Abstract

Microseismic data generated during hydraulic fracturing contain a wealth of information, but currently, we are extracting only a small fraction of that information. The primary objectives of this novel microseismic data analysis methodology are three-fold:

1. Accurately image natural and stimulated fractures

2. Image and distinguish stimulated zones from under- and un-stimulated zones

3. Provide accurate flow and fracture parameters to be input into reservoir simulation

Microseismic waves travel through the whole subsurface; therefore, they are sensitive to elevated pore pressures (stimulated and hydraulically-connected zones) and elevated confining stress (un-stimulated and bypassed zones). Here, I present a novel methodology to accurately map the pressure and stress changes in detail within and around the reservoir using inversion of microseismic data. The methodology reduces the volume of microseismic data and then performs an efficient inversion to yield a 3D highresolution image of the effective stress changes in the subsurface. Inversion is performed using an advanced algorithm called waveform-inversion that generates an optimal map of the subsurface moduli that can accurately predict the microseismi waves passing through it. The same inversion procedure is performed for different stages of hydraulic fracturing to yield a time-lapse image of the reservoir that each stage stimulates. This enables more informative decision making in reservoir stimulation even in realtime. In addition, one can analyze the moduli image to determine the fracture opening and tortuosity to be input into the reservoir simulator. Moreover, such an image can provide a more accurate estimate of the stimulated reservoir volume.

Introduction

The permeability of reservoir rocks can be enhanced through hydraulic fracturing, wherein high-pressure fluids and proppants are pumped into rock formations to create fractures. These fractures create pathways for the oil and gas to flow from the stimulated portions of the rock to the wellbore and then to the surface.

The above process breaks up the rock at many locations in the formation. The disintegration or fracturing of the rocks are associated with a significant drop in the P-wave and S-wave moduli. At other locations, no fractures are created; instead the pore pressure increases because of the fluids pumped at high pressure. These zones will experience a drop in P- and S-wave velocities as can be seen from Figures 1 and 3. Yet at other locations, where the formation has neither been fractured nor has experienced a pore-pressure increase (because of the lack of hydraulic communication with the well bore), the rocks might be subjected to an increase in confining stress. The rock formations above and below the hydraulic fracture zone are a prime candidate for confining stress effects only. The P- and S-wave velocities would increase in these zones (Tosaya, 1982, Figure 1).

This content is only available via PDF.
You can access this article if you purchase or spend a download.