Using production logs (PLT) to estimate the size of fracture networks

Abstract In naturally fractured reservoirs, determining fracture properties, such as size and permeability, is difficult due to limited data about the fractures. The primary information that is available, mainly from image logs or core, is known only at the wellbore; however, while fractures can be local to the wellbore region, they often extend hundreds of feet from the well. If fractures can be characterized more accurately, then flow paths and flow behaviors in the reservoir can be better delineated. The goal of this work is to combine dynamic data with static data to better characterize fracture properties, in particular, the size of fractures away from the wellbore. Production log (PLT) information is used in conjunction with borehole image logs (BHI) to more accurately define fracture properties. A single-well flow-simulation model is constructed, and discrete fractures are included at each location where wellbore inflow (observed on PLT logs) is correlated with fractures (observed on BHI logs). An optimization method is developed to automatically perturb the lengths and heights of model fractures until their flow response matches the actual measured response. This method is designed for wells that intersect multiple fractures. The technique is applied to two different examples where fracture sizes are estimated. Fracture lengths range from a few feet to thousands of feet. To provide parameter sensitivity for the fracture sizes, the examples are matched with multiple models. Different options are used for the different models to demonstrate the various capabilities of the program. The optimization is efficient as it generally finds solutions in about 10–20 flow simulations. Knowledge of fracture size can be used to estimate potential for fracture interconnectivity among wells in a field, reservoir drainage patterns, and implications of injection and production patterns, all of which can impact reservoir management decisions. Furthermore, fracture size distributions can be used as an input for flow-simulation modeling, thus providing an improved physical basis for modeling overall reservoir performance.