With the domestic natural gas demand increasing, the deep Cambrain Qiongzhusi formation is becoming the new exploration target in the Sichuan basin of China. The gas in place is the key indicator to reflect the reservoir quality in the shale gas reservoir. The traditional gas-in-place estimation methods require the inputs of effective water saturation, effective porosity, total organic carbon, pore pressure, formation temperature, Langmuir isotherm experiments, etc. to estimate the free gas and adsorbed gas separately. As a new exploration target, those key parameters are not well determined in a time frame relevant for testing or production decisions in different exploration locations. In this paper we present the direct determination of the gas-in-place method using nuclear magnetic resonance (NMR) T1-T2 logs to reduce the evaluation uncertainties of reservoir quality in the early exploration stage.
In an unconventional reservoir, the traditional NMR with 1D T2 measurements is not sufficient to determine the fluids quantitatively because of its complex minerals assemblages, organic matters, pore types, pore structures, etc. The new-generation NMR tool provides continuous 2D T1-T2 measurements. It enables the separation and quantification of different fluids in the pores by the 2D data analytics technique. And it can directly determine the continuous gas-filled porosity, water-filled porosity, and water saturation which circumvents bias using resistivity methods that result from the uncertainties of Archie equation parameters. Because NMR are intrinsically sensitive to the density of hydrogen atoms, the volume of gas in standard cubic feet of gas per cubic meter of formation can be readily determined with the gas-filled porosity based on the ideal gas law. Then the quantity of gas in place can be computed via the inclusion of the density log. Compared to the traditional free gas and adsorbed gas model, this method is more robust and faster for testing or production decisions.
The case study shows that the accurate fluid component volumes were estimated quantitively such as gas, bound water, and free water from the NMR T1-T2 logs through the 2D data analytics technique. And the gas-in-place was determined directly by the gas-filled porosity and water-filled porosity from T1-T2 measurement eventually. The clay-bound water volume, gas volume, and water volume from NMR T1-T2 logs match well with core measurements by tight rock analysis(TRA). The gas in place from T1-T2 logs is a bit more than the traditional free gas and adsorbed gas model, but it is closer to the core measurements. The study verified the value of NMR T1-T2 logs for robust and quick gas-in-place determination in shale gas reservoirs.
In this paper presents the novel application of NMR T1-T2 logs in shale gas reservoirs in the Sichuan Basin of China, which helps the operators make decisions on the testing or production more quickly and confidently with the direct gas-in-place determination by NMR T1-T2 logs.