Jørn Stenebråten

Stress-Induced Fracturing of Reservoir Rocks: Acoustic Monitoring and μCT Image Analysis

Stress-induced fracturing in reservoir rocks is an important issue for the petroleum industry. While productivity can be enhanced by a controlled fracturing operation, it can trigger borehole instability problems by reactivating existing fractures/faults in a reservoir. However, safe fracturing can improve the quality of operations during CO2 storage, geothermal installation and gas production at and from the reservoir rocks. Therefore, understanding the fracturing behavior of different types of reservoir rocks is a basic need for planning field operations toward these activities. In our study, stress-induced fracturing of rock samples has been monitored by acoustic emission (AE) and post-experiment computer tomography (CT) scans. We have used hollow cylinder cores of sandstones and chalks, which are representatives of reservoir rocks. The fracture-triggering stress has been measured for different rocks and compared with theoretical estimates. The population of AE events shows the location of main fracture arms which is in a good agreement with post-test CT image analysis, and the fracture patterns inside the samples are visualized through 3D image reconstructions. The amplitudes and energies of acoustic events clearly indicate initiation and propagation of the main fractures. Time evolution of the radial strain measured in the fracturing tests will later be compared to model predictions of fracture size.

Stress-induced versus lithological anisotropy in compacted claystones and soft shales

Shales are anisotropic. Most definitions of shale in-corporate this attribute, either by referring to fissility and existence of cleavage planes, or to anisotropic texture resulting in anisotropy of physical properties on many length scales. Definitions of shale scatter though; some focus on a high content of clay minerals as characteristic of a shale, while others consider a large amount of fine grains (< 2μm) as sufficient. In a rock mechanical context, it is natural to define shale as a rock in which clay minerals constitute the load-bearing framework. This means that “gas shales” in oil-field terminology are, strictly speaking, not shales according to a geological or a geomechanical perspective. Still, these materials have a lot in common with classically defined shales (e.g., low permeability) and anisotropy.

Static and dynamic behaviour of compacted sand and clay: Comparison between measurements in Triaxial and Oedometric test systems

In rock mechanics and rock physics, like in many other branches of research, it is important to compare results obtained in different kinds of apparatus that are meant to measure the same properties. Differences may in general be due to differences in samples, or in test procedures. Here we compare uniaxial compaction experiments in oedometric and triaxial tests systems, using brine-saturated samples made from pure kaolinite or from Ottawa sand. Small differences in sample manufacturing or in initial loading of the specimens were found to cause significant differences in static behaviour and in ultrasonic velocities during the tests. The influence of differences in sample geometry (wide and thin samples in the oedometer versus long and slim samples in the triaxial set-up) and the influence of different boundary conditions caused by the confining medium (steel in the oedometer, thin soft sleeve in the triaxial system) were studied, amongst others with the use of discrete particle modelling. Although the boundary conditions may have an influence, the most significant sources of discrepancy in our experiments were associated with the manufacturing and preparation of the samples to be tested. The test data show that the drained static compaction modulus for sand is close to its dynamic counterpart, while for kaolinite, the dynamic modulus is significantly larger than the static one.

Controlled laboratory experiments to assess the geomechanical influence of subsurface injection and depletion processes on 4D seismic responses

Laboratory experiments are performed with soft synthetic reservoir sandstone cemented under stress and with synthetic overburden (caprock) material consisting of compacted clay (kaolinite) in brine. The rock-like materials are loaded mechanically under stress paths representative of stress changes occurring in the subsurface as a result of injection (increasing pore pressure) or depletion followed by injection into a storage reservoir. Static stress-strain behaviour and multidirectional P- and S-wave velocities are monitored during the tests. The tests with sandstone are performed on dry material and simple poroelastic modelling is performed to relate these data to the behaviour of fluid (water / CO2) saturated samples under the same stress paths. The focus is on identifying 4D seismic attributes that may be used in the field to interpret monitoring measurements. This could help diagnose stress changes in the overburden, signalling the risk of CO2 leakage from a reservoir if the compressive or tensile strength limit of the overburden is reached and of course to help quantify amounts of CO2 stored.

Static and Dynamic Characterization of a Deep Overburden Shale

Since shales are the far most abundant overburden formation, understanding the geomechanical and rock physics behavior of shales is essential. We discuss a particular deep overburden shale in light of a thorough static and dynamic multi-directional data acquisition, as a basis for complete static and dynamic characterization assuming transverse isotropy. The stress-and strain-path dependence of the principal P- and S-wave velocities are also analyzed. These quantities are of great interest for interpretation of seismic or sonic-log overburden signatures upon reservoir depletion (or injection), as geomechanical simulations show significant variations of stress and strain changes throughout the overburden.

Mechanical Anisotropies of Shale

Laboratory experiments have been performed with a number of shale samples, including commonly studied outcrops like Pierre and Mancos Shale. The angular dependences of unconfined strength, friction angle, static E-modulus and P-wave velocity have been measured and modelled. The anisotropy of the ratio between dynamic and static moduli is found to represent plastic / brittleness anisotropy. Stress and stress path dependent velocity anisotropy will be shown and related to possible 4D seismic applications. Implications for field applications to borehole stability and fracturing will be discussed.

Experimental Studies of Stress Dependence,Static vs Dynamic behaviour and Mechanical Anisotropies of Shale

Laboratory data from controlled rock mechanical tests with various shales have been used to understand and quantify stress effects on wave velocities of relevance for 4D seismics, and to quantify the difference between static and dynamic mechanical properties and their anisotropies.