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Fractures are considered to be both problems and opportunities for exploration and production from petroleum reservoirs. The main aim of fracturing an oil medium is the stimulation of oil and gas production by inducing highly conductive channels leading to a well, from which hydrocarbons will flow. Many applications of hydraulic fracturing take place in soft sandstones using moderate to low viscosity fluids. Soft 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. The transitional nature of sandstones, in particular, presents some challenges to the safety of these operations and the understanding of the mechanical response of these materials under a variety of conditions is poor. The motivation of this research was to gain understanding of the fracture mechanics of soft media under low viscosity fluid injection. Synthetic rock specimens were generated via microbially induced carbonate precipitation (MICP), providing virtually limitless quantities and had customisable characteristics, allowing relevant structural parameters to be varied independently, and hence isolating their effects. The method was applied to produce realistic weak sandstone-like materials from a base sand through a bio-process that builds up calcium carbonate cementation around the particles. Once a single recipe was developed and the uniformity of the specimens and repeatability of process were assured, the mechanical and physical properties of the specimens were assessed with varying base materials (grain size, shape, width of particle size distribution) and levels of cementation. The artificial rocks sufficiently resembled soft sandstones to be used as a substitute in the subsequent laboratory investigation of the hydraulic fracturing experiments. Fracturing experiments were conducted with both cohesionless sands and bio-cemented samples at various confining conditions, flow rates, fluid viscosities, cementation levels and various base materials. The main aim was to get an insight into how each individual factor affects the fracture patterns and pressure response. For those experiments, two experimental setups were developed explicitly for this project that were able to apply actively or passively stresses to the medium which was repre- sented in three dimensions. The transitional behaviour of weakly cemented sandstones was clearly seen on both the fracture patterns and pressure responses obtained when conducting the hydraulic fracturing tests. The fractures transitioned from plane-like to string-like as the cementation level increased, while the deviation between the breakdown and propagation pressures was also more pronounced. Many observations in the literature concerning the fluid properties and rates were confirmed in this work, but also new information was added because of the ability to visualise the fracture initiation and propagation. The work conducted in this research bridges previous works on two-dimensional media, where visualisation of the fracture was possible, and three-dimensional media, where only post-test observations could be made., The author would like to acknowledge the funding and technical support from bp through the bp International Centre for Advanced Materials (bp-ICAM) which made this research possible

Peer reviewed: True, Fluid injection in a porous medium is the underlying mechanism for many applications in the fields of groundwater hydraulics, hydrology and hydrogeology, and geo-environmental engineering and in the oil and gas industry. Fluid flow experiments in porous media with a viscous fluid at varying injection rates were conducted in a modified Hele-Shaw setup. The granular media were three-dimensional bio-cemented sands of various grain sizes across various cementation levels, generating a matrix of various hydraulic conductivities, porosities, and strengths. The fluid injection experiments showed that a cavity-like fracture developed, which transitioned to crack-like fractures at higher cementation levels (hence, higher strength). As the flow rate increased, less infiltration was evident and higher breakdown pressure was observed, with propagation pressure reducing to zero. It was harder to induce an opening in cemented specimens with higher hydraulic conductivity and a larger pore network despite their lower strength due to excessive infiltration dominance, which inhibited the build-up of pressure required to generate a fracture. The results of this study suggest that, when designing fluid injection programs, the combined effects of hydraulic conductivity and strength need to be carefully considered.

Funder: bp International Centre for Advanced Materials (bp-ICAM), Artificially bio-cemented sands treated with microbially induced calcite precipitation are weakly cemented rocks representing intermediate materials between locked and carbonate sands. Variations in cementation significantly affect the strength of sample, particularly tensile stregth. The modes of fracture and the surface characteristics resulting from the indirect tensile strength tests (Brazilian tests) are strongly correlated with the specimen strength and consequently the degree of cementation. This study examines the tensile strength of bio-cemented fine and coarse sands (average particle diameter 0.18 and 1.82 mm, respectively) and investigates failure modes by recording fracture evolution at both sides of specimen and surface characteristics of the reconstructed surfaces. The dimensionless slope parameter Z2 provided the best fit with respect to tensile strength while the power spectral density was a good indicator of surface anisotropy. Finally, wavelet decomposition allowed for comparison of fracture surface characteristics of the two sands ignoring the grain size effects.

Funder: BP International Centre for Advanced Materials (BP-ICAM), Protocols for microbially induced carbonate precipitation (MICP) have been extensively studied in the literature to optimise the process with regard to the amount of injected chemicals, the ratio of urea to calcium chloride, the method of injection and injection intervals, and the population of the bacteria, usually using fine- to medium-grained poorly graded sands. This study assesses the effect of varying urease activities, which have not been studied systematically, and population densities of the bacteria on the uniformity of cementation in very coarse sands (considered poor candidates for treatment). A procedure for producing bacteria with the desired urease activities was developed and qPCR tests were conducted to measure the counts of the RNA of the Ure-C genes. Sand biocementaton experiments followed, showing that slower rates of MICP reactions promote more effective and uniform cementation. Lowering urease activity, in particular, results in progressively more uniformly cemented samples and it is proven to be effective enough when its value is less than 10 mmol/L/h. The work presented highlights the importance of urease activity in controlling the quality and quantity of calcium carbonate cements.

We study the effect of stress inhomogeneity on the behavior of fluid-driven fracture development in weakly consolidated granular systems. Using numerical models we investigate the change in fracture growth rate and fracture pattern structure in unconsolidated granular packs (also referred to as soft-sands) as a function of the change in the confining stresses applied to the system. Soft-sands do not usually behave like brittle, linear elastic materials, and as a consequence, poroelastic models are often not applicable to describe their behavior. By making a distinction between ``cohesive'' and ``compressive'' grain-grain contact forces depending on their magnitude, we propose an expression that describes the fluid opening pressure as a function of the mean value and the standard deviation of the ``compressive stress'' distribution. We also show that the standard deviation of this distribution can be related with the extent to which fracture ``branches'' reach into the material.