models and reactions can best be applied to the above problems and other similar problems in the drilling operation.

For the purpose of this presentation, we will focus on presenting the models and how to use them in the above applications.

Formulas

The grouting industry has vast experience on the ability of sand/silts and fractures to accept products of various sizes and gradations. From their research comes two formulas that aid the design engineer to optimize placement. The first formula is for injection into a fracture.

frac width/d95 grout with a grout ratio of > 5 normally you should be able to inject (formula 1)

The second formula is utilized to estimate the injection of cements into the formation matrix.

d10 soil /d95 of grout you can easily grout where this ratio is > 11 and if it is < 6 it is very difficult (formula 2)

The formulas can now be utilized to calculate a minimum frac width and formation particle size that will allow injection.

The formulas for the Class G cement and microfine cement equate to the following minimum fracture width and minimum formation sizes to allow for both fractured and matrix permeability.

Various generic particle products can fit into the above formulas and can be presented graphically as figure 1. since liquids do not fit the formulas above they are added from field experiences to represent their relative injectivity.

When application, environmental stability and longevity are all considered one material consistently is utilized in situations where normal Portland materials cannot be applied successfully. This material is microfine cement.

Although no specific definition for microfine cement exists the general definition is two-fold. The first part of the definition specifies that the particle size is approximately 10 times finer than the particle size of normal cement. The second part of the definition is that a substantial portion of the cement particles must be less then 10 um. Microfine cements are rarely manufactured from pure Portland cement and normally contain a mixture of Portland cement and a pozzalonic secondary additive. A very large portion of all microfine cement produced is utilized in the prevention of unwanted water movement in mining, water control structures (dams spillways and canals) and tunnels. Another major use of microfine cements is in injection into sand, silts and cracks to stabilize building structures at risk. The oilpatch use of microfine cements has been overlooked by manufacturers for years, because of the relatively small market size and the high demands the market places on product quality over a broad temperature range.

Microfine cement has gained a lot of interest in the past few years. This can be attributed to:

Understanding of environmental risk of other injection products.

Over the last 50 years various families of silicates and polymers have been used to inject into micro systems such as cracks, sands and silts. Today use of these products is being scrutinized on their potential reaction with fresh water reservoirs as well as their questionable permanence. Normal Portland based cement including microfine cements, however, are the standard for a permanent environmentally sound hydraulic barrier in all industries.

Understanding the environmental risk of failure to control water flows or gas flows.

Uncontrolled water or gas flows can result in contamination between normally unconnected reservoirs. A large percentage of cross flow between zones occurs through narrow annular channels and as the formulas above demonstrate microfine cements can be very effective in sealing off these channels.

Increasing the value of your asset.

In the past having water flow past a dam into the river below wasn't very important. Today the value of water to produce power and support civic projects is significant and all attempts are made to reduce water leakage. New understanding of how soils react under various loads can determine if a building is at risk. Today many buildings in earthquake zones are stabilized to assure risk is reduced. With very few new reservoirs being found and pressure to maximize the value of reserves in place, it is imperative to assure hydraulic isolation of your assets is maintained. Microfine cements are increasingly being utilized to assure assets keep their value.

Various techniques are utilized in manufacturing microfine cements and product quality is a direct result of production technique.

The most common and cost effective method is to extract dust from cement plant and then add a secondary pozzalonic product. This process has the advantage of being less costly, but has a huge disadvantage in that the finer materials do not necessarily represent all the components of cement powder. Softer materials that grind easier also are extracted more easily. This inconsistent process leads to varying product quality and set times.

The second method is a vibratory ball mill with classification. This is a high energy process with very small output volumes. Due to the process, the ground materials are generally similar to any process where one product is rubbed against another to create a round or oval product. This leaves very little reactive surface area and thus poorer strengths.

The third process utilizes an impact grinder that takes all the cement products and shatters them creating a high surface area reactive cement product. The cement and additives are pre-blended and then injected into the grinder creating a continuous uniform material. The uniformity allows for successful placement in broad temperature ranges and applications.

Why Microfine?

In the drilling operations when is microfine cement utilized?

1.Stabilizing wellbores.

Since microfine can go directly into a gravel, silt or sand, it can stabilize the formation from within. Microfine cement is injected directly into the formation and allowed to set. Once set, the formations permeability is reduced and strength increased.

2. Water inflow or gas inflow.

Most cement squeezes designed for water or gas shutoff simply bridge the formation face and do not inject into the fractures of permeability where the water/gas is originating from. Microfine cement, with its small particle size, can be squeezed directly into the rock, thus shutting the water/gas off at its point of origin, permanently repairing the problem.

3.Wellbore or casing leak.

Microfine cement is utilized for squeezing casing or wellbore bleeds or leakoff. In most cases when minor leaks are encountered and must be dealt with, materials such as epoxies are very problematic to place and normal Portland cement is too coarse to fill the small leak. Microfine cements is fine enough to inject into the leak and can be placed in the wellbore in a normal cement plug technique.

In the case of injecting into a microannulus, one must first calculate the potential annular gap. An annular gap is formed when either the casing is expanded and contracted or the annular material shrinks back from the casing. The expansion and contraction of steel is very well understood and this may occur at any time during the life of the wellbore. Once a casing is set, the biggest influence on the annular diameter is the change in pressure or change in temperature. A typical expansion or contraction curve is demonstrated in figure 2 based on pressure changes and figure 3 for temperature changes.

Let's go back to the scenarios in the introduction and see how we can apply the formulas and make see why our initial recommendation may not have been successful.

Scenario A:

A 244.5 mm casing is cemented in place and prior to drilling out the casing shoe, the casing is pressure tested to 25 Mpa and shortly after the pressure test gas is encountered on the annulus.

Solution:

From figure 2, the casing expansion would have been 1 mm during the pressure test. In order to penetrate the microannulus, we need to calculate the maximum particle size that would be able to penetrate the microannulus. Using formula 1 for crack or slot penetration, we divide the expansion by 2 to obtain an average slot width on each side of the casing. Now the 0.25 mm is divide by 5 to obtain a maximum particle size that can be penetrate into the annulus. The final value is now 0.05 mm or 50 um.

Class G cement with a D95 particle size of 100 um would not penetrate this microannulus and would bridge fairly readily. Microfine with a D95 less then 15 um would penetrate the microannulus and placement could be accomplished.

Application:

The well would be perforated and a cement injection placed into the microannulus. Since microfine cement is not designed to bridge against the formation, a given volume would be placed at the lowest pressure possible and the cement left to set for the minimum set time. In some circumstance, the microfine would be followed with normal Portland cement which would bridge at the perforations to assure displacement is obtained.

Scenario B:

While drilling a surface casing a flowing sand is encountered. It is determined the D10 of the sand is 220 um.

Solution:

From formula 2, in order to inject anything into the sand, the D10 to D95 ratio of the sand must be greater than 11.

Thus, 220 um divided by 11 is 20 um. In order to stabilized the sand, the maximum (D95) size of the particle to be injected must be less then 20 um. Normal Portland cement with a maximum particle size of 90 um would bridge against the formation face and not stabilize the sand. Microfine cement with a maximum particle size less the 15 um would penetrate the sand and stabilize the formation sand.

Application:

The drillpipe would be placed above the flowing sand and microfine cement injected directly into the sand. The cement would be displaced with wellbore fluid and left to set. The injection would be placed at a low rate to assure the cement injects into the sand in a radial pattern.