02-bhs tool & acquisition
DESCRIPTION
asTRANSCRIPT
OCTOBER, 2010
HO CHI MINH CITY UNIVERSITY OF TECHNOLOGYRESEARCH CENTER FOR TECH. & INDUSTRIAL EQUIPMENT
training course
Borehole Seismic Tool and Acquisition
VERTICAL SEIMIC PROFILE PROCESSING AND INTERPRETATION VIETSOVPETRO PETROLEUM
JOINT VENTURE
Borehole Seismic Survey
1 Borehole Seismic Introduction
2 Borehole Seismic Tool and Acquisition
3 VSP Processing
4 Sonic Calibration and Synthetic Seismogram
5 VSP Examples
Kieu Nguyen Binh
HCMC-2010
#2
Borehole Seismic Tool
and Acquisition
Downhole Tools
Downhole Seismic Tool40 level VSI tool
Downhole Seismic Tool40 level VSI tool
Gimbaled and Non-Gimbaled Geophones
Gimbaled
Z geophone is always vertical
Easier to interpret
Non-gimbaled
Geophone is fixed to the tool
Requires omni-tilt geophones
Mechanically simpler to implement
Data often projected back to vertical – horizontal reference frame
Gimbaled Non-gimbaled
Logging tool
Borehole seismic tool considerations
• X, Y and Z geophones
• Number of shuttles and inter-shuttle spacing.
• Clamping mechanism and force.
• Data transfer rate on logging cable, and time
between shots.
• This is a big factor when using large array tools.
• Tool diameter
• For thru tubing or thru drillpipe operations
• Temperature rating
• Interface with seismic source
4 shuttle configuration
15 metres
Tool Conveyance Methods
Tool conveyance method for standard logging tools
In a highly deviated well, may need drill pipe conveyed logging (TLC).
Difficult to use an array seismic tool with Pipe conveyed technique. Need a “stiff bridle”.
VSP in High Deviation Wells Thru-Drill-Pipe VSP (open hole or cased hole)
VSI tool ID is 2.5”
VSI tool can be pumped down inside the drill pipe
Example from
Malaysia
Downhole TractorMaxTrac
Downhole tractor to pull the VSI tool
Only for cased hole.
This is a better option for data quality, than thru-drill-pipe.
MaxTrac is routinely used with 4-level and
8-level VSI tools in the Middle East.
Sources & Source control
Examples of Good
and Bad VSP data
The 3 main factors that influence VSP
data quality:
1. Downhole environment
2. VSP tool
3. Surface energy source
In this example the downhole
enviroment is noisy, and the
airgun array has problems
Examples of Good
and Bad VSP data
Same as previous slide but
with increased trace overlap
A good quality VSP requires
A perfectly repeatable signature
G and GI gun configurations
G-gun 150 cu in (only used for CS surveys)
Double G-gun 300 cu in
Triple G-gun 450 cu in
GI gun 255 cu in (105+150)
G-GI gun 405 cu in
3 gun array for offshore
3 Gun Array
Top right shows a 3 gun array with
an “in-sea” Trisor gun controller
Airgun pits on
land
Typically recommend a pit
4-5 metres deep
4-5 metre wide
Land Airgun pit and gun deployment
Source type: G-GI air gun cluster
Azimuth: 80 deg Offset:17 m
Source Elevation:2.0 m below GL
Hydrophone elevation : 1.0 m below GL
Offset and Walkaway VSP surveys
Additional hardware required
A boat from which to deploy the airgun.
A crane on the boat.
A navigation system on the boat interfaced to the wireline logging unit.
A radio controlled airgun firing system.
– Fire the airgun
– Record navigation for every shot
A rig tender for a deepwater semi-sub in Malaysia.
The crane boom can extend to about 10 or 12 metres
Safe distance of airgun from the boat hull
3 gun array = 8m
2 gun array = 6m
Single gun = 4m
Vertical Incident V.S.P or
Walkabove VSP
Source Vessel
Well Bore
Downhole
Seismic Receiver
Drilling Rig
Require:
- Remote radio control of airgun
- Navigation system
• Differential Global Positioning System
• For accuracy positioning the Seismic Source
• Mainly used for offshore applications using
supply boats
Navigation System
Deviated well VI-VSP - Target Circles
Receiver Positions
Rig
Target Circles
Well Track
Typical circle diameter 1/40 or 1/80 TVD
At 2000 m depth, 1/40 TVD = 50 metres
At 500 m depth, 1/40 TVD = 12.5 metres
Typically would tell boat 10 metres tolerance for entire survey, if
weather conditions poor, can got to 20 metres
WALKABOVE
Walkaway VSP - Line Example
Off Track error lines (Tolerance)
Ideal Shot positions
Actual shot positions
Air Gun Source
25m
(Typical)
Boat goes at constant speed = 4 knots = 2 metres/sec
At this speed, can fire the gun every 12.5 seconds gives 25m shot spacing
WALKAWAY
VI-VSP using single level tool
Example is a single level tool, doing 12
levels per hour
The time for 4-level VSI survey will be
about 3 times quicker.
Vertical axis is the borehole depth
Horizontal axis is in hours
1 hour grid
VI-VSP Acquisition Efficiency
Using a VSI-4 tool
Each point corresponds to one setting of VSI-4 tool
Survey speed is about 9 settings per hour, or 36
levels per hour
Vertical axis is the depth of the 4 level VSI tool
Horizontal axis is in hours
Choice between a rig source and VI source in a deviated well
Rig Source VSP
Compute vertical time-depth.
However, this may be approximate
as it will be effected by refraction,
which are difficult to correct for.
Provide VSP image under the well
trajectory. For a rig source VSP,
this will require migration.
VSP image to identify structure
under the well trajectory.
Rig Source & Deviated well
Choice between a rig source and VI source in a deviated wellVI-VSP
Compute vertical time-depth. Since the ray-path is vertical, then better measure of time depth is obtained.
If lateral velocity variations are present in the upper sections, the time-depth will still be correct at each shot point, but the computed interval velocity, may not be representative, since this computed from two adjacent shot points.
Provide a VSP under the well trajectory. For flat formations, migration is not required. If formations are dipping, migration can be considered.
VSP image to identify structure under the well trajectory.
Generally, both the time-depth and P-wave VSP image are considerably better for the VI-VSP configuration.
VI-VSP will provide an image that follows directly below the curved well path.
VI-source & Deviated well
This is an example from ray trace modeling
Left plot shows the well trajectory.
Right plot is the difference in the “vertically corrected transit time”, computed from a rig source VSP and a VI-VSP. This curve shows the OWT error that will result from a rig source VSP.
Error is 3 msec OWT (6 msec TWT) at 3000 metres.
At TD, the straight line direct raypath angle is 15 deg from vertical. The error occurs, since the raypath is not straight line, but follows a curved refracted path.
This error can increase or decrease, depending on the average formation structural dip.
Error in OWT from a Rig Source VSP
VSP data behind casing
This example is deviated well so
we can expect better results for
the VSP data if poor cement.
13.3/8 inch shoe
20 inch shoe
Top of cement ?
Cemented 9.5/8” casing
VSP behind casing
A single cemented casing string is
OK. Often well cemented single
casing is better quality than
open hole.
Un-cemented surface casing
generally give poor quality data
Double casing with one un-
cemented will generally give
poor quality data.
If well is deviated, then casing ring
will be more attenuated, than in
a vertical well.
VSP behind casing #2
General rule:
- VSP works well in open hole
- VSP works well in cemented cased hole
- Results cannot be guaranteed in casing with no cement (particularly large diameter surface casings)
- Deviated well with casing and no cement may work OK
Casing shoe
Poor cement / bad hole
behind casing
Free pipe
VSP behind casing #3