NUMERICAL MODELING AND DIAGNOSTICS OF INTER- AND INTRA-WELL INTERFERENCE IN TIGHT OIL RESERVOIRS: TOWARDS OPTIMAL WELL SPACING DECISIONS

Abstract

The main objective of this dissertation is to provide sound understanding and mitigation solutions to the perplexing problem of well interference and its implications on completions design, well-spacing decisions, and ultimately the economics of unconventional reservoirs. Optimal fracture spacing has eluded reservoir and completions engineers since the inception of multi-stage hydraulic fracturing. Very small cluster spacing results in fracture-to-fracture interference and higher completions cost, whereas very large cluster spacing leads to inefficient oil recovery which is detrimental to the economics of the well. Furthermore, when US onshore oil producers transitioned from appraisal to development, they were surprised not only by oil price volatility, but also by the magnitude of infill degradation due to well interference. In simple words, lucrative results from appraisal efforts were not representative of infill operations. Hence, several numerical models were constructed to better understand inter- and intra-well interference based on finite-difference, and finite-volume methods. The physical principals utilized in these models are conservation of mass, Darcy's law, thermodynamic equilibrium of fluid components between phases, and the definitions of phase saturation and mole fraction to complete the system. Numerical models presented in this work are three-phase, transient, and consider compressible fluid flow. Fluid thermodynamics were addressed via equation of state. iii Since the numerical solutions are non-unique, actual production data were utilized from different basins to calibrate the numerical models via history-matching process. Additionally, available geologic data were integrated to construct fit-for-purpose geologic models that were used as flow domains. Findings from intra-well interference simulation suggest that drawdown strategy is more impactful to short-term oil productivity than hydraulic-fracture spacing. Drawdown strategy is even more impactful on short-term oil recovery than 20% error in porosity, or water saturation. Results suggest that the profile of producing gas-oil ratio depends on fracture spacing and has been interpreted within the context of linear-flow theory. Also, results clearly show that drawdown strategy and magnitude of intra-well interference can be optimized based on the desired economic metric (NPV, or IRR). For instance, if the objective is to maximize rate of return, then tighter fracture spacing may be accepted Simulation results from inter-well interference show that the unpropped fracture geometry could be higher than matrix permeability by a factor of 10 to 20. History-match results confirm that hydraulic fracture half-length could exceed 2,000 ft depending on the completions design. Results also show that a certain level of inter-well interference improves oil recovery. Based on observations from sector modeling, the acceptable level of inter-well interference is dependent on the business commercial objectives, oil pricing, and well cost structure.