An unconventional idea

Open to Interpretation

American Association of Petroleum Geologists (AAPG)

Production from unconventional resource plays, principally shale deposits, is transforming North America into a major energy force.

To get to this point, however, has been a supreme challenge to geoscientists, drillers and many other industry participants.

Unconventional tight sand plays have been around for some time, but the fairly recent proliferation of E&P in numerous different shale formations represents a kind of whole new – and complex – world for these experts.

For starters, production variability among these dense low permeability rocks can be extreme – even between neighboring wells.

After years of conventional resource E&P, operators who opt to take on the unconventional targets quickly find that this entails a kind of back-to-the-books effort to determine what exactly is going on.

Valuable Data

The obstacles and pitfalls involved in this switch are very familiar to the folks at Fasken Oil and Ranch Ltd., which has been a part of the oil and gas industry since the 1940s, when oil was discovered on the west Texas C Ranch, long owned by David Fasken. The company is celebrating its centennial anniversary.

After decades-long production from conventional carbonate and sand reservoirs, the company shifted its focus to a plethora of drilling targets in low permeability formations such as the Spraberry, Wolfcamp and Cline Shale. Fasken also owns large acreage blocks in the high profile Eagle Ford shale and the Bone Spring horizontal play.

AAPG member Glenn Winters, chief geophysicist at Fasken, noted that geophysics plays a major role in optimizing production performance by well placement, especially in tight oil reservoirs. When it comes to staying in zone and monitoring the drilling process, geophysical data are invaluable.

“Getting to depth is a critical duty in order to generate well paths for horizontal wells,” Winters noted. “Using different techniques in combination, such as converting volumes to depth by stacking velocities by the processor or utilizing p-wave inversion data and taking seismic horizons to depth several ways, helps to create the boundaries for the drilling engineer for the well path.”

Different Strokes

Fasken is using different geophysical techniques to evaluate key factors in its three play areas.

“For the everyday interpreter, there are obstacles and pitfalls in dealing with unconventional resource plays,” Winters said. “These can pop up in the realm of seismic data reprocessing, which data volumes to use (and when), depth conversion and integration of microseismic in 3-D volumes.

“It’s imperative to choose the right data set for interpretation,” he said. “It’s fairly common that I work with four or more data volumes that could consist of structural, high bandwidth, inversion, depth, and attributes such as coherency, geobodies or others.”

After choosing the appropriate volume, Winters explained that they begin looking at the best way to display the data as slices, line and crossline, or else look at the combination of volumes in visualization software.

“We need to determine which attributes can maximize efficiency for the task at hand,” he commented. “If I just want to identify locations for major structural elements in multiple formations, I may use one volume to interpret and just draw faults, in contrast for placement of a horizontal well in a single formation where the fault contacts and position are crucial to the well path.”

Regarding the exceptional production variability often observed between adjacent wells in shale horizons, Winters noted that one of the lessons learned from analyzing seismic data in the Wolfberry play is that the geology can change somewhat abruptly. Intervals found in the stratigraphy of one well may not exist in the next one; this can be seen using the appropriate seismic data.

He emphasized that optimizing well locations and drilling the best wells at the beginning enhances production and cash flow.

Working Together

The Fasken team also is looking closely at integrating microseismic data into the interpretation process.

Winters indicated that a lot of small companies are very interested in doing this. Yet accessing high-end microseismic software requires big bucks, and many of the smaller companies likely won’t use the product often enough to justify the cost.

He is a big believer in time lapse microseismic monitoring, noting that Fasken at one time laid out a permanently bedded array comprised of cemented geophones buried 200 feet deep over 16 square miles in the Wolfberry. This was done in an attempt to allow them to monitor completed wells in the area and to be prepared to monitor horizontal completions in selected formations in the future.

This can provide the key to determine how those formations fractured differently, according to Winters.

When the time comes to refracture a tight oil zone with declining production, time-lapse microseismic monitoring can be the big factor in helping to detect where the fluids exited the reservoir following the initial injection.

“We have to understand the fracture treatment before we can effectively refracture and enhance the stimulated reservoir volume,” Winters said. “We have to know the results of that initial treatment in order to design a refracture program.

With all things shale, it’s imperative that engineers, geologists and geophysicists all work together in these unconventional plays, in order to accomplish their objective(s).

Winters emphasized that this includes communicating with one another in an understandable language so that everyone is on the same page.

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