When cement is in transition period (forming the bond), you will lose some hydrostatic pressure because cement becomes solid phase, therefore only water in the cement will provide hydrostatic pressure. In many cases happened, there is no issue while performing the cement job, however; once the cement is set after period of time, there is a casing pressure indicating that there is hydrocarbon in the annulus.
This example will demonstrate you why the well is in underbalance condition while waiting on cement.
Well information: Previous 9-5/8” casing shoe at 4000’ ft.
The vertical well (8.5” hole) is drilled to TD at 10,000ft with 12.1 ppg mud and the pay zone is at 9,800’ TVD with reservoir pressure of 11.6 ppg. The cement is planned to cover 3,500 ft in the annulus above the casing shoe. Water used to mix cement is 8.3 ppg weight.
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)