A systematic study of internal gas generation in shale source rocks using analog experiments

Abstract

A two-dimensional Hele-Shaw cell was created to study and visualize hydrocarbon source rock (i.e., shale) elastic-brittle fracture and fracture network propagation mechanisms triggered by internal gas generation and gas build up during maturation of organic components. An analog immature source rock was created using a mixture of gelatin, sugar, and live yeast in pre-determined proportions. This mixture was poured into the Hele-Shaw cell and allowed to gel. The internal gas generated as a result of sugar fermentation by yeast caused pressure build up within the gel that ultimately resulted in the activation of dissipation mechanisms such as gas diffusion and deformation of the gel matrix. These mechanisms are analogous to shale maturation and fluid generation. Image analysis was employed to study and characterize key events such as fracture nucleation, propagation, and coalescence versus time. Derivative parameters such as fracture growth rate, fracture tip pressure, and fracture spatial distribution, among others, were calculated using the image-sourced data and applying Linear Elastic Fracture Mechanics (LEFM) principles. Results showed that the experiments are repeatable within a consistent response range and additional parameters were identified to be useful input for numerical modelling, including nucleation sites, fracture merging angle, merging to nucleation ratio, and fracture connectivity. For a volume of about 100 cm3 of 11.6% wt. food-grade gelatin solution with 0.25% wt. yeast and a sugar/yeast ratio of 3, measured average total gas production was 35 cm3 (at standard conditions). On an average gelatin matrix area of 400 cm2, the average total fracture length and fracture density are 106 cm and 0.15 fractures/cm2, respectively. Average maximum calculated fracture tip pressure and maximum fracture velocity are 0.92 psi and 5.85 × 10−4 cm/s, respectively. Fracture connectivity was more frequent in the second half of the gas generation process and greater for the older and longer fractures. Average final fracture spacing was 4.42 cm. Connection angles between fractures tended to be closer to 0° than to 90°. Photoelasticity imaging techniques were applied to the experimental setup to visualize the stress field within the sample. The difference of principal stresses was estimated at 3.70 × 103 Pa for the tested material late in the maturation process. Results suggest interdependence of gas pressure, gas diffusion, and transient stress field states. The range of laboratory-scaled data are useful as benchmarking parameters for numerical modelling of source rock maturation including geomechanics.