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

Abstract

Abstract Connection and total gas measurements have been used qualitatively for decades in drilling oil and gas wells to identify over-balanced, under-balanced or near-under-balanced pressure conditions (mud hydrostatic relative to shale pore pressure). Results often conflicted with sand direct measured pore pressures and the pre-existing perceptions of shale pore pressure. We propose a technique that allows mud gases to be used in a quantitative manner, given that: (1) there is an increased understanding of the relationship between mud gases and shale pore pressure, (2) the development of the LWD based annular pressure measurement allows real-time monitoring of down hole hydrostatic pressures, and (3) differences between sand and shale pore pressure may exist as a result of the sand's structural position. This paper presents time-based ECD behavior of block movement, flow rate, and total gas from Gulf of Mexico wells, and explains the relationship to shale pore pressure. These examples are quantitatively analyzed for shale pore pressure. The results are compared to 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 assignment to specific mud gas observations as a function of recorded annular mud pressure. The pore pressure estimates in shale can be used as calibration points for traditional shale pressure indicators such as resistivity, velocity or Dxc measurements. The result of this integrated technique yields an improved characterization of shale and sand pore pressures and it can be used as input for well designs to increase reliability, safety and improve drilling efficiency.