Alan F. Baird

Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy

Here we present new measurements of an anisotropic ice fabric in a fast moving (377 ma−1) ice stream in West Antarctica. We use ∼6000 measurements of shear wave splitting observed in microseismic signals from the bed of Rutford Ice Stream, to show that in contrast to large-scale ice flow models, which assume that ice is isotropic, the ice in Rutford Ice Stream is dominated by a previously unobserved type of partial girdle fabric. This fabric has a strong directional contrast in mechanical properties, shearing 9.1 times more easily along the ice flow direction than across flow. This observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers, suggesting it is essential to consider anisotropy in data-driven models to correctly predict ice loss and future flow in these regions. We show how passive microseismic monitoring can be effectively used to provide these data.

Field Estimates of Fracture Compliances using Active & Passive Seismics

Fracture networks provide a major control on the permeability of petroleum reservoirs, thus the ability to remotely characterize them is of great importance. Seismic anisotropy is a useful attribute to determine fracture orientation and provide some indication of the fracture density, however it is often difficult to constrain their ability to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: the stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Although P-wave anisotropy is primarily controlled by ZN and S-wave anisotropy by ZT, we find that for waves propagating oblique to the fracture plane, both P- and S-velocities are sensitive to the ZN/ZT ratio. Thus we can exploit these sensitivities to can gain some insight into ZN/ZT of subsurface fractures. We demonstrate this by estimating ZN/ZT using: (a) Azimuthal variations in P- and S-velocities estimated from a near-surface refraction survey, and (b) shear wave splitting measurements from a hydraulic stimulation microseismic dataset. In both cases we found that the natural fracture ZN/ZT was relatively low (~0.2-0.3), but in the case of the hydraulic stimulation ZN/ZT appeared to increase after the initial stage of the stimulation. We suggest this may be due to the increase in fracture connectivity and the generation of new clean fractures

Integrated Hydro-mechanical and Seismic Modelling of the Valhall Reservoir - Predicting Subsidence, AVOA and Microseismic

In this paper, we describe a workflow that integrates geomechanical, fluid-flow and seismic modelling that can be used to predict the stress distribution and evolution as well as various seismic attributes before and during production. Poroelastoplastic constitutive model is used to incorporate matrix failure during simulation, allowing strain hardening and weakening to develop within the model. This is especially important for modelling reservoir stress path and stress path asymmetry. The material model also enables the prediction of when and where failure occurs in the model, allowing us to model the likely microseismic response of a reservoir. Specifically, the finite element code ELFEN is coupled to the production simulation code VIP. The workflow is applied to the data-rich Valhall reservoir, North Sea to model seafloor subsidence. Furthermore, the output from the hydro-mechanical simulation is linked with a stress-dependent rock physics model to predict seismic attributes, allowing for an additional assessment of hydro-mechanical simulation via comparison and calibration with observed seismic. The results of the subsidence predictions compare well with field observations. For instance, the AVOA response calculated from the output of the hydro-mechanical simulation modelling closely resembles that measured from field seismic data, despite the limited calibration of the rock physics model to the Valhall reservoir rocks.

Monitoring the evolution of fracture compliances during hydraulic stimulation using passive seismic data

Seismic anisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs. While many techniques to estimate anisotropy have been successful in inferring fracture density and orientation, they generally provide little constraint on the ability of the fractures to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: the stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Here we demonstrate a method to infer ZN/ZT using shear wave splitting measurements on two microseismic datasets from hydraulic stimulations. Both examples show apparent increases in ZN/ZT during the stimulation process. We suggest that this may be produced by the development of new, clean fractures that have a greater normal compliance than their natural counterparts, combined with increases in fracture connectivity and permeability. The ability to monitor ZN/ZT during stimulations provides a means to gain insight into the evolving flow properties of the induced fracture network, and may be beneficial for assessing the effectiveness of stimulation strategies.

Monitoring Fracture Network Stimulation Using Micoseismic Data

The successful exploitation of many reservoirs requires fracture networks, sometimes naturally occurring, often hydraulically stimulated. Microseismic data acquired in such environments hold great promise for characterising such fractures or sweet spots. The loci of seismic events delineate active faults and reveal fracture development in response to stimulation. However, a great deal more can be extracted from these microseismic data. Inversions of shear-wave splitting data provide a robust means of mapping fracture densities and preferred orientations, useful information for drilling programs. They can also be used to track temporal variations in fracture compliances, which are indicative of fluid flow and enhanced permeability in response to stimulation. Furthermore, the frequency-dependent nature of shear-wave splitting is very sensitive to size of fractures and their fluid-fill composition. Here we discuss a range of methods for extracting spatial and temporal variations in sub-seismic scale fractures.

Tracking Fracks: Evolution of Fracture Normal/Tangential Compliance During Hydraulic Fracture Stimulation

S-wave splitting (SWS) from microseismic events may be used to estimate anisotropy in the region around hydraulic fracture stimulations. The anisotropy can then be used to characterize the distribution of fractures in a reservoir. In addition to fracture orientation, SWS can be used to estimate the ratio of normal to tangential compliance (ZN/ZT). ZN/ZT is sensitive to (1) the stiffness of the infilling fluid, (2) fracture connectivity and permeability, and (3) the internal architecture of the fracture (e.g. fracture roughness, degree of cementation). Here we demonstrate the use of SWS to infer the evolution of ZN/ZT from two hydraulic stimulation datasets from tight gas reservoirs. In both examples we observe an apparent increase in ZN/ZT as the stimulation progresses. We suggest that this increase may be produced by the development of new, clean fractures that have a greater normal compliance than their natural counterparts, combined with increases in fracture network connectivity and permeability. The ability to monitor ZN/ZT during stimulations provides a means to gain insight into the evolving flow properties of the induced fracture network, and may be beneficial for assessing the success of drilling and stimulation strategies.

Microseismic Monitoring of Fracture Networks During Hydraulic Stimulation: Beyond Event Locations

The successful exploitation of tight-gas reservoirs requires fracture networks, sometimes naturally occurring, often hydraulically stimulated. Borehole microseismic data acquired in such environments hold great promise for characterising such fractures or sweet spots. The loci of seismic events delineate active faults and reveal fracture development in response to stimulation. However, a great deal more can be extracted from these microseismic data. For example, inversions of shear-wave splitting data provide a robust means of mapping fracture densities and preferred orientations, useful information for drilling programs. They can also be used to track temporal variations in fracture compliances, which are indicative of fluid flow and enhanced permeability in response to stimulation. Furthermore, the frequency-dependent nature of shear-wave splitting is very sensitive to size of fractures and their fluidfill composition. Here we demonstrate the feasibility of using such analysis of shear-wave splitting measurements on data acquired during hydraulic stimulation of a tight-gas sandstone in the Cotton Valley field in Carthage, West Texas.

Monitoring and Modelling Hydraulic Fracture Stimulation: Future Directions

Multiple hydraulic fracturing along horizontal wells has proved to be a game changer that has led to the economic recovery of a vast amount of natural gas from shale resource plays in the USA. Optimization of hydraulic fracture stimulations has generally been achieved using a trial-and-error approach; although the microseismic monitoring of event locations has over the last decade proved to be a key enabling technology. Reductions in gas price, combined with the push to exploit resource plays in highly populated areas without a well-developed supply chain, mean that there is increasing pressure to optimize hydraulic fracture stimulations. Use and integration of advanced microseismic monitoring and geomechanical modelling offers the potential to make a step change in the optimization of hydraulic fracture stimulation. In particular, interpretation of microseismic attributes such as the magnitude and frequency dependence of shear wave splitting can be used to track temporal and spatial changes in fracture density, compliance and potentially size. Geomechanical modelling of the stress distributions prior to and following fracture stimulation can potentially help optimize the spacing and sequencing of individual stages of a fracture treatment as well as identifying the optimal time to conduct workovers (i.e. refracturing).