In situ stress orientations and magnitudes in the Liard Basin of Western Canada

Abstract

Part 1 of this report presents the conclusions reached from identifying and recording the orientations of borehole breakouts recorded on 4-arm dipmeter logs from 47 wells in the Liard Basin of western Canada. Two borehole imagery logs were also examined and they provided orientation data for breakouts and drilling-induced fractures. Twenty-seven of the forty-seven wells exhibited 2 breakout populations. In the majority of wells, the major breakout populations exhibited mean azimuths that cluster between NW-SE and NNW-SSE. These orientations are interpreted to reflect the direction of SHmin, the smaller horizontal principal stress. A few wells exhibited significant breakout populations that were at high angles to this regional trend. It is suspected that, in these cases, nearby fault zones may have deflected the stress trajectories. A table of results has been prepared and all the supporting data are recorded in the Appendix. Figures showing breakout population axes, stress trajectories, and areas where fault zones may be deflecting stresses are included in the text. All the data presented in the report are available in digital format. Part 2 of this report describes gathering and processing subsurface pressure data from the records of wells drilled in the Liard Basin in northwestern Canada. Part 2 of the Liard Basin Study is concerned with establishing principal stress magnitudes at as many locations as possible across the basin and mapping how these parameters vary laterally at strategic horizons. Information from laboratory tests and field observations in numerous oil and gas fields worldwide indicate that oil and gas reservoir permeability is inversely related to rock stress. The less a rock body is compressed, the greater will be its permeability, all other factors being equal. That is the operational justification for this investigation, but it is also anticipated that the findings will contribute to an improved understanding of the tectonic evolution of that part of the Canadian landmass occupied by the Liard Basin. All the data analyzed here are in the public domain and come from well logs, drilling histories and well history reports. These sources of information provided data that was used to estimate the magnitudes of the vertical stress (SV) and the smaller horizontal stress (SHmin). The interpretation of vertical stress variation across the Liard Basin employed density logs from 64 wells. Analysis of these logs allowed a lateral variation in vertical stress magnitude (SV) to be mapped at various depths. The smaller horizontal stress (SHmin) was mapped using leak-off test pressure and fracture breakdown pressures. The larger horizontal stress (SHmax) was determined via numerical simulation of breakout failure, as well as being calculated algebraically. Horizontal and vertical effective stresses were calculated and mapped and their relationship to seismic activity was speculated upon.