Interpretation of Fluid Injection Experiments in Poorly Consolidated Sands
Authors: Konstantinou C. , Biscontin G. , Papanastasiou P. 
Published: September 2022

ABSTRACT:
The transitional nature of the so-called soft sandstones presents challenges to the understanding of their mechanical response and to the safety of storage and extraction operations. This paper aims to analyse and interpret fluid injection experiments in artificially cemented granular rocks of low strength under true triaxial stresses. The rock utilised was generated via microbially induced carbonate precipitation, a bio-cementation method that allows for controlling the mechanical properties of the resulting rock based on the cementation level. Results show that the Brittleness Indices that are derived from the failure criteria are more reliable in capturing the brittle/ductile behaviour and therefore predicting fracability of these rocks. The breakdown pressures obtained in the experiments can be reliably predicted from the analytical expressions of the limit pressure of the cavity expansion theory.

1 INTRODUCTION
Weakly-cemented and poorly-consolidated sandstones represent the host rock for a large portion of active aquifers and oil and gas reservoirs because their high porosity both enhances storage and facilitates extraction. These reservoirs can be used also for gas storage including CO2 sequestration. The transitional nature of the so-called soft sandstones presents challenges to the understanding of their mechanical response and to the safety of these operations. The main challenge is the risk associated with the ability to maintain production and fully recover reserves as maintaining injectivity by fracturing is not working as expected in soft sands. Key operational issues encountered include the higher than expected initial fracturing pressures which often exceed the operating limits.
Any improving of the process required the development of a more complete understanding of the grain scale materials science that underpins the processes governing fracturing behaviours in fluid injection in soft sandstones. The behaviour of weakly cemented media under fluid injection deviates substantially from the classical fracture geometry that appears in competent rocks.This finding results from hydraulic fracture simulation studies in weak formations that were carried out originally to investigate the highnet pressures that are often observed in field operations and the differences between simulators and field measurements (Papanastasiou, 1997). Experimental studies have shown that the generated fractures propagate perpendicular to the least compressive stress in a similar manner to competent rocks. However, the injection into cohesionless sand may induce other failure modes such as shear failure or mixed modes. The most striking evidence that proves the deviation from the opening mode (tensile failure) is the fracture morphology observed in the previous studies. Fracture-like patterns in soft materials are described as a main conduit (cavity) with multiple offshoots, chaotic or dendritic, with excessive leak-off causing rapid dissipation of the pressures and back-stresses generation (Khodaverdian and Mcelfresh, 2000; Bohloli and de Pater, 2006; Chang, 2004; Hurt et al., 2005; Konstantinou et al., 2021c).