Drawing a ‘Walk-Away Line’
3-D VSP Proving Its Deep Value
Vertical seismic profile (VSP) has been around for some time as a tool to provide time-to-depth for seismic well-ties.
But VSP for time-to-depth is so yesterday.
Today, VSP has moved beyond its roots to include a number of other sophisticated applications. Think 3-D imaging and inversion using geophones temporarily deployed in newly drilled wells.
“There’s growing interest in 3-D VSP imaging,” said Brian Hornby, senior adviser at BP. “There have been extensive surveys acquired on land and also offshore -- mainly in the deep water. These are huge efforts that have focused on trying to see the value of 3-D VSP to get higher resolution and image where you can’t image with surface seismic.”
Simply described, an offshore 3-D VSP imaging project is comprised of receivers in the borehole and a 2-D surface source geometry using a seismic shooting vessel. The downhole array acquires signals reflected off the subsurface structure, and these data typically are migrated using a prestack depth migration algorithm to create a 3-D image volume around the wellbore.
Hornby noted it is important pre-survey to evaluate what survey is required to attain the potential imaging “prize” and to examine the trade-off in survey parameters and expected results.
For example, rather than sensors deployed throughout the wellbore, would a less expensive limited array suit the purpose?
“My ultimate vision for 3-D VSP imaging is to have receivers everywhere in the borehole,” Hornby said, “from reservoir top to mudline, as much as possible. You get huge coverage and potentially get the best images away from the wellbore.
“Standard arrays give us a very limited geometry for VSP,” Hornby noted. “My push is to get very large arrays at reasonable cost.”
A Direct Image
VSP long has been used to determine the location of salt flanks. The traditional approach requires up-front information that is not always available, e.g., accurate knowledge of the shape of the salt.
Recent improvements in salt flank imaging methods are noteworthy.
In fact, if you never thought of interferometry as being cool -- cool being an apt description for a technique using VSP to image salt flanks sideways -- you’re missing the boat.
The interferometry method redatums the surface sources to the receiver array, resulting in virtual sources in the borehole at each geophone location.
“You end up with a series of common shot gathers where each receiver becomes the source one after another in the borehole,” Hornby said. “The big advantage there is, for example, imaging sideways to a salt flank, you don’t have to know what the overburden is or worry about anisotropy -- and you don’t worry about what raypath to get through the salt.
“You just directly get the image -- sideways.”
Safety in the Salt
Speaking of salt, there’s a concerted effort under way using VSP to help drillers safely exit salt with the drill bit. The potential impact of this application is huge, particularly in the Gulf of Mexico where such a high percentage of deepwater wells are drilled sub-salt.
A goodly number of these wells have problems exiting the salt, resulting in lost wells, sidetracks and other problems. It typically costs about $5 million to $20 million-plus each time that happens, according to Hornby.
Pore pressure beneath the salt is always a big unknown, and there’s an ongoing effort to look at the pore pressure aspect of the sub-salt environment using a variety of techniques that basically revolve around putting the VSP tool in the well.
“The driller stops just before the base of salt,” Hornby said. “They normally plan to set casing there, so they pull out, and we go in the well with the VSP tool and take a walk-away line. This is a 2-D survey comprised of a single line of source positions where the source vessel goes past the rig to some distance on each side of the rig and gives us a 2-D set of data.
“We take the data and use them to try to estimate the velocity below base of salt,” Hornby said. “After getting the numbers, we come up with an opinion on what the velocity is, and we convert that to pore pressure.”
The group then meets with the drilling engineer to provide input on mud weight, so the engineer can formulate a plan for the mud weight.
“This has been successful in several cases where they’ve changed the mud weight according to these measurements,” Hornby said, “and that has saved the well.”