In this paper we present the workflow, and a case study, for optimizing wellbore spacing and completion design in unconventional reservoirs by integrating reservoir characterization techniques with numerical reservoir simulation. The case study is from a pilot project for which extensive microseismic, core, PVT, and well-log data have been collected and analyzed. The pilot project includes two horizontal wells targeting the Cleveland formation in Oklahoma, with wellbore spacing that increases from 750 ft at the heel to 1,500 ft at the toe. Different completion designs (e.g. stage length, and number of perforation clusters per stage) have been used for the two wells.
A discrete fracture network (DFN) was constructed from the location, magnitude and focal mechanism of the captured microseismic events, which are assumed to represent rock failure caused by hydraulic stimulation. The proppant volume pumped at each stage was then distributed into the DFN to obtain a subset of the total DFN that is likely to be propped. This allows for the application of different compaction curves (i.e. conductivity reduction due to pressure drop from depletion) for propped and un-propped fractures in the model. The DFN model was then integrated with petro-physical well logs, core data, interpreted horizons, and PVT lab measurements to build a detailed reservoir model. Permeability enhancement in x-, y-, and z-direction was estimated based on the number of fractures, their orientation and geometry in each cell of the reservoir grid. History matching of production data was subsequently performed to calibrate the reservoir model. The reservoir drainage pattern obtained from the history-matched model was used to determine the optimum wellbore spacing, as well as the preferred completion design for different scenarios of oil price.
The reservoir model obtained in this work captures the variations in fracture geometry and intensity along the wellbore, which is fundamentally different from traditional models that assume bi-wing fractures with uniform fracture spacing, half-length and conductivity along the wellbore. The quantified permeability enhancement for each cell, and the subsequent reservoir drainage pattern obtained from reservoir simulation, provide a success measure for treatment design of each stage, as well as the spacing among the wells.