During a drill well on paper phase, wells might be planned based on available data from offset wells or nearby areas. This information may deviate from the actual drilling phase due to several reasons therefore it is very critical to have good monitoring and evaluating pore pressure while drilling. The actual pore pressure will dictate where to set the casing, how much the actual mud weight should be, what potential problems are, etc.
The actual reservoir pressure obtained while drilling will be used to determine right mud density in order to ensure an adequate primary well control. Besides the well control, the proper mud weight can help us in several aspects such as minimizing lost circulation and pipe sticking and maximizing a rate of penetration. You can achieve the trouble free drilling operation.
The following indicators can demonstrate abnormally pressured reservoirs;
- Higher porosity
- Higher permeability
- Higher sonic velocity
- Lower shale density and resistivity
- Lower water salinity in formation
The concept of evaluating pore pressure while drilling is to measure the changes in these properties which vary with different lithology and area of drilling. You need to take into account about the lithological variation when interpreting data. Moreover, measuring of these parameters can be used to quantitatively estimate. It will give you the relative information when compared to the overall trend.
Let’s take a look at shale density as an example. The shale density is typically denser over depth; however, a high pressure zone tends to have higher porosity and lower bulk density because the reservoir pressure inside will push the rock grains. While drilling, the shale density should be plotted against the depth because you can see the normal trend. When you see decrease in the shale density trend, it shows you that the abnormally pressured zone is there (see Figure 1).
Figure 1 – Shale Density Plot
Nowadays, Logging While Drilling (LWD) tools are widely used because they can measure reservoir data and send to the surface real time. This will help us closely and accurately monitor the well to see any changes in the parameters. These tools can assist you to make adjustment in mud weight while drilling effectively. If you want precise reservoir pressure, some service companies can provide formation tester service while drilling. It might take time but you will know exactly where you are and how accurate of the pore pressure curve.
Surface real time logging is another way to evaluate the pore pressure and there are some mud logging companies also provides pore pressure analysis. These following changes can be a possible high pressure zone;
- Abnormal change in ROP
- Increase in return line temperature
- Increase in drilling drag and torque
- Increase in cutting size and shape
- Increase in trip, connection and/or background gas
- Decrease in d-Exponent
Conclusion: Monitoring and evaluating the formation pore pressure is the key to achieve drilling free operation especially in unknown areas where you have the limited information. With real time data monitoring both downhole and surface, you can detect changes very quickly and the right decision regarding the mud weight can be made on time. Always listen to the well and you will be fine.
Reference books: 
Well Control Books