James Kear

Sustained acoustic emissions following tensile crack propagation in a crystalline rock

We show that simple breakage of a crystalline rock (gabbro) in tension begets further breakage of rock in the area around the first crack that is self-sustaining and spontaneous and that is detected via sustained acoustic emissions (AE). The result is a sequence of AE events that is statistically similar to aftershocks from earthquakes, that scales with the size of the main crack, and that we were able to observe for days following the initial breakage in laboratory-scale experiments. A new model for aftershock generation that is based on residual strain relaxation is shown to be consistent with the observed hyperbolic decay of the event rate with time and with the manner in which the decay law scales with the size of the main rupture.

Three Dimensional Forms of Closely-Spaced Hydraulic Fractures

When creating arrays of hydraulic fractures in close proximity, stress field changes induced by previously placed hydraulic fractures can lead to deflection in subsequent fracture paths and coalescence between fractures. Any fracture coalescence can compromise the effective‐ ness of the treatment array and the fracture geometry will not be appropriately account‐ ed for in reservoir or caving models. Here we present the results of an experimental study consisting of arrays of 4 closely spaced hydraulic fractures grown sequentially in 350x350x350 mm blocks of a South Australian Gabbro under different initial stress states and for notched and un-notched wellbores. In particular we focus on insights gained from 3-dimesional serial sectioning and digital reconstruction of the hydraulic fracture pat‐ terns that were formed. The results show that the curving hydraulic fractures typically do not exhibit a high degree of radial symmetry in their paths even though the fractures grew by radiating outward from a centrally located wellbore. The results also confirm model predictions that a subsequent fracture will curve towards a previous fracture when the minimum stress is zero and that this curving is suppressed when the minimum stress is sufficiently large. Finally, fracture initiation is shown to be critical to the symmetry of the fracture pattern and preponderance of branching and therefore effective notches that lead to initiation in the eventual plane of favored propagation have a profound impact on the hydraulic fracture geometry.

Dependence of Static Fatigue Tests on Experimental Configuration for a Crystalline Rock

This paper presents static fatigue laboratory experiments conducted in three different configurations. The experiments are designed so as to cause delayed tensile failure in dry Gabbro specimens after the sustained application of a static subcritical load. Results from the static fatigue experiments give a time to failure of the specimen related to the applied static load. In the presented experiments, results spanning up to six orders of magnitude of time to failure were collected for three-point bending, four-point bending, and indirect tensile (Brazilian) specimens. The data supports an exponential relationship between tensile stress and time to failure, noting that a power law relationship is also supported by the data. The salient difference among the configurations is hypothesized to be the size of the region of the specimen that is subjected to a close approximation of the maximum tensile stress. The time to failure at a given nominal tensile stress, the decrease in time to failure associated with a given increase in stress (i.e. the slope in a semi logarithmic plot), and the magnitude of the scatter of the data about the best-fit curve are all observed to correlate inversely with the nominal size of the region subjected to the maximum tensile stress.

The Role of Specific Surface Area and Cation Exchange Capacity in Determining Shale Rock Properties

Surface area, surface charge and the exchangeability of cations have long been known as factors in determining the physical and mechanical properties of shales. Such properties are related to the fine grain size of the various clay minerals and also the particle shapes, edge-face microstructural arrangements and are impacted by the likes of salinity and depositional environment. In general, the siltier a given clay or shale, the lower their cation exchange capacity (CEC) and specific surface area (SSA). CEC and SSA have direct impacts on mechanical and flow properties. Wellbore instability can be caused by the development of osmotic pressures resulting from differences between pore fluid composition and drilling fluid composition and the resultant time dependent effects based on permeability. Such instability can be mitigated by the use of oil-based muds or for example high KCl water-based muds. Electrical and dielectric properties are also dependent on mineral surface charge and related directly to the CEC of a given shale, especially properties such as resistivity (or conductivity) and permittivity (dielectric constant). This paper will detail SSA and CEC values of shales from multiple basins worldwide and also from individual boreholes to demonstrate their role for shale rock properties.

Blue Shift in the Spectrum of Arrival Times of Acoustic Signals Emitted during Laboratory Hydraulic Fracturing

We discuss a method of detecting localised fracturing that potentially requires only one channel. The method is based on the notion that the fracture propagation involves generation of acoustic events from its contour. It is proposed that the number of events (microcracks) generated at each step of fracture propagation could be proportional to the fracture size to a certain power called the localisation exponent. This dependence of the number of generated events on the fracture size (the event coherence) leads to a shift to higher frequency (the “blue shift”) in the combined spectrum of the events as compared to the spectrum of randomly generated events. This concept was applied to the results of a laboratory test in which hydraulic fracture was driven by injecting glycerine into a 200x200x120mm block of polycrystalline gabbro. We show that there is indeed a blue shift in the spectrum of the arrival times at any one sensor that seems to correspond with the growth of a localized hydraulic fracture. The localisation exponent is able to distinguish between the cases of the fracture contour length roughly proportional to, and more slowly than proportional to, the nominal fracture radius.

Perspectives on successful coal seam gas well decommissioning

Decommissioning of wells and well pads (plugging and abandonment of the well and rehabilitation of the well pad) is an important activity in coal seam gas developments due to the large number of wells drilled and their co-location with other land uses. Successful decommissioning will need to be conducted in a way that is effective, efficient, complies with regulatory requirements and is socially acceptable. This study sought to understand the perspective of industry, government and local stakeholders on the decommissioning process and asked the question ‘what does successful decommissioning mean’? The study was conducted through workshops held in Camden, Chinchilla and Narrabri involving industry, government and local residents. All stakeholders agreed that the goal of successful decommissioning is to effectively have no legacy issues arising from the abandoned well or well pad, in perpetuity. However, there were differences in the confidence that stakeholders had in the process of achieving this outcome, largely based on the varying levels of understanding of well abandonment. This lack of confidence and understanding contributes to cross-cutting themes that emerged from the workshops. Local stakeholders expressed a strong desire for independent oversight and assessment of well abandonment processes, with increased transparency and access to information on abandoned wells in an understandable form. They also stated that long-term monitoring of wells post abandonment would assist in providing them with confidence that wells can be successfully decommissioned.