The Use of Connection and Total Gases Quantitatively in the Assessment of Shale Pore Pressure

Abstract

Summary Connection and total gas measurements were used qualitatively for decades in drilling oil and gas wells to identify overbalanced, underbalanced, or near-underbalanced pressure conditions (mud hydrostatic pressure relative to shale pore pressure). Results often conflicted with directly measured sand pore pressures (PP) and the pre-existing perceptions of shale PP. We propose a technique that allows mud gases to be used in a quantitative manner when (1) there is an increased understanding of the relationship between mud gases and shale PP, (2) the development of the logging-while-drilling (LWD) -based annular-pressure measurement allows for the real-time monitoring of downhole hydrostatic pressures, and (3) differences between sand and shale PP may exist as a result of the sand's structural position. This paper presents time-based equivalent-circulating-density (ECD) behavior relative to block movement, flow rate, and total gas from Gulf of Mexico (GOM) wells, and explains the relationship to shale PP. These examples are quantitatively analyzed for shale PP. The results are compared with direct formation-pressure measurements within the sands. Six principles guide pore-pressure interpreters on the quantitative use of mud gases. The principles define the pore-pressure values assigned to specific mud-gas observations as a function of recorded annular mud pressure. One can use the pore-pressure estimates in shale as calibration points for traditional shale-pressure indicators such as resistivity, velocity, or corrected drilling-exponent measurements. This integrated technique yields an improved characterization of shale and sand PPs and can be used as input for well designs to increase reliability, safety, and drilling efficiency.