Low-Frequency Seismic Illuminates Shale Plays

?Low-frequency (LF) 3-D surveys take surface measurements of changes in the earth’s ambient wavefield that can be related to properties of hydrocarbon reservoirs. This variability has been observed consistently in oil, gas, and tight gas reservoirs through surface measurements.

There are many benefits of LF seismic compared to other E&P monitoring technologies. LF seismic is influenced directly by the reservoir fluid system and not structural variations and is low cost and environmentally friendly with a light environmental footprint through the use of standalone recording stations.

Testing for variations or anomalies in LF attributes has significant applications in unconventional reservoirs. This is achieved through LF seismic’s ability to potentially identify sweet spots with higher concentrations of hydrocarbons, decrease costs through improved fracture planning, and reduce the risk of uneconomic wells. LF seismic has the potential to be particularly effective in shale plays.

Predrilling stage

Crews lay out equipment to shoot a survey in Wyoming. (Images courtesy of Spectraseis)

The cost of failure is high, particularly in fields where hydraulic fracturing is an integral part of the production process. For example, a conventional well with a budget of US $10 million can be plugged and abandoned if, while drilling down to the zone of interest, it is deemed to be uneconomic. The operator then has the option of cutting its losses, limiting cost to $2 million and saving the remaining $8 million for another drilling program.

However, with unconventional resources such as shale gas, operators do not have the luxury of cutting their losses at an interim evaluation point. They normally are not sure of the economics of the well until they have completed the costly fracturing operation and brought the well on production. This makes a failure rate of only 20% to 30% in shale gas (where wells can cost between $10 million and $15 million) prohibitively more expensive than a failure rate of even 60% in conventional wells.

By helping to identify higher accumulations of gas prior to drilling, LF seismic can help operators reduce the risk of uneconomic wells and costly fracture stimulation programs. More than $30 billion was invested in 2010 by the industry in acquiring shale gas assets, and it is vital that these assets are made economically viable from the pre-drilling stage onward.

Fracture monitoring

The working hypothesis in unconventional reservoirs today is that the reservoir fluid flows into the fractures created. Most or all of the effort in shale E&P focuses on engineering fractures and monitoring the fracture network through the use of microseismic techniques.

Whereas microseismic techniques are intuitive and might give the operator an indication of the fracture network distribution as well as fracture growth characteristics, such techniques cannot tell the operator whether the reservoir fluids are behaving in the way expected.

This EUR map over the Jonah and Pinedale tight gas fields in Wyoming highlights the fault systems that control reservoir productivity.

If LF can monitor the reservoir gas plume directly before, during, and after fracturing, then over time valuable information on the fluid can be used to calibrate the reservoir and production models. Reservoir monitoring and time-lapse techniques provide critical information on the reservoir fluids’ behavior in these non-homogeneous reservoir systems.

When integrated with other reservoir data, LF signals directly affected by the fluid in the reservoir could provide important information on (a) conditions affecting the reservoir fluid flow, (b) fracture performance, and (c) decline curve analysis. LF fluid measurements are, therefore, complementary to conventional 3-D seismic and microseismic measurements.

Environmental implications of shale reservoirs

LF seismic also has potential applications to help shale reservoirs meet environmental requirements. The last few months have seen considerable controversy in the US about the fact that hydraulic fracturing is exempt from the Clean Water Act. There is an ongoing debate in the industry as to the environmental impact, if any, of the fracturing process, from the frac fluid itself to the gas released as a result of the process.

Through direct monitoring of the multiphase fluids integrated with knowledge of the fracture network, LF seismic would provide the data that might help to resolve the debate and provide the means to catch problems before they occur. Testing this concept would be required and could involve a multiphase fracturing fluid with an additive that provides an LF signal (e.g., diesel in water).

The LF attribute PSD-IZ (shown in red) correlates well with the measurements of the reservoir in the field.

Testing the hypothesis

Spectraseis is planning a test over a shale gas field in the US where it will be running a traditional 2-D LF seismic profile over a large area to determine if there are any variations and to see if the gas sweet spots can be mapped.

Many geoscientists believe good shale reservoirs have better porosity and less clay minerals than the less-productive shale zones, with a compressional-to-shear wave velocity (Vp/Vs) contrast and porosity similar to a tight sand reservoir. The natural permeability is expected to be low, but the best zones might show evidence of some overpressuring. It is possible that 3-D seismic and other geophysical methods could identify areas of Vp/Vs contrast, but reservoir fluid measurements are needed to better qualify reservoir potential and reduce economic risk.

Results obtained from a tight gas field project in Wyoming suggest that low permeability and porosity combined with overpressuring generate an LF signal. These varying effects also could be observable in shale reservoirs. One should expect to see changes in the LF attributes if a large enough area (covering poor to good) is surveyed. Long 2-D profiles or 3-D grids should be used, depending upon the nature of the reservoir targets.

With operators needing to (a) reduce risk at the predrilling stage before committing to expensive drilling and fracturing programs, (b) generate more information during fracturing for input into the reservoir and production models, and (c) meet environmental concerns, the LF seismic measurement of the fluid locations in the reservoir is a technology with significant applicability to shale plays today and in the future.