Analytical Tracer Interpretation Model for Fracture Flow Characterization and Swept Volume Estimation in Unconventional Wells

Jain, Lokendra (Chevron Corporation) | Doorwar, Shashvat (Chevron Corporation) | Emery, Daniel (Chevron Corporation)

OnePetro 

Abstract Tracer surveillance has long been established as a proven method in direct characterization of the dynamic heterogeneity, swept volume and remaining oil saturation estimation of the reservoir volume. It is the only surveillance technique where measurements are made at the scale of the entire reservoir volume contributing to flow. Such application of tracer surveillance has primarily been constrained to the conventional floods. The direct translation of the established interpretation methods for conventional tracer tests (interwell/single well tracer tests) is not possible for unconventional reservoirs. Such translation is limited because for unconventional reservoirs, huff and puff technique is used where boundary conditions during injection vs production are different. For an unconventional offset well the tracer flow is like a conventional producer (interwell tracer test). This work aims to showcase an analytical tracer interpretation model for unconventional tracer flow for huff and puff/injection wells. The analytical model estimates the swept volume associated with the injection well and the offset well while quantifying the dynamic heterogeneity of the swept volume as a Lorenz coefficient to help characterize flow through the fracture volume. The analytical tracer model developed for the unconventional wells is based on the Method of Moments (MoM), the same method used for conventional reservoir characterization. The MoM is independent of any flow regime (only fracture dominated flow, fracture and matrix flow or just matrix flow and any combination thereof) conceptually applied to flow in unconventional reservoirs. This is based on material balance applied over the entire control volume. This novel analytical method estimates the flowrate from the tracer swept part for an injection well during flow back and for the offset well(s) after tracer injection. The flowrate estimation from the tracer swept reservoir volume is critical for MoM based analytical method for evaluation of the swept volume and the dynamic heterogeneity characterizing flow. The analytical model was verified against numerical simulations with varying fracture and matrix flow characteristics. The calculated reservoir swept volumes were within 5% of the simulation reservoir swept volume. The analytical model also quantified dynamic heterogeneity in the form of a Lorenz coefficient which shows direct correlation with parameters that characterize fracture conductivity (aperture distribution, half length) in simulation models. The model has been successfully applied to field data for flow volume characterization. The new tracer interpretation model allows for EUR optimization because recovery from unconventional reservoirs is directly corelated with the stimulated rock volume, fracture conductivity and how it is distributed. This novel tracer analytical method provides ranges for the swept volume and dynamic heterogeneity which could be critical for completion design optimization (well spacing, stage spacing, frac cluster spacing etc.) as well as potential IOR/EOR optimization for unconventional wells. The dynamic heterogeneity also allows for the direct estimation of flow conformance and addressing such conformance issues might help to recover additional oil.

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