Cementing a primary casing involves many aspects that the cement engineer has no control over unless he/she is consulted from the start of the project. It is very important that the Drilling / Mud and cement engineer work together from the start to maximize the chances of a good cement job. Here are the main issues:
Hole Size Vs Casing Size
Hole Size Vs Casing Size – To maximize displacement rates and increase velocity of fluids in the annulus a smaller hole size is required however if the hole size is to small compared to casing then the increase n ECD can be prohibitive and CAN lead to fracturing of rock and partial or total loss of cement. As a rule of thumb the optimum hole size is 1.5 to 2.5 inches larger than casing, however a simulator needs to be used to effectively plan casing and hole size.
The hole size and casing size is typically decided by the drilling engineer, if at all possible it is very important for the cement engineer to get involved in the planning phase of the well with the drilling engineer. Below are two examples run in a simulator, the first is a 7-5/8″ casing in a 9-5/8″ hole, the second is a 7-5/8″ casing in a 10-5/8″ hole:
As on can see the difference a one inch increase in hole size increases our pump rate by roughly 60%, to get similar results. Getting these rates might not be possible depending on the equipment available.
Centralization: This is one of the most important parameter when it comes to primary cementing. It has been found through different forms of testing that approximately 67% standoff at a minimum is need to keep channeling at a minimum. The other parameter of high importance is mud that I will cover later.
Circulation efficiency is defined as the difference between hole volume (which we get from a caliper ) and volume of liquid measured to circulate the hole. In other words a circulating efficiency of 1 means 100% of mud in hole is moving. A 0.5 efficiency means only 50% of mud is being moved.
These graphs illustrate the Circulation efficiency for standoffs of 50,67 and 100%. On the X axis we have how many annular volumes that were circulated and on the Y axis is the Circulation efficiency.
The graph below shows Standoff on the X-axis and channeling on the Y axis . (The higher the standoff the more centralized, 100% standoff means perfectly centralized )
The results are from full bore model studies and field applications where segments of cemented casing were cut and examined for cement annular fill and bonding. In all cases, whenever centralization fell below 67%, at any flow rate, some form of channel or solids bed resulted. A minimum of 67% standoff is basically the industry standard.
Discussion on Mud Channeling Vs Standoff: As one can see that at 80% standoff and above we have no channeling, below 60 we have channeling at majority of points. What does this mean? It is very hard to get a good cement job with insufficient centralization. (a minimum of 67% is recommended). Work with the provider of Centralizers to achieve 67% standoff. Most service providers have a centralizer program, however these can be limited due to limited centralizer database. Work with drilling engineer and centralizer provider to calculate standoff, this information is a must to run a simulation. For more information on centralizers click here
Conditioning of mud
Before cementing the mud company will “Condition the mud”, this means they will get the mud properties as optimal as possible for the cement job. Increasing circulating velocity, reducing YP (Yield Point), PV (Plastic Viscosity) and Gel strength, are some of the options mud companies have to condition mud. Typically after the casing is in place the cement company will rig up the cement head and circulate out any gas / gel and cuttings as part of conditioning of mud. Mud is designed to gel up to a certain extend to stop or slow down cuttings from falling, therefore its important to minimize the time mud is not moving, I have included some points below for conditioning mud:
1. Run mechanical caliper & fluid caliper: compare the results of mechanical and fluid caliper to ensure that we have over 90% movement of fluid.
2. Lower Yield Point (YP) and Gel strength if possible. By doing this we can minimize pockets of gel down hole and ensure more of these pockets can be circulated out, this will lead to minimization of channeling and mud contamination. The reduction of YP and Gel strength is done gradually to ensure we do not by pass the gelled pockets. Reduction of YP also benefits cleanup in that fluids will transition to turbulent flow at a lower rate.
3. As a rule of thumb we want velocity of above 240 ft/min, use a simulator to ensure proper cleanup is achieved, that we do not go above FG (frac gradient). Also use knowledge of the geological profile to minimize possibility of hole erosion.
4.Degas, Meaning circulate out all remaining gas out of the well. Any remaining gas in the mud can lead to mud density below pore gradient and influx of additional gas, which can lead to a blow out.
5. Have less than 10% Drill Solids: Make sure we have less than 10% of drilling solids remaining in the hole, slow down drilling if you need to. Use wiper trips (which is necessary especially when drilling with a mud motor and pipe is not turning), viscous pills, and reciprocation to ensure minimal cuttings in the hole.
6.Break circulation often while while tripping the bit out of the hole. DO NOT let mud sit static for long periods of time especially in HTHP wells.
7. For Cementing purposes we wish to have a mud with gel properties that are Low flat and non progressive, this means we have little gain of gel strength with time.
8. Have mud with Low Fluid Loss (FL), this will lead to a thin tough filter cake which will enhance cement bond, High fluid loss will lead to dehydrated immobile mud.
One of the most important things an operator can do to ensure a good cement job is casing movement, there are two main ways to move casing one is rotation the other is reciprocation.
The most effective way is to rotate, this can only be done with a Top Drive. Top drives are highly recommended for getting a good cement job. Not only does it help move the pockets of mud but with centralizers and scratchers it will actually break the gel strength.
Spacers and Flushes
- Have a differential weight of at least 0.5 ppg for each fluid, preferably a 1 ppg differential if Frac Gradient will allow for it.
- Design a Spacer that has a higher frictional pressure gradient than the mud. The ideal situation is to have a hierarchical pressure gradient for all the fluids, this can be achieved by adjusting the YP and PV of fluids. The below graph is generated by an excel sheet that I created. Program is available for free, use the contact me button to receive a copy.
- Consider using flush to thin mud and get turbulent flow. Some flushes such as SAPP (sodium acid pyrophosphate) will help thin and break up WBM
- Use simulators to help design spacer.
- Compatibility testing.