Application of Nano-Scale Geomechanics Using PeakForce Quantitative Nano-Mechanical Mapping to Improve Hydraulic Fracture Design in Highly Heterogeneous Reservoirs

Abstract

The production of hydrocarbons from unconventional reservoirs is nowadays essential to meet the rising the demand for energy in the world. Unconventional reservoirs have a low porosity and an extremely low permeability, which requires horizontal drilling and multi-stage hydraulic fracturing. As hydraulic fracturing strongly depends on the principal stresses, the identification of geomechanical properties is key for the assessment of effective hydraulic fracturing design. The assessment of geomechanical properties through macro-scale testing such as true triaxial testing has been performed for decades. However, heterogeneities on a nano-scale in addition to the possibility of having non-intact samples lead to the application of nano-scale geomechanical testing. PeakForce Quantitative Nano-mechanical mapping in atomic force microscopy (AFM PF-QNM) maps the geomechanical properties on a nano-scale requiring a sample size that is as small as drill cuttings. The testing reveal s the 3D heterogeneities within the sample indicating spots of highest and weakest strength. This way the anisotropy of the tested material can be assessed. However, only few studies with application of AFM PF-QNM on reservoir rocks have been performed. Therefore, AFM PF-QNM testing has been performed and compared to the results from performed true triaxial testing for the Eagle Ford Formation. The results show a multi-scale comparison of geomechanical properties. As AFM PF-QNM testing assumes a preset value of Poisson's ratio, an iterative model that solves for Poisson's ratio in dependency of Young's modulus has been developed for the Eagle Ford Formation, as these properties are strongly lithology dependent. The new model corrects the Young's modulus in dependency of the Poisson's ratio. The results from AFM PF-QNM indicate many hundred thousand measurements. In order to reduce the large data set, exploratory factor analysis has been deployed. It determines if unmeasured factors could explain the variance from the data set, which is caused by different minerals, macerals, textures, or pores. It shows that 34% of the data set can be explained by 6 factors. These different factors represent groups that are ductile, intergranular or of high strength. The comparison with the results from true triaxial testing on a macro-scale indicate that AFM PF-QNM could yield similar results. True triaxial testing reveals the macro-scale anisotropy indicating a decreased velocity in Z-direction. The results show that different testing methods performed at different spatial scales and different loading conditions, lead to different results. The dynamic Young's modulus from true triaxial testing indicates the highest value (69.7 GPa) and the static Young's modulus from true triaxial testing yields to the lowest value (42.6 GPa). The results from AFM PF-QNM show an average modulus (52.28 GPa) in between the static and dynamic results from true triaxial testing. This thesis demonstrates a multi-scale geomechanical comparison for unconventional reservoirs. It improves the evaluation for AFM PF-QNM testing by correcting the Young's modulus iteratively in dependency of the Poisson's ratio. It reveals the advantages of AFM PF-QNM as it can be performed on samples that are as small as drill-cuttings determining the 3D heterogeneity of the material.