Since the advent of horizontal wells, the oil and gas industry has come a long way to ensure optimal drilling and well placement practices. The proper placement of these wells has meant that Measurement and Logging While Drilling, MWD and LWD, have come to play a primary role in geosteering, placement and evaluation of highly deviated or horizontal wells.
This paper presents a case study from a complex carbonate reservoir that has low permeability zone in the middle of the reservoir and rimmed by a tar mat, which complicate the optimal placement of the horizontal producers and injectors. Conventional triple-combo logs lack the critical data to geosteer in this multifactorial environment. A comprehensive logging program was developed to integrate additional petrophysical data such as Formation Pressure While Drilling (FPWD), cuttings analysis, Nuclear Magnetic Resonance While Drilling (NMR-WD), and neural-network processing.
Formation pressure while drilling is used in real-time to detect penetrated low mobililty/low permeability zones. Pressure build-up and drawdown measurements along with fluid mobility are used to modify the well trajectory. NMR while drilling is applied for placement of power-water injection wells to detect zones bearing heavy immobile oil where the ability to inject is unlikely. Algorithms were developed to estimate the in-situ oil viscosity. Moreover, neural-network models that incorporate legacy well log and core data were used to predict litho-facies and rock permeability in real-time.
The workflow developed for this study demonstrates the necessity of additional petrophysical data and vigilance for real-time solutions to geosteer extended reach horizontal wells in heterogeneous geological systems.