Microsiesmic Monitoring & Fracking: Downhole or Surface?

Growth in global energy demand encourages oil and gas companies to use secondary oil recovery methods for higher production rates – in particular, hydraulic fracturing. The efficiency of stimulation, especially in terms of ensuring high oil production rate, depends on the quality of EOR operations, on conformity of the actual fissure zone geometry and achieved filtration mode to the model parameters as set out by frac design. Undoubtedly, such “aggressive” stimulation techniques as fracking have to be monitored to make sure the process is under control. 

Taking into account the most common problems oil producers face when performing a frac, these are the key monitoring tasks:

• Identifying discrepancy between the frac design and the factual geometry and size of the fracture zone (including detection of frac asymmetry)
• Anticipating negative scenarios of fracture expanding beyond the target reservoir (including the area of neighboring water-saturated horizons)
• Identifying the causes of premature emergency pumping stops
• In situ control of fissure filtration properties
• Getting the data for instant correction of subsequent operations in multi-fracking projects
• Possibility of dynamic 3D visualization of the reservoir and development of the fissure zone in real time
• Fracturing quality diagnostics

 

Microseismic Technology

Microseismic monitoring is the best bet here, judging from the experience of oilfield services providers who develop monitoring technologies for fracking. This is confirmed by comparing different sensor monitoring methods for hydrofrac geometry ( see table). Microseismics provide the operators with a long-distance tool for defining the fracturing geometry (downhole or on the surface) and for receiving diagnostic 3D imaging in the process of fissure formation and development. This underlines the distinction between this method and acoustic techniques such as cross-dipole shear imaging used to estimate fissure azimuth only near the wellbore. The key advantage of microseismic monitoring is its higher reliability in determination of most geometrical parameters, therefore any research work on microseismic monitoring for hydrofrac is important.

The technology is based on registering seismic processes that follow the formation of the fissure zone and uses some special tools, such as continuous monitoring, tailored equipment and software. In the oil and gas industry, it has been successfully used for over 20 years, providing reliable data for real-time correction of hydrofrac design while minimizing risks and optimizing the production rate on difficult deposits.

There are various technologies of downhole and surface microseismic monitoring based on registering deep microseismic waves both directly in the fractured well and in neighboring monitoring wells, or on the surface using areal seismic setups. Given the different objectives and efficiency of monitoring, the technological risks and difference in the cost of such operations, the producer often faces the difficult choice of a suitable tool for monitoring the hydrofrac operations. This article discusses the features and capabilities of these technologies, as well as the conditions required for their successful application.

Concepts for monitoring systems are shown in figures (for downhole technology, we would use only the standard monitoring technique as low-traffic equipment for direct monitoring of the frac well currently provides insufficient volume