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