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Defining Geologic Structure Encountered in Horizontal Well and Its Impact on Petrophysical Evaluation

In horizontal wells, the traditional formation evaluation can be effectively carried out only after the geometric relationship between the well trajectory and the target reservoir is correctly interpreted. With the omnipresence of horizontal wells nowadays, a priority in petrophysical uncertainty evaluation is to control the spatial uncertainty between the well path and the target formation. Because of the cost and technology access constraints, the set of logs in a typical horizontal well alone may not be adequate in fully defining the geometry. Consequently, constraints from the offset/pilot well logs, seismic images, and other geologic information are utilized to achieve an integrated geometric understanding. Through the analysis of a set of horizontal wells with extensive suite of LWD logs in a pilot study, this paper discusses the workflow to determine the geologic structure and fluid contact around the wellbore under complex geologic environment. The project evaluates the effectiveness of various combinations of LWD measurements to understand the role of geologic constraints in place of additional measurements. The investigation starts with the commonly applied log-correlation between neighboring wells, progresses to the inclusion of images for the geometric relationship between the well and bedding， and the addition of the advanced boundary-detection curtain sections around the wellbore to quantify the reservoir thickness and continuity wherever feasible. The lateral variation observed in horizontal wells improves our understanding of the reservoir extension and spatial variation. Modeling and inversion of logging tool responses and the understanding of the underlying response characteristics in various geologic environments also enable us to correct the environmental effects on some measurements to minimize the uncertainty in reserves computation and to understand the continuity of the fluid barrier. Field examples are presented with the focus on accounting for the complexity of the reservoir. In particular, the reality of gradual change in the saturation and/or shalyness does not fit nicely with the distance-to-boundary or DTB inversion model of step-variation in resistivity. In this pilot study, the service company provided a complete set of azimuth propagation resistivity logs for boundary-detection purpose. Therefore, the contribution of information content to the petrophysical evaluation by using various types of data is experimented. The project tells that the petrophysical evaluation must consider the spatial location of the wellbore inside the reservoir. The study provides workflow and examples to guide the interpretation sequence not commonly practiced otherwise.