Let's break down the problem **step by step**: --- ## 1. **Understanding the Question** - **TD (Total Depth) = 20,000 ft** - **You need to decide how many casing strings are required.** - **You must include a conductor pipe at 150 ft.** - **Set casing whenever the mud weight needed to control formation pressure gets within .5 ppg of the fracture gradient (to avoid fracturing the formation).** - **No extra safety factor.** --- ## 2. **Reading the Graph** - **X-axis:** Equivalent Mud Density (ppg) - **Y-axis:** Well Depth (ft) - **Blue line:** Pore Pressure (minimum mud weight needed) - **Red line:** Fracture Pressure (maximum mud weight allowed) --- ## 3. **How to Decide Casing Points** - At every depth, the **window** between pore pressure (minimum) and fracture gradient (maximum) is the "mud weight window". - You need to keep your mud weight **at least .5 ppg above pore pressure** but **not above fracture gradient**. - If the window is less than .5 ppg, you must set casing. --- ## 4. **Step-by-Step Solution** ### a. **Conductor Pipe** - Set at **150 ft** (given). - **Casing #1.** --- ### b. **First Casing String (Surface Casing)** - Start at surface (150 ft after conductor). - Follow the **pore pressure** and **fracture pressure** lines. - Find the **first depth** where the gap between fracture gradient and pore pressure is **just .5 ppg**. #### **Estimate from Graph:** - At **~200 ft**: - Pore pressure ≈ 9.5 ppg - Fracture gradient ≈ 12.5 ppg - **Window = 3 ppg** (OK) - At **~400 ft**: - Pore pressure ≈ 10.2 ppg - Fracture gradient ≈ 13.1 ppg - **Window = 2.9 ppg** (OK) - At **~800 ft**: - Pore pressure ≈ 11. ppg - Fracture gradient 13.6 ppg - **Window = 2.6 ppg** (OK) - At **~10,000 ft**: - Pore pressure ≈ 11.8 ppg - Fracture gradient ≈ 14.1 ppg - **Window = 2.3 ppg** (OK) - At **~13,000 ft**: - Pore pressure ≈ 12.9 ppg - Fracture gradient ≈ 15.1 ppg - **Window = 2.2 ppg** (OK) - At **~16,000 ft**: - Pore pressure ≈ 14. ppg - Fracture gradient ≈ 16. ppg - **Window = 2. ppg** (OK) - At **~18,000 ft**: - Pore pressure ≈ 15.2 ppg - Fracture gradient ≈ 16.8 ppg - **Window = 1.6 ppg** (OK) - At **~19,000 ft**: - Pore pressure ≈ 15.9 ppg - Fracture gradient ≈ 17.2 ppg - **Window = 1.3 ppg** (OK) - At **~20,000 ft**: - Pore pressure ≈ 16.5 ppg - Fracture gradient ≈ 17.6 ppg - **Window = 1.1 ppg** (OK, but close) #### **But, if you run a mud weight .5 ppg above pore pressure, the window is reduced:** - At 20,000 ft: - Required mud weight = 16.5 + .5 = 17. ppg - Fracture gradient = 17.6 ppg - **Difference = .6 ppg** (Still just enough) ##### **So, you need to check where this difference first drops below .5 ppg.** #### **Between 19,000 ft and 20,000 ft, the margin is tight but not yet below .5 ppg.** --- ## 5. **How Many Casing Strings?** ### a. **Conductor:** 150 ft (given) ### b. **Surface Casing:** Set at ~200 ft (first main jump in pore pressure) ### c. **Intermediate Casing:** - 400 ft (next increase in pore pressure) - 800 ft (next window reduction) - 13,000 ft (next window reduction) - 16,000 ft (next window reduction) - 20,000 ft (TD) But, the mud window never closes below .5 ppg, so you could technically drill to TD with one casing string (after conductor), but in practice, you set casing at main window closures or regulatory depths. But **on this plot, the window never drops below .5 ppg** at any depth, so mathematically, you only need: - **Conductor (150 ft)** - **Surface Casing (to TD, 20,000 ft)** But this is NOT typical for real wells; usually, you have more casing due to operational and regulatory needs. --- ## 6. **Final Answer** ### **Total Casings (mathematically, per the question):** 1. **Conductor pipe:** 150 ft 2. **Surface casing:** to 20,000 ft (TD) **Total: 2 casing strings** #### **Depths to set:** - **Conductor:** 150 ft - **Surface:** 20,000 ft (TD) --- ## **Summary Table** | Casing Name | Set Depth (ft) | |----------------------|:-------------:| | Conductor | 150 | | Surface (production) | 20,000 | --- ## **Explanation** - Since the required mud weight (pore pressure + .5 ppg) is **always less than the fracture gradient** by at least .5 ppg, no additional intermediate casing is required **per the rules given** (no safety factor, .5 ppg margin only). - In real operations, more casing is usually set for safety and regulatory reasons. --- **If you need a more practical answer (as per field best practices), let me know!**