Abstract
Adsorption isotherms of light hydrocarbons on reservoir rocks are key data used to quantify the total gas content in reservoirs and isotherms are now being used to improve our understanding of the processes affecting subsurface gas flow associated with gas injection from Enhanced Oil Recovery techniques. This project combined elements of the traditional pressure-volume gas adsorption isotherm technique and an NMR-based adsorption isotherm approach to determine the adsorption isotherms of light hydrocarbons on to tight rocks from oil and gas reservoirs. The new approach allows isotherms to be derived from NMR data. First, a T2distribution of the gas is determined over a range of gas pressures. Next, the volume of pore gas is estimated using the pore volume of the rock and the Van der Waals gas equation. The adsorbed gas content is then calculated by subtracting pore gas content from the total gas content. This is repeated for a range of gas pressures to determine the adsorption isotherm. This project used the NMR method described above and measured the gas pressure decay in the NMR cell. This combined approach includes the advantages of the NMR method but it also produces a pressure-time curve that can be used to identify when equilibrium is attained in low permeability rocks and can be used to compare adsorption kinetics of different gases. The advantages of our approach are that 1) the samples remain intact and the measurements provide information on the pore size distribution; 2) analyses can be carried out at reservoir pressures; 3) isotherms can be measured for any gas containing hydrogen atoms; and 4) the results can be used to examine the processes controlling gas flow through the rock. Future work to develop this technique will improve our quantification of the amount of pore gas in the cell, which will improve our partitioning between adsorbed gas and pore gas as well as allow for an improved analysis of the pressure response of the sample after degassing.
Highlights
Adsorption of gases to rocks is an important process for evaluating natural gas resource potential, coalbed methane recovery, as well as secondary and tertiary enhanced oil recovery techniques
Gas transport by diffusion is a slow process while transport by advection, or Darcy flow, may be orders of magnitude greater
Adsorption isotherms for rocks commonly report data on the volume of gas contained within the pore space, referred to here as pore gas, and the gas adsorbed on to the surface of the pore walls referred to here as adsorbed gas
Summary
Adsorption of gases to rocks is an important process for evaluating natural gas resource potential, coalbed methane recovery, as well as secondary and tertiary enhanced oil recovery techniques. The expansion of the gases is repeated at numerous pressures to generate isotherms The disadvantages of this method include 1) the sample is crushed for the measurement which alters the structure of the sample; 2) no information is acquired on the size of the pores occupied by the gas; and 3) many analyses do not examine the processes controlling gas flow through the rock. The advantages of NMR measurements include 1) the samples remain intact and the measurements provide information on the pore size distribution; 2) analyses can be carried out at reservoir pressures; and 3) isotherms can be measured for any gas containing hydrogen atoms; and 4) the results can be used to examine the processes controlling gas flow through the rock. We combine pressure measurements with NMR measurements to carry out an initial assessment of the transport processes controlling gas flow in a shale
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