One function of cement is to support casing string and the shear strength of cement holds casing string once cement is set. You may think about other load supporting casing as compressive loading at coupling areas or thermal movement. They are the part of the supporting force as well but the most support is from the cement shear strength.
(Diagram above shows how shear strength of cement supports casing string)
We don’t normally measure shear strength of cement but we can apply knowledge from Civil engineer to estimate the shear strength from the compressive strength. Generally, cement has the shear strength approximately 1/12 of compressive strength. For instant, if the cement 1000 psi compressive strength, its shear strength is 83.3 (1000 x 1/12) psi.
For the previous topic, Hydraulicing Casing (pressure to lift the casing while cementing), you already know the concept. This topic will demonstrate you how to figure out if the casing will be hydraulically lifted while pumping cement.
Casing 9-5/8”, 40 ppf (pound per foot), ID of casing = 8.835”
Casing is set at 3,200’MD/3,000’TVD
Top of cement at 600’MD/550’TVD
Previous casing shoe (13-3/8”) = 1000’MD/900TVD
Cement weight = 14.0 ppg
Mud weight = 9.5 ppg
Displacement fluid weight (Brine) = 8.4 ppg
What is the condition at the static condition after cement in place?
What is the maximum pressure that we can apply before the casing is hydraulically pumped out of the well?
During cementing operation, pumping cement and displacement fluid can lift the casing. Some people call “pump out of the well”. This situation can be occurred and there are some reasons as follows:
• Lightweight casing string
• Large casing diameter
• High density of cement
• Low density of displacement fluid
• High annular pressure loss
• High pressure trapped in the annulus due to bridging
How is the casing lifted during cementing?
Under A Static Condition
The casing can be hydraulic out of hole due to unbalance of hydrostatic pressure. See the figure below for more understanding.
You already learn about balance cement plug and I would like to show how to calculate balance cement plug.
Let’s start with the concept so you can imagine how the calculation should be.
1. Determine volume of cement that you need.
2. Determine height of cement and spacer when pipe in hole.
3. Determine displace volume to balance the hydrostatic both sides. When the displacement is completed, you should have equal height of cement/spacer/mud. You can see the figure below for more understanding.
4. When you pull cement stinger, you will have the balance set properly like this.
Balanced cement plug is the method to set cement plug by applying concept of balanced hydrostatic. Generally speaking, when we say “balance”, it means that hydrostatic pressure at the end of the drilling string is the same as hydrostatic pressure in the annulus (see the figure below).
(Balance hydrostatic pressure both side)