Abstract

Shell and PetroChina are partnering in a pioneering Joint Cooperation Project in the Sichuan Basin targeting the Lower Silurian Longmaxi shale gas play. Since 2010, over 20 wells, including both verticals and horizontals, have been drilled and completed with various hydraulic fracturing technologies. Unlike most of the North American shale plays, the Sichuan Longmaxi shale play is characterized by abnormal formation pressure, closure pressure close to overburden, low stress anisotropy, and regionally active tectonics, all of which contribute unique challenges to hydraulic fracturing stimulation. The combination of these geological challenges results in high treating pressures, difficulty in proppant placement, constrained fracture height growth and complex fracture geometry. Under such subsurface complexity, an optimal completion design requires insightful understanding of the geological settings and reservoir characteristics, as well as a large number of wells for technology trials in a "trial and error" approach, which often results in a large capital investment and a long value realization cycle time.

To meet these challenges, an advanced workflow that integrates geomechanics modelling for hydraulic fracture simulation and 3D reservoir modelling for well performance using reservoir simulator is developed in collaboration with Shell global experts in unconventional resources. This workflow provided a much more economical and efficient solution in understanding the reservoir responses to hydraulic fracturing, quantification of estimated ultimate recovery (EUR) ranges, and potential maximum EUR uplift by increasing completion intensity or technical limit EUR.

The geomechanics modeling workflow (Bai et al. 2016) is a truly 3-D hydraulic fracture simulation platform that integrates structural framework, stress setting, formation characterization, flow dynamics, hydraulic fracturing monitoring, and well completion parameters into a finite-element based modeling environment. Key capabilities that distinguish this platform from conventional hydraulic fracture simulators are the ability to model complex fracture initiation and propagation as a consequence of natural fracture systems, hybrid stress regimes, and formation heterogeneity. Once calibrated with hydraulic fracturing field diagnostics, the model performs hundreds of numerical runs accounting for multiple subsurface realizations and well completion design scenarios. The optimized well completion design is selected based on the modelled Stimulated Rock Volume (SRV) that corresponds to increased EUR at optimized costs. Two suites of models were built for this study, each of which represents key play segment in the concession block in terms of profitability and materiality. EUR uplift potential is identified through optimizing well placement and completion design, such as well orientation, landing interval, pump rate and job volume, perforation spacing, and stage spacing.

With the hydraulic fracture or SRV geometries derived from the geomechanics modelling, the reservoir simulation workflow was then performed to evaluate EUR uncertainty and estimate the EUR uplift based on geomechanics modeling results hydraulic fracture or SRV geometries from both the reference and optimized cases. These derived hydraulic fracture geometries and their corresponding SRVs were built in the reservoir simulation model and history matched to generate EUR ranges through multiple-realizations.

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