# Research on Pore Pressure Detection While Drilling Based on Mechanical Specific Energy
## 1 Introduction  
Pore pressure is the pressure of the formation fluid within the pores of the soil or rock. In drilling engineering, accurate knowledge of pore pressure changes can provide a basis for the optimization of drilling fluid density and well structure design to avoid drilling incidents (e.g., well kicks/blowout and borehole collapse). There are three aspects of pore pressure analysis: pre-drill prediction, detection while drilling, and post-drill analysis [1]. The pre-drill prediction of pore pressure is usually based on seismic data combined with logging data from the offset wells. Detection while drilling mainly relies on logging while drilling (LWD), measurement while drilling (MWD) and drilling parameters. Post-drill analysis summarizes pore pressure in a developed region using logging data from drilled wells, which is an after-the-fact technique that can be used for pre-drill prediction in future wells. [PERSON] and [PERSON] [2] were probably the first to use logging data (sonic and resistivity) to make predictions of overpressure in shales. They found a linear relationship between the common logarithm of sonic transit time or resistivity and depth in hydrostatic pressure intervals, which they called the normal compaction trend (NCT). When entering anomalous pressure intervals, the sonic transit time or resistivity trend will be deflected. [PERSON] [3] was the first to apply seismic interval velocity data to the prediction of pore pressure. Under normal conditions, interval velocity increases with depth, and when an overpressure interval appears, it is often accompanied by a decrease in interval velocity. Based on data provided by [PERSON] and [PERSON], [PERSON] et al. [4] proposed an empirical equation that can be expressed as a direct form of pore pressure (Equation (1))  
\[P_{\rm p}=\sigma_{\rm v}-\frac{\left(\mathrm{A}-\mathrm{B}\ln\Delta t\right)^{3} \left(\mathrm{O}BG-P_{\rm ng}\right)}{Z^{2}} \tag{1}\]  
where \(P_{\rm p}\) is the formation pore pressure (psi); \(\sigma_{\rm v}\) is the normal overburden pressure (psi); \(\Delta t\) is the sonic transit time (\(\mathrm{\SIUnitSymbolMicro s}/\mathrm{ft}\)); \(\mathrm{O}BG\) is the overburden pressure gradient (psi/ft); \(P_{\rm ng}\) is the normal fluid pressure gradient (psi/ft); \(Z\) is the depth (ft-TVD). [PERSON] [5] proposed three sets of prediction equations for pore pressure based on resistivity, sonic transit time and the dc-exponent, respectively (Equations (2)-(4)). Estimation of pore pressures using [PERSON]'s model requires an accurate NCT and reliable input data, so that accurate pore pressure can be obtained. \[P_{\rm pg}=\mathrm{O}BG-\left(\mathrm{O}BG-P_{\rm ng}\right)\times\left(\frac{ R_{\rm o}}{R_{\rm n}}\right)^{1.2} \tag{2}\]  
\[P_{\rm pg}=\mathrm{O}BG-\left(\mathrm{O}BG-P_{\rm ng}\right)\times\left(\frac {\Delta t_{\rm n}}{\Delta t_{\rm o}}\right)^{3} \tag{3}\]  
\[P_{\rm pg}=\mathrm{O}BG-\left(\mathrm{O}BG-P_{\rm ng}\right)\times\left(\frac {d_{\rm co}}{d_{\rm cn}}\right)^{1.2} \tag{4}\]  
where \(P_{\rm pg}\) is the formation pore pressure gradient (psi/ft); \(R_{\rm o}\) is the observed shale resistivity at a given depth (ohm-m); \(R_{\rm n}\) is the normal compaction shale resistivity at a given depth (ohm-m); \(\Delta t_{\rm o}\) is the observed shale transit time at a given depth (\(\mathrm{\SIUnitSymbolMicro s}/\mathrm{ft}\)); \(\Delta t_{\rm n}\) is the normal compaction shale transit time at a given depth (\(\mathrm{\SIUnitSymbolMicro s}/\mathrm{ft}\)); \(d_{\rm co}\) is the computed dc-exponent from the measured data at a given depth; and \(d_{\rm cn}\) is the dc-exponent from the normal compaction trend at a given depth. [PERSON] [6] proposed an empirical equation for predicting pore pressures through a comprehensive study of seismic, logging and drilling data in the Gulf of Mexico region (Equation (5)). The new empirical equation does not rely on the NCT and can be used as an effective pre-drill pore pressure prediction method [7].