It is easy to justify 4-D surveys offshore. Land surveys provide a few additional hurdles.
Four-D seismic technology has become a routinely applied reservoir surveillance tool that is demonstrated to increase reserves and recovery by locating bypassed and undrained reserves, optimizing infill well locations and flood patterns, improving reservoir characterization, identifying compartmentalization, and mapping permeability pathways. In addition, 4-D seismic technology can help optimize field development plans by decreasing operating costs with fewer dry holes and reducing reservoir model uncertainty, which results in more effective reservoir management.
However, the vast majority of 4-D surveys have been offshore. Land 4-D surveys are far more limited. Most of these surveys are designed to monitor enhanced recovery processes, either thermal or CO. These processes are expensive, and surveillance is critical for efficient reservoir management. Most land 4-D surveys have been acquired over a relatively small area, many are research or pilot studies, and results are mixed. Interpretations of 4-D data over thermal and CO2 floods are most compelling . Other applications can have less than convincing results.
Why has 4-D technology been so successful offshore and so limited onshore? What are the technical and business challenges that impact land 4-D application? And what forces are at work to overcome these obstacles?
Repeatability is a key factor in 4-D success, and land 4-D has one distinct advantage over marine 4-D: source and receiver positions can be repeated with a high degree of accuracy. This is important because rapid near-surface variations found onshore demand accurate source-receiver repositioning to minimize differences in backscattered, shot-generated noise. But near-surface properties also are subject to seasonal and water-table changes, making repeatable surveys challenging.
In general, land seismic data have poorer quality compared to marine data, often dominated by ground roll and refractions. Working oil fields have high ambient noise. Infrastructure is more spread out than in offshore fields and can impact a greater area of the survey. New construction can prevent reoccupation of baseline source and receiver positions on monitor surveys. There also can be permitting issues that limit access to parts of the survey area and, in some fields, rough terrain. Because of the size of land crews, there is greater HSE exposure and a larger environmental footprint compared to marine surveys.
Developments in land seismic acquisition could help overcome some of these challenges. Cablefree recording systems result in more efficient field operations with greater accessibility and a smaller footprint. Increased channel counts result in a higher signal-to-noise ratio. Limitations of the near surface might be overcome by permanent and continuous monitoring systems and by concepts like the virtual source. Indeed, results have shown that with careful attention to detail in acquisition and processing, land 4-D seismic data can be repeatable.
Still, onshore reservoirs present additional hurdles compared to their offshore counterparts. Increasing activity in unconventional and fractured reservoirs will require developments in rock physics to better understand the relationship between field depletion and the 4-D seismic response. The rocks in onshore fields tend to be older and, in many parts of the world, dominated by carbonates. As a result, the sensitivity of the onshore reservoir's elastic properties to fluid saturation and pressure changes is generally less than what is seen in offshore clastic reservoirs. But there have been many successful marine 4-D surveys where the changes in elastic properties are as small as those predicted for tight rocks and carbonates. This suggests that if noise and data quality issues can be overcome, the application space for 4-D onshore can grow substantially.
Finding the value proposition
But the greatest challenge facing land 4-D application is economic. It can be difficult to define the value proposition for land 4-D when the unit cost for land seismic is significantly greater than for marine seismic and when well costs are significantly less expensive. It is easy to justify a deepwater marine 4-D survey where individual wells can cost more than US $100 million. It is harder on land, where well costs might range from $3 million to $20 million.
The global onshore resource base also is shifting away from oil toward gas. Conventional gas has high recovery rates, averaging 60%, which means there is less incentive for seismic monitoring compared to oil fields. And while passive microseismic monitoring of unconventional gas development is commonplace, conventional 4-D seismic monitoring of these reservoirs is nearly nonexistent.
There are, however, several forces acting in the industry that could help overcome these economic hurdles. Production costs associated with depletion strategies used for many onshore fields are higher than for offshore fields. According to the International Energy Agency (IEA), conventional oil and gas (outside deep water and ultra-deep water) costs about $5/bbl to $40/bbl to produce. For COfloods, heavy oil and bitumen recovery, and other EOR processes, production costs range from $30/bbl to $80/bbl, requiring more efficient operations and increasing the incentive for reservoir surveillance.
In situ production of heavy oil sands and bitumen, which already represents the most common application of land 4-D, is expected to grow from about 1 MMb/d in 2010 to more than 5 MMb/d in 2030. According to one IEA scenario, enhanced oil recovery will similarly grow from about 1.25 MMb/d in 2015 to 5.5 MMb/d in 2030. These projects will mostly involve COinjection and are almost exclusively onshore.
Additional opportunities for land 4-D could be substantial. Onshore fields represent the bulk of the current world resource base – 55% of the fields but more than 80% of the proved and probable reserves. The outlook for the future is essentially more of the same. By 2030, the IEA estimates that more than 60% of daily production is expected to come from onshore fields. And carbonate fields will dominate this production. Their inherent complexity practically demands 4-D seismic to monitor fluid movements.
There is one additional incentive for land 4-D – environmental monitoring. There is likely to be a significant increase in carbon capture and sequestration (CCS) projects. Current CCS pilots are almost all located onshore and are spurring the growth of 4-D seismic to monitor the COinjection, driven in part by regulatory requirements. In addition, about 60% of Middle East gas reserves have high HS content, potentially spurring demand for monitoring sour gas injection.
Is there a future for land 4-D seismic? There are certainly technical challenges and economic barriers. But with increasing production from onshore fields, increasing exploitation of heavy oil, increasing EOR, and the increasing need for environmental monitoring, the opportunities for land 4-D are significant. Perhaps we are asking the wrong question. If there is a future for 4-D seismic, is it going to be on land?