Implications of Wellbore-Fracture Separation on Temperature Log Interpretation Studied Through Numerical Modeling and Field Measurements and Observations

Abstract

Temperature logging is an old technique that quantifies fracture height based on cooldown anomalies. Warm anomalies are very frequently observed in post-fracturing measurements. One of the reasons for these anomalies is misalignment of the wellbore with the fracture, which depends on the geometry of wellbore and preferential fracture plane. A systematic study is presented here to avoid misinterpretation of fracture height. Two mathematical tools were coupled: (1) a geometrical resolution of the 3D space around the wellbore and (2) a numerical scheme solving the heat transfer partial differential equation (PDE) in dimensionless form to simulate temperature evolution around the wellbore. Finally, the findings were tested and corroborated with a few field cases in deep, hot, clastic reservoirs. The temperature log was conducted with three passes and was used for interpretation in deviated wellbores. The first tool utilized the wellbore deviation, wellbore azimuth, and fracture azimuth to resolve the relative positions and detailed geometry in 3D space. The tool yielded the fraction of total fracture height that will be coincident with the wellbore for a given set of inputs. The outputs were then coupled with the numerical tool with an explicit finite difference code to solve the relevant PDE with appropriate boundary conditions for the given geometrical space for the angled/separated fracture. The results showed that the further the fracture separates from wellbore, the more difficult it is to observe cooldown if the temperature logging is conducted soon after fracturing. Delaying the temperature passes allows the cold front from fracture to move towards wellbore and is a viable solution to capture cooldown, as seen from field measurements and validated by the model. The field cases demonstrated some complicated temperature behaviors, and the understanding developed from the modeling tools aided in interpreting the anomalous trends. The possibility of constructing pseduo temperature logs, lowering the number of passes, and extending the approach for multiple applications is discussed. The innovative approach avoids pitfalls of false indications of fracture containment in deviated wells. It can be used to improve the utility of high-resolution temperature logging data to enhance efficiency.