# Research on Pore Pressure Detection While Drilling Based on Mechanical Specific Energy
## 1 Introduction
When entering the overpressure intervals, the d-exponent will tend to deviate from the NCT in a decreasing trend. The degree of deviation of the d-exponent could reflect the degree of overpressure. During drilling, the increase in drilling fluid density will cover up the change in the d-exponent, and a corrected d-exponent was proposed (Equation (11)) [14]. However, based on shale compaction, it is not applicable for the d-exponent concept to detect pore pressure in carbonate formations. \[P_{\rm pg}=\mathrm{O}BG-\left(\mathrm{O}BG-P_{\rm ng}\right)\times\left(\frac{ R_{\rm o}}{R_{\rm n}}\right)^{1.2} \tag{2}\]0  
where \(ROP\) is the rate of penetration (m/hr); \(N\) is the rotary speed (rpm); \(WOB\) is the weight on bit (kN); \(D_{\text{b}}\) is the bit diameter (mm); \(NPP\) is the normal pore pressure (sg); and \(ECD\) is the equivalent circulating density (sg). [PERSON] and [PERSON] [15] used the concept of sigmalog (rock strength parameter) to detect the pore pressure (Equation (12)), which was based on the relationship between rock strength, pore pressure and formation lithology. Likewise, the variation of sigmalog with depth can be plotted, and a trend line can be created. There are a number of factors that affect sigmalog, which can be divided into two aspects: geological factors and drilling parameters. The effect of factors other than pore pressure and porosity on the sigmalogcan be excluded by correcting the equation and adjusting the trend line intercept. In this way, the pore pressure abnormality can be reflected by the inversion of the sigmalog curve. Based on the drilling strength of the rock, it is usually not obstructed by lithology in the detection of pore pressure. However, the application of the sigmalog method in carbonate rocks is practically not ideal [16]. \[P_{\rm pg}=\mathrm{O}BG-\left(\mathrm{O}BG-P_{\rm ng}\right)\times\left(\frac{ R_{\rm o}}{R_{\rm n}}\right)^{1.2} \tag{2}\]1  
where \(\sigma\) presents the rock strength parameter; \(n\) is the hole cleaning coefficient; \(\Delta p\) is the pressure difference between the drilling fluid and the hydrostatic pressure, kg/L; and \(h\) is the drilling depth, m.  
Recently, research on pore pressure detection about drilling parameters has begun to use the mechanical specific energy (MSE) model. The original model of MSE was proposed by [PERSON] [17], which reflects the energy required to break and remove a unit volume of rock. It is widely used for real-time detecting of drilling efficiency, optimization of drilling parameters, downhole identification of inefficient working conditions, and lithology identification [18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. [PERSON] [28] was probably the first to use the MSE concept to detect pore pressure. Subsequently, [PERSON] et al. [29] proposed a method to detect pore pressure through downhole drilling parameters and in situ rock data using the concept of drilling efficiency and mechanical specific energy (DE-MSE). The pore pressure was expressed as a function of equivalent circulating density (ECD), DE, MSE, angle of internal friction, and uniaxial compressive strength (UCS) of the rock (Equation (13)). Among them, the angle of internal friction and UCS of the rock are computed from the compressional velocity and based on empirical equations (Equations (14) and (15)).