Denis Heliot

A robust technique for well-log data inversion

Inverse problems associated with many geophysical measurements are often ill-conditioned, nonunique, and multimodal. Consequently, the gradient-type optimization methods to obtain model parameters become ineffective since the accuracy and convergence of these methods depend highly on initial position and search direction in the parameter space. Evolutionary algorithms which employ direct search global optimization technique are well suited for such problems. In this paper, we apply differential evolution algorithm to invert for the Earth formation properties from measured phase and attenuation by an electromagnetic propagation-type sensor and compare the results with those obtained by conventional method. Results indicate that differential evolution algorithm provides robust results although computationally the method is less efficient.

Discrimination of fracture and stress effects using image and sonic logs for hydraulic fracturing design

Selecting and designing the proper completion in naturally fractured reservoirs is always a challenge because of the mechanical and flow heterogeneities due to the fractures. Furthermore, when hydraulic fracturing is used to enhance the recovery, the interplay between the 3D stress field and the existing natural fracture systems becomes an important factor. Three mechanical scenarios might occur while fracturing the medium (Figure 1). First, the natural fractures may have no influence and the hydraulic fracture will propagate in a direction orthogonal to the minimum principal stress as expected in a classical model (Figure 1a). Second, only the system of natural fractures will be reactivated and eventually extended (Figure 1b). Third, both newly generated hydraulic fractures and natural fractures will intersect and propagate in a complex manner (Figure 1c). The tortuosity of hydraulic fractures will be greatly controlled by the anisotropy of the effective elastic medium due to 3D stress and natural fractures.

Application of wavelets in estimating dip angles and conductivities of Earth formation

Earth formations can be modeled as a layered medium with each layer having its own dip angle and conductivity. In a typical exploration environment, a wellbore (about 15 to 30 cms in diameter) is drilled to a depth that may extend to a few kilometers. Formation properties are measured using a sensor that moves along the wellbore trajectory. The sensor is remotely controlled from the surface. Formation resistivity is an important property, which is measured by a sensor consisting of several transmitters and receivers and operating at a frequency of a few MHz. Estimating dip angles provides valuable information in positioning the wellbore trajectory. We present a novel method for estimating dip angles and formation properties using a wavelet-based processing of well log data in conjunction with an inversion method.