Abstract

A workflow is presented in this study named Fracture Evaluation & Design System (FEDS) that couples established technique of nodal analysis (well performance evaluation) and Quasi-3D (type of Pseudo-3D) hydraulic frac models. This methodology of hydraulic frac simulation is more robust in predicting post-frac production rates from fractured wells specifically in unconventional reservoirs like shales and has several advantages over utilizing frac models in isolation. The results from FEDS are compared with commercial frac simulators and discrepancies are noted. This workflow is divided in two segments, flow in frac (or reservoir) and flow in wellbore. The frac element is calculated through Quasi-3D frac model, a published hydraulic frac model that calculates frac geometry (frac length, width & height) in asymmetric multilayer formations such as Pakistan's Shales. This calculation is used to estimate the dimensionless fracture conductivity (FcD) which is a measure of effectiveness of the frac. These calculations are combined with reservoir parameters such as pressure, permeability and skin to generate deliverability profile or Inflow Performance Relationship (IPR). This IPR generated from calculated fracture geometry & conductivity inherently accounts for uncertainty of formation stresses, frac height implications, effect of permeability variation etc. This is a numerically calculated IPR, while fracture growth is being modelled; IPR is constantly updated based on fracture model results in a fully coupled setting. Second element of wellbore hydraulics or Vertical Lift Performance (VLP) is calculated using several published correlations such as Gray et al. The idea behind incorporating VLP in frac simulation is to model effects of water holdup, slippage, multiphase flow etc. Most commercial frac simulators utilize correlations of FcD to estimate post frac production, such as cinco-ley et al correlation. However, often production at surface is hampered due to wellbore effects such as water slippage. This is one of the major reason, despite having reliable input data, design post frac profile is much higher than realized production. The working of this workflow is validated by applying on two field fracture treatments. One of this treatment is in conventional sandstone reservoir while other is unconventional. The design & post frac production prediction is conducted in published frac models (that are used by commercial simulators) and using Quasi-3D frac model in FEDS. In all cases, production predicted by FEDS is significantly lower than commercial simulators. Main frac treatment is conducted as per design in these two reservoirs and actual post frac production is measured. The instantaneous gas production from both reservoirs is in better agreement with production predicted by FEDS validating the calculations of this workflow. This workflow brings together two well established techniques of petroleum engineering to evaluate effectivity of fracture treatments in the system as a whole. The modular nature of this system allows the utility of any vertical lift correlations, even calibrated or mechanistic models that best replicate the well in question. Further, with system optimization option available, several sensitivities can be readily run to evaluate range of uncertainty in post-frac production.

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