BCGS April Technical Talk – Thursday April 17^th, 2014

Speaker: Peter Fullagar, Fullagar Geophysics

Title: 3D magnetic modelling and inversion incorporating self-demagnetisation and interactions

Date/Time: Thursday April 17^th, 2014 at 4:30pm.

Location: Room 451, 409 Granville St (UK Building at Granville and Hastings), Vancouver

Abstract:

Self-demagnetisation can significantly reduce the amplitude and modify the shape of the response from highly magnetic bodies. The direction of magnetisation rotates in a manner determined by the shape and orientation of the body. Furthermore, when highly magnetic bodies are in close proximity, the magnetisation induced in one body is affected by the magnetisations in all the others. When modelling highly magnetised bodies, it is important to take both self-demagnetisation and interactions into account. Inverting for magnetisation vector has become popular recently. However, this is not a substitute for physically valid magnetic modelling. Magnetisation inversion is highly nonunique, with the result that any particular solution must be interpreted with care. An inverted magnetisation rotated from the ambient field is not necessarily indicative of remanence. Moreover, relating an inverted in situ magnetisation to rock properties is often problematic. In this respect, magnetisation inversion complicates ground truthing. Potential field modelling and inversion software “VPmg” has been upgraded to account for self-demagnetisation within, and interaction between, 3D magnetic bodies. Remanence can be taken into account. The algorithm computes H-field perturbations at the model cell centres in two stages: initialisation and optimisation. During initialisation, a first estimate for the H-field perturbation is derived from the demagnetisation tensor computed for each cell.

During optimisation, the H-field perturbation is refined iteratively via an inversion procedure. The algorithm has been validated for homogeneous spheres, spheroids, slabs, and cylinders. It has also reproduced magnetic interactions between two horizontal cylinders, published by Hjelt (1973). Explicit verification for complex heterogeneous bodies requires a suitable independent algorithm for benchmarking. The application to inversion in highly magnetic environments is illustrated on field data examples.

BCGS March Technical Talk – Tuesday March 18^th, 2014

Speaker: Sarah Devriese, PhD Student, UBC

Title: Using Electromagnetic Methods to Image SAGD Steam Chambers

Date/Time: Tuesday March 18^th, 2014 at 4:30pm.

Location: Room 451, 409 Granville St (UK Building at Granville and Hastings), Vancouver

Abstract:

Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery method, used to produce bitumen from oil sands. The success of this technique is dependent upon the propagation of steam throughout the bitumen layer. However, heterogeneity in the reservoir rock can cause the steam to not always propagate as desired. Monitoring the location and growth of steam chambers provides vital information about the production efforts and has traditionally been done using 4D seismic. While successful in many cases, it may be limited by low sensitivity to changes in fluid content, saturation, and porosity. Alternatively, electrical conductivity is significantly affected by the injection of steam into a bitumen layer. Thus electromagnetic (EM) surveys have greater potential in imaging the steam floods while providing higher resolution compared to traditional 2D DC resistivity surveys. The design of an EM survey for monitoring SAGD is crucial in achieving the highest possible resolution at a minimal cost. This talk will introduce a practical methodology for survey design that addresses location and geometry of electromagnetic transmitters. The technique is applied to a synthetic model that emulates the Athabasca oil sands environment and the results are compared to inversions of 2D ERT data. Two-dimensional inversions of the ERT data produce a low quality image with artifacts but the image is improved by carrying out a 3D inversion. However, a superior image is obtained by working with a modest number of transmitters and inverting multiple-frequency EM data in 3D. The results provide optimism for the potential of using 3D EM for imaging subsurface steam chambers.

POST-TALK: Attendees are encouraged to grab refreshments together at Moose’s Downunder afterwards