Abstract
The Jurassic Najmah-Sargelu of west Kuwait can be thought of as a "hybrid" between a conventional and an unconventional reservoir. These systems form an increasingly important resource for operators, but their performance is unpredictable because matrix permeability is in the micro-Darcy range and production depends on natural fractures. Success depends on how well the static models are aligned to the dynamic production, and the effectiveness of a fit-for-purpose multistage completion on project economics. In this work we present our lessons learnt in production modelling these reservoirs and the coupling between reservoir simulation and the discrete fracture network (DFN).
Our reservoir models were constructed using a highly integrated approach incorporating data from all scales and disciplines (drilling, geophysical, geological, reservoir and production) and the production simulations were run using dual porosity and black oil models. As expected, the DFN played a key part of this effort. An iterative approach was used to adjust the DFN so that it was consistent with production observations. However, in all cases care was made to ensure the new DFN honoured the seismic, geological, well log and drilling data from which it was generated. Final, smaller adjustments were made to the simulation model at the log scale to match PLT data.
We used uncertainty analysis to run hundreds of simulation cases and found that the character of the natural fractures is quite well imprinted in the observed production data, particularly pressure buildup data. This gave us a better understanding of whether the natural fractures are diffuse and laterally extensive away from the wellbore or if they are localized close to the wellbore. Where reservoir simulation history matches inferred laterally extensive natural fractures, an good correlation was obtained with the natural fracturing from the DFN. This correlation was poor where natural fracturing was confined to a smaller depth interval (as observed from PLT), and is a result of the limitation in seismic resolution to resolve these natural fractures.
The lessons learnt from our work helps towards improved understanding of production mechanisms of these reservoirs and their natural fracture networks. This, together with higher resolution azimuthal seismic, advanced wellbore characterization data and multistage completions are the desired key ingredients for technically enhancing production in these reservoirs.
Natural Fractures Characterization and In Situ Stresses Inference in a Carbonate Reservoir—An Integrated Approach