Case 1: Reservior identification
SDVL technology can be applied to a reservior development while drilling. Vibro-acoustic signal is based on material hardness, and different hardness rockies have different vibro-acoustic frequiencies. the harder the material, the higher the frequency.
The liquid in rockies can change material vibro-acoustic feature, therefore, a slight shift of frequency can be showed in SDVL logging map.
Please see the following figure that was logged at a vertical well endded at depth 3880m. The figure shows that a reservior characterist from 3820 to 3888m is very different from the others of formation rockies. - in 1994
Case 2: Formation boundaries identification
The following figure shows that different rockies have different vibro-acoustic frequency behaviors, logged at a top-drived rig in a vertical well of Northern-China oil field in 2007.
Case 3: Reservior identifications in a directional well.
While drilling into reservior zone, the vibro-acoustic signals have a large change on frequency zone, showing a reservior appearence. It was logged at a directional well of Jidong oil field of China desiged ended at the depth 2400m in 2008.
Case 4: Laboratory experiment report
Dr. Yan Gao reported on Jan. 1, 2008
Experiment Date: Dec. 26~27, 2007
Experiment Location: Rock Water Shooting Tech Lab, University of Petroleum, China (Huadong)
l Experiment purpose
While drill bit cuts rock samples, different vibro-acoustic signals will be excited depending on individual sample characteristics (cementation structure, permeability, porosity, drillability, compressibility). Previous tests (Rock Mechanics Lab, University of Petroleum, China (Beijing), 1994 by author) showed that vibro-acoustic frequency spans very wide-band spectrum, but a special scope (1k~10khz) can cover most formation rocks characteristics. Using a special formation information filter (see Fig 2), filtered signals demonstrate that every individual rock sample has unique frequency behaviors, so the test purpose focuses on validating that
1. rock vibro-acoustic (fracturing wave) frequencies vary with individual rock sample characteristics under PDC bit cutting condition,
2. individual sample frequencies are independent of WOB and RPM changing, and
3. lithological identification can be fulfilled at top of drillstring by DVL technology.
l Equipment description
Figure 1 shows mini-rig equipment for the test that simulates proportionally real drilling procedure by WOB and RPM indicators, and rotary drilling through hydraulic pressure drive. For test purpose, we use a 4-inch PDC bit (cutting tooth diameter is 19.05mm, see Figure 3.).
The test is based on the hypothesis that failure vibro-acoustic waves can travel along drill pipe up to drillstring top while PDC bit cuts the rock samples. A vibro-acoustic sensor is rigidly clipped at the top of drill pipe for abstracting rock information in tangential direction because steel pipe is good conductor of sound wave. Following is a flow chart describing the DVL hoop’s data system for drillstring vibro-acoustic logging technology in real drilling procedure.
While drilling, drilled rock formation emits failure sound, so-called vibro-acoustic wave. In general, useful fracture wave belongs to higher frequency wave and has less energy than lower mechanical wave does. In order to pick up useful weak signals from all range vibrations, a formation information filter needs to be installed before data acquisition unit so that all these weak energy signals can get hi-fi amplification and higher energy mechanical waves can be cut off.
In Figure 2, a vibro-acoustic sensor is rigidly clipped on one side of pipe for picking up tangential signals from drill bit, and a formation information filter and a data acquisition unit with wireless transmission node are bound on the other side of pipe for data acquisition and data wireless transmission, and all the parts are controlled by a remote computer while drilling.
While drilling, the wireless node will receive a command for data sampling, and the data acquisition unit will work at sampling rate of 22 kHz to get a set of 512 length and 12 bit data, and send to central monitoring system (CMS) connecting with remote computer by RS232 port for data further interpretation.
l Rock samples
Detailed description for testing rock samples is not the purpose of the test, so roughly estimation is made in the following table by author:
We select five different samples, sandstone, shale, two type cements and natural marble. The sandstone sample is a homogeneous, possessing of well porosity and permeability, but compressibility strength is less than others. The shale sample is very similar to the cement #2, holding strong compressibility, but both samples have less porosity than the sandstone’s. Cement #1 is roughly made by stone chips, and ratio of cement: sand: water is 1:2:0.5, possessing miscellaneous cementation structure. The marble sample is very hard, apparently impossible in drillability.
l Experiment method
We apply a varying force to every individual sample from 1 kN to 2 kN, and RPM from 60 to 120, sandstone first, and then shale, cement #1, cement #2, marble, and half hour drilling for each sample.
Basically, the faster bit (RPM) is running and the higher force (WOB) is applied to PDC bit, the stronger the signal waves behavior, but the vibro-acoustic frequency location and spectrum-band of each samples maintain the same.
Figure 4 shows a time-running frequency-waterfall spectrum for the test. Obvious interfaces for these samples can be distinguished according to vibro-acoustic energy distribution from the spectrum, and each individual sample has its relative steady frequency distribution.
l Spectrum analysis
Sandstone sample possesses lower frequency peaks, and vibration energy converges mainly in low frequency zone showing its looser cementation structure.
Shale has a denseness cementation and lower porosity which behaviors a hard drillability, so fracture frequency should be higher than sandstone. The cement #2 is very similar to shale sample in the test and both behavior the same feature in frequency zone, but maximum peak location is different.
Cement #1 has wide spectrum-band scope because of its miscellaneous cementation structure.
Marble behaviors higher compressibility and lower drillability. While drilling in the test, you can hear screech sound indicating its instability frequency distribution, but some rule can be followed that its vibro-acoustic energy focuses mainly on higher frequency zone.
Following table gives out a set of rock samples behaviors in time zone and its frequency zone. Their characteristics exhibit out well repetition and relative stabilization in the test (see Figure 4). Further applications in real drilling sites for PDC and rolling cone bit have displayed DVL technology abroad foreground at
l Lithology identifacation
l Strata interface measurement
l Horizontal reservoir guidance
l Well to well anti-collision measurement
