*1. What is casing pressure in oil and gas?**
Casing pressure refers to the pressure trapped in the annular space between two concentric strings of casing or between the casing and the tubing. It's measured at the wellhead. Monitoring this pressure is critical for well integrity; unexpected pressure buildup can indicate issues like leaks in casing strings, tubing, packers, or sustained casing pressure (SCP) from formations communicating with the annulus.
**2. What is the difference between casing and tubing in oil and gas?**
Casing is large-diameter pipe cemented in place during well construction to line the drilled hole. It stabilizes the wellbore, isolates formations, and contains pressure. Tubing is a smaller-diameter pipe run *inside* the casing through which oil or gas flows to the surface. It's removable for maintenance, while casing is permanent. Tubing handles produced fluids, while casing provides structural integrity and zonal isolation.
**3. How is well casing measured?**
Well casing is primarily measured by its **outer diameter (OD)**, specified in inches (e.g., 4 ½", 9 ⅝", 13 ⅜"). This nominal OD is the standard reference size. The **inner diameter (ID)** is also important and is calculated by subtracting twice the wall thickness from the OD. Casing length is typically measured in joints (usually around 30-40 feet per joint) or total measured depth.
**4. What is the thickness of casing?**
Casing thickness (wall thickness) is not a single value. It varies significantly based on the casing size (OD), the specified **weight per foot** (lb/ft), and the **steel grade** (e.g., H-40, K-55, N-80, P-110). Heavier weights per foot and higher-strength grades correspond to thicker walls for a given OD. Thickness is precisely controlled to meet API standards for required strength (collapse, burst, tension).
**5. How do you size a casing pipe?**
Sizing casing involves selecting the appropriate **outer diameter (OD)** and **weight/grade** for each section (conductor, surface, intermediate, production) based on:
* **Hole Size:** The casing OD must fit inside the drilled hole, allowing space for cement.
* **Depth & Pressure:** Deeper sections with higher formation pressures require larger OD, heavier weight, and higher-grade casing to withstand collapse (external pressure), burst (internal pressure), and tension (string weight).
* **Future Strings:** Each casing string must allow sufficient clearance inside for the next smaller bit size and subsequent casing string to be run.
* **Well Purpose:** Production casing sizing must accommodate the desired tubing size and downhole equipment.
* **Standards:** API specifications define dimensions, weights, grades, and performance properties.
