Centralization Myth, Centralization Myth!

Question, does the industry truly calls for 70% standoff as an accepted recommendation? Has it been recognized as an recommended practice in the Oil and Gas Industry? Or is this just a common practice due to not understanding why 70% standoff has been termed as an acceptable standoff value in the industry for so long. Lets start by defining "Standoff" What is standoff? Standoff is defined as the smallest distance between the outside diameter of the casing and the wellbore.

Well today we are going to address the reality of fact and fiction when it comes to recommendations made concerning standoff percentages for proper centralization. The reality is, there isn't a industry standard that specifies the actual standoff percentage required for proper centralization. Just know centralization is an art form more than a scientific fact.

*NOTE* The overall standoff depends more on spacing of a centralizer than restoring force. However, it is recommended to achieve the highest standoff possible without compromising ECD values. Why? For starters we know there isn't a such thing as a perfect circular drilled open hole section, its safe to say its more of an oval shape due to tectonic forces and such.

So as we work toward our goal of obtaining adequate centralization, we will fight to overcome the effects of eccentricity. Eccentricity?? Eccentricity determines how off centered a tubular is within another tubular or open hole. It's usually expressed as a percentage (%). For an example a tubular would be considered to be 100% eccentric if it were lying up against the inside diameter of an enclosing tubular or open hole.

On the other hand, a tubular would be considered concentric (0% eccentric) if it were perfectly centered in the outer tubular or open hole. In other words, A perfect circle has an eccentricity of zero, which means its a 100% concentric. Adequate standoff plays a huge factor when it comes to eccentricity, simply because casing, drill pipe and other tubulars has the tendency to lie up against the wall of the wellbore caused by eccentricity values greater than zero.

This will result in low velocity fluid flow which effect mud removal and displacement efficiencies. So hopefully we can see why proper centralization is important in a wellbore. *NOTE* Eccentricity has greater effects in high inclination wellbores.

After knowing this, could you imagine without any sort of standoff what it would look like downhole? In my opinion some standoff is better than none at all. So higher is better to a degree. But can we really say 70% standoff is adequate and should we accept it as a recommended industry practice?

However API doesn't even specify it as a industry recommended practice because there is no mentioning of it in API 10TR4 or API 10D Spec. So my question is, where did this number come from? Does anyone know?

I believe a lot of us heard it so much throughout our careers, we just conformed and accepted the idea as a general practice. We say the more standoff the better, right? Well one might argue the fact and others might agree, but I disagree to an extent and probably so should you.

Ideally we would think in this manner because of the idea to have perfectly centered casing in the wellbore is a good thing, but in reality this is far more difficult to achieve than we think. Especially taking in consideration variables such as displacement efficiency, friction pressures, ECD's, wellbore geometry, pump rates and etc. these all have an affect on centralization and standoff values.

Benefits of Centralization:

• To help get casing to bottom (TD). It helps reduce potential possibilities of differential sticking.
• To help provide uniform wall and annular clearances to achieve optimum fluid flow paths to increase mud removal and displacement efficiency.
• Reduce the effects of channeling.
• Reduce drag forces while running casing.

Here is something you might find surprising, and also quite shocking. The idea of 70% standoff being a recommended industry practice comes from a testing parameter of a centralizer in API. Surprised yet? The testing parameter is used to determined the restoring force of a bow spring centralizer which is tested at 67% standoff using a total of 12 compressional bend without failure and load deflection.

Therefore this is where the myth originated and rose to fame, interesting huh? I thought so myself when I was informed. Still don't believe it? Well, just have a look yourself and research it.

Testing Parameters for Restoring Force according to API:

• Flex each spring 12 times.
• Record load deflection in 1/16 inch to 67% standoff.
• Average load deflection for both test positions.
• Restoring force - load deflection at 67% standoff.

Now lets take a look at a few definition concerning centralization:

1. Starting Force - represents the maximum force required to insert the inner pipe into the outer pipe. Also known as the force require to compress a centralizer in the previous ran casing body. API requires the "Starting Force" of a centralizer is less than the average weight of a 40' joint of casing.
2. Restoring Force - represents the force exerted by a centralizer to keep it away from the wellbore wall. Also known as the force exerted by the centralizer perpendicular to the casing pushing it away from the wellbore.
3. API Restoring Force - Required amount of restoring force under 67% (Bow Spring Centralizer) deflection to be API certified.
4. Running Force - is the maximum force required to move a centralizer through specified wellbore under load (weight above). To simplify, is the amount of drag resistance exerted by a centralizer under load.