Drainage-Fracture-Height Characterization Using Rayleigh Frequency Shift Distributed Strain Sensing in a Vertical Monitoring Well from HFTS-2

Summary

Understanding drainage fracture height is critical for optimizing multibench unconventional reservoir development and improving forecasting reliability. Rayleigh frequency shift distributed strain sensing (RFS-DSS) can measure strain changes along fiber during hydraulic fracturing and production. RFS-DSS data acquired from fiber installed in vertical offset wells show potential for characterizing drainage fracture height. This study revisits the Hydraulic Fracturing Test Site-2 (HFTS-2) data set and provides new insights into the potential of RFS-DSS for drainage-fracture-height characterization through cross-disciplinary data analysis and numerical modeling.

This study first conducts a series of synthetic simulations using a fully coupled reservoir geomechanics model to characterize the strain responses induced by pressure depletion. Then, a cross-disciplinary analysis of the HFTS-2 data set was performed, including RFS-DSS, low-frequency distributed acoustic sensing (LF-DAS), microseismic, pressure gauge, geochemistry, and geological lithology data. It enables a comparison between drainage height interpreted by RFS-DSS and created/drainage fracture height determined by other surveillance methods. This is followed by a field-scale fracturing and production simulation to evaluate the physical consistency of the interpretation.

Simulation results from synthetic cases with various fracture and reservoir configurations were used to build a reference catalog that supports the interpretation of RFS-DSS data in field applications. The RFS-DSS data in HFTS-2 were acquired during a well interference test. Pressure gauges show continuous pressure decrease in far-field even when wells are shut-in, enabling the interpretation of drainage height using this data set. After 13 months of production, the RFS-DSS-interpreted drainage height was approximately 40% of the created fracture height. This interval is effectively bounded by calcite-rich layers above and below. A field-scale model of HFTS-2 successfully reproduced the observed RFS-DSS strain pattern during the interference test, supporting that the compressive strain zone is associated with the drainage height.

This paper demonstrates the potential of RFS-DSS in characterizing drainage fracture height and supporting production allocation. RFS-DSS in vertical monitoring wells provides a more complete picture of vertical drainage profile compared with pressure gauge arrays and geochemistry methods, thanks to its spatially continuous measurement. Its application in multibench unconventional reservoirs has the potential to support the optimization of development strategies and completion designs, as well as the improvement of production forecasting.