Transparent gelatin as a reservoir analogue: Dimensional scaling for hydraulic fracturing laboratory experiments

Abstract

Gelatin is frequently used as a reservoir analogue for modeling hydraulic fracture (HF) growth in brittle, organic-rich rocks; its transparency enabling direct, visual observation of the HF growth. However, not many works have assessed the scalability of laboratory experiments on gelatin blocks with-respect-to “real-world” HF stimulation treatments. In this work, lab-scale parameters are compared against typical slickwater and crosslinked-gel stimulation treatment parameters from the Barnett Shale in northeastern Texas, for comprehensive (geometric, kinematic, and dynamic) dimensional-scaling evaluations.Dimensionless groups relating lab-scale physical models that display HF growth to the field-scale parameters are used to quantify the representation of elastic deformation, crack formation energy, treatment time-duration, and stress confinement, honoring the governing principles for fluid-driven fracture propagation in linearly-elastic media (applicable to HF growth in rocks, as well as magmatic intrusions, such as dykes or sills). The analysis demonstrates gelatin as a reservoir analogue to exhibit enhanced scaling against crosslinked gel compared to slickwater stimulation treatments, with the laboratory conditions accentuating toughness over viscous effects in comparison to field conditions, something which if neglected can lead to biased conclusions. Henceforth, a set of laboratory parameters is proposed for gelatin blocks of a practically-attainable powder concentration that yields improved scalability against the prototype parameters, minimizing the discrepancy between the lab-scale and the corresponding field-scale values for each dimensionless group, inside the limits of standard laboratory equipment.Identical dimensional-scaling evaluations are performed on lab-scale parameters from past experimental studies reported in the literature where gelatin samples were used, to the extent that the published data allows, indicating poor scaling of the physical phenomena. Results and conclusions from such inadequately-scaled experiments should be treated with suspicion.