Rayleigh Frequency Shift based Distributed Strain Sensing (RFS-based DSS) is a novel fiber optic diagnostic technique capable of measuring strain changes with a spatial resolution as low as 20 cm and measuring sensitivity as small as 1 με. RFS-based DSS measurements rely on the frequency shifts of the Rayleigh backscattered spectrum, which is sensitive to temperature and mechanical strain changes. This work uses in-well RFS-based DSS and DTS measurements acquired during a single-stage stimulation. The goal is to quantify the mechanical strain changes by removing the temperature effects from the RFS-based DSS measurements using the DTS measurements and gain insights into the in-well stress shadow and near-wellbore fracture connectivity.
A new stage was stimulated in an already stimulated fiber well above the heel-most perforation. A cast iron bridge plug (CIBP) was placed between the heel-most perforation and the new stage to prevent fluid leakage. The RFS-based DSS measurements exhibit high spatial variations at the new stage depth than the DTS measurements due to the combined effects of temperature and mechanical strain effects. The temperature change from DTS measurements is converted to corresponding RFS-based DSS measurements using a scaling coefficient. The scaled temperature change is subtracted from the RFS-based DSS measurements to obtain the mechanical strain change. A geomechanical model is used to understand the strain and strain-rate changes during a single fracture propagation from the fiber well.
The temperature change obtained from the DTS measurements shows an adiabatic heating effect of wellbore fluid at the depths below the new stage and the CIBP. The RFS-based DSS measurements show a compression effect at the depths between the new stage and CIBP, contradictory to temperature change. The mechanical strain change obtained after removing the temperature effect shows a compression effect up to ∼300 ft below the injection depth at the end of the pumping, whereas the temperature change measurements show the cooling effect due to injection exists for ∼60 ft below the injection depth. This difference demonstrates the intensity of the in-well stress shadow effects.
This work is the first analysis to utilize high-resolution in-well RFS-based DSS measurements to understand the stress shadow effects and near-wellbore geometry. This work provides key information to understand the relation between strain and treatment pressure responses and their association with near wellbore fracture conductivity.
