What are the best practices for using petrophysical data to improve wellbore stability in shale formations?

1 Understand the shale heterogeneity

Shale formations are not homogeneous, but rather consist of various layers and facies with different petrophysical and geomechanical characteristics. To improve wellbore stability, it is important to identify and characterize the shale heterogeneity using petrophysical data from logs, cores, and seismic. For example, you can use density and porosity logs to estimate the shale brittleness and ductility, which affect the fracture propagation and borehole collapse potential. You can also use resistivity and sonic logs to evaluate the shale anisotropy and stress orientation, which influence the wellbore failure modes and mud pressure requirements.

2 Apply the effective stress principle

The effective stress principle states that the strength and deformation of a rock depend on the difference between the applied stress and the pore pressure. Therefore, to improve wellbore stability, you need to balance the effective stress around the borehole using appropriate mud weight and fluid properties. You can use petrophysical data to estimate the effective stress and the minimum mud weight needed to prevent wellbore failure. For example, you can use porosity and sonic logs to calculate the pore pressure and the overburden stress, and then use empirical or analytical models to derive the effective stress and the collapse pressure.

3 Monitor the shale-fluid interactions

Shale formations are sensitive to fluid invasion and hydration, which can cause swelling, softening, and dispersion of the shale matrix, and reduce the wellbore stability. To prevent or minimize these effects, you need to monitor the shale-fluid interactions using petrophysical data from logs, tests, and samples. For example, you can use resistivity and gamma ray logs to detect the fluid invasion depth and the shale reactivity, and then adjust the mud salinity and additives accordingly. You can also use sonic and caliper logs to measure the borehole enlargement and the shale damage, and then modify the mud rheology and filtration properties accordingly.

4 Evaluate the shale fracture network

Shale formations often contain natural or induced fractures, which can affect the wellbore stability in different ways depending on their orientation, aperture, and connectivity. To improve wellbore stability, you need to evaluate the shale fracture network using petrophysical data from logs, images, and tests. For example, you can use resistivity and acoustic logs to identify the fracture locations and orientations, and then align the wellbore trajectory accordingly. You can also use image logs and pressure tests to characterize the fracture aperture and connectivity, and then design the mud system and the cementing job accordingly.

5 Integrate the petrophysical data with other sources

Petrophysical data alone cannot provide a complete picture of the shale wellbore stability, as there are other factors that need to be considered, such as the drilling parameters, the operational practices, and the environmental conditions. To improve wellbore stability, you need to integrate the petrophysical data with other sources of information, such as the drilling data, the geosteering data, and the real-time data. For example, you can use the drilling data to monitor the torque, drag, and vibration, and then adjust the drilling speed and direction accordingly. You can also use the geosteering data to optimize the well placement and the reservoir contact, and then update the mud weight and the casing program accordingly. You can also use the real-time data to detect any anomalies or problems, and then take corrective actions accordingly.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?