Fluid-Filled Fractures Mapping Using S-Wave Attenuation on Microseismic Events

Abstract

Abstract

Summary

We disclose a method based on S-wave attenuation of microseismic events to map fluid-filled fractures. Since S-waves do not propagate through fluids, they offer a way to distinguish between wet (reached by frac fluid and proppant) and dry (caused by local stress and pressure changes only) fractures. We applied this principle on a hydraulically fractured horizontal wellbore to indirectly map the stimulated fractures.

Introduction

Recording the microseismicity triggered by the hydraulic fracture treatment is one of the few tools that allow us to estimate the volume in which the formation was affected by the frac treatment. Nevertheless, this “activated” volume calculated from the microseismic monitoring does not provide an exact value of the actual Stimulated Reservoir Volume (SRV), which contributes to the long-term well production.

Microseismic monitoring can show the volume stimulated by the hydraulic fracturing (induced microseismic events), as well as showing pre-existing faults surrounding the hydraulic fractures that slip when the pore pressure increases, triggering microseismic events. The SRV is then embedded within, and smaller than, the microseismic volume.

The microseismic monitoring experiment

A 5-stage horizontal well was hydraulically fractured and monitored using both a dense surface network of 30,000 single-component geophones and a string of 24 3-component geophones deployed in a nearby vertical observation well. The two networks were GPS time synchronized.

Principle

A microseismic event will generate both P and S-waves. When propagating through a fluid-filled fracture, the S-wave will be strongly attenuated.

Methodology and Results

The first step is to detect, locate and invert for the focal mechanism of the microseismic events recorded by the surface array.

An S-wave attenuation factor is then obtained for each microseismic event by comparing the amplitudes measured on the borehole records with what should theoretically be recorded according to the inverted focal mechanism.

It is then possible to invert the S-wave attenuation factor to map the zones that are producing the attenuation using a Monte Carlo approach, and this highlights the fluid-filled fractures. For this process we introduce the term shear wave attenuation tomography.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016