Publications

Various arrays of pole-pole and dipole-dipole electrical logs of standard up to ultra-long spacings have been modelled in an environment of conductive borehole and resistive formation. The forward modelling has been done with a 2D finite element code solving Laplace's Equation. As the frequency of the measure currents is sufficiently low, the skin depth is much larger than the distances involved, and therefore the additional complication of a full Helmholtz Equation solution is unnecessary.The code has been run on various synthetic conductivity models in order to determine the effects of resistive and conductive shoulder beds, of conductive invasion, and of the conductive borehole.Logs of the same electrode configurations and spacings have been run in a well drilled in a hot dry rock (HDR) geothermal reservoir in Hijiori, Honshu, Japan. The existence of both local mechanical- and thermal-stressrelated fractures close to the borehole as well as natural fractures intersecting the borehole has been confirmed by sonic, natural gamma, flowmeter, and temperature logs. This fracturing requires the introduction of a conductive invaded zone.The electrical logs have been inverted by iterative forward modelling using the same 2D finite element code in order to derive a model of rock conductivity which is horizontally lay ered, invaded by wellbore fluid, and incorporating varying borehole diameter. Iteration has continued until the short-, intermediate-, and long-spacing electrical logs match with those derived from forward modelling the layered model.Residual differences on the ultra-long spacings have then been interpreted in terms of conductive fractured zones remote from the wellbore by tens of metres. This interpretation has been done by further application of the 2D finite element code with a remote axisymmetric conductive zone. This has finally been interpreted in terms of distance to the remote fractured zone in a 3D geometry by means of a 3D Laplace's Equation finite element code.