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The Effect on Permeability of Step-Wise Removal of Organic Material Using Digital Rock Methods

Abstract

The organic material (OM) in shale samples brought to the lab for analysis includes kerogen, bitumen, pyrobitumen, “dead” oil, and other compounds. Some fraction of the OM is solid and not mobile under in-situ conditions. There is often another fraction composed of non-volatile hydrocarbons and some bitumen that may be mobile in-situ and should be removed before permeability measurement. Unfortunately, due to its viscous nature and low permeability (perm) of shales, it is difficult and slow to solvent-extract this material, especially from intact plug samples. Digital rock methods offer a solution because the OM is a 3D object and any portion of it can be changed to porosity. Organic shale samples were analyzed with a focused ion beam scanning electron microscope (FIB-SEM). Each 3D volume was analyzed to obtain the volume fraction of OM and porosity. The porosity was further analyzed and quantified as connected, non-connected and porosity associated with organic matter (PAOM). Connected porosity was used to compute perm of the 3D volumes using a lattice-Boltzmann fluid flow algorithm. The samples were imaged “as-received” meaning no solvent cleaning or oven drying. Samples were placed in a vacuum chamber which removed most liquid water and volatile hydrocarbons but left behind non-volatile oil and bitumen. After porosity and perm computation from the original imaged volume, we digitally removed a portion of the OM from the segmented 3D volume and replaced it with additional PAOM. The initial volume removed was determined from geochemical S1 and S2 values. We then recomputed perm, and the process was repeated several times until no OM was left in the system. The porosity-perm trend from the five data points was plotted for each sample. Application This work helps to solve the difficult problem of getting reliable perm values for reservoir simulation and production forecasting. For some shales, removal of just the likely mobile fraction of OM increased perm by a factor of 5 to 10. Conclusion The remnant non-volatile oil and bitumen in lab samples may significantly reduce the measured perm and digital rock methods provide a fast and simple solution to the problem, enabling more reliable forecasts of production and EUR. Technical Contribution This work demonstrates a method to digitally convert organic material to porosity in a step-wise fashion, up to and including removal of all OM.