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About Fracturing Monitor by VSP

Overview

Micro-seismic fracturing monitoring technology inside well is a key technology for the exploration and development of unconventional fields such as shale gas and tight oil and gas. Compared with ground micro-seismic fracturing monitoring technology, fracturing monitoring inside well is more closer, more accurate and more clearly reflect the fracture length, fracture height, and real-time extension in the fracturing process, so that, technicians analyze and study the formation transformation more accurately, evaluate the fracturing effect in real-time, and guide the fracturing parameters efficiency. The adjustment of the plan reduces the period and cost of reservoir reform monitoring, it is one of the most accurate, timely and informative monitoring methods in the fracturing process.

Applications

Real-time monitoring: Check the fracturing effect, analyze the fracture morphology in real-time, adjust the fracturing parameters (such as pressure, sand volume, fracturing fluid, temporary ball plug, etc.), monitor the casing change, guide the fracturing construction in real-time, optimize the fracturing plan.

Fracturing evaluation: Provide fracture network geometry, comprehensive analysis by combined with well logging, rock geophysical parameters, seismic data and other information to evaluate the fracturing effect and estimate the available oil and gas Stimulated Reservoir Volume (SRV).

Development application: Provide fracture space shape and maximum main in-situ stress direction, etc. And provide important reference for the layout of oilfield development well patterns (horizontal well spacing, horizontal section length, fracturing classification and fracturing section length, etc.).

Tool features

MultiVSP is long operation time designed. Downhole geophones work at 0.25 millisecond sampling rate with up to 12-level detectors. The geophones finish the monitoring items with more fracturing intervals such as horizontal well fracturing and longer periods.

Micro-seismic data processing

Wellsite micro-seismic monitoring and processing flow

Taking the wellhead of the monitoring well as the origin of the coordinates, a unified fracturing monitoring coordinate system of the fracturing well trajectory and the monitoring well trajectory is established, and the relative coordinates of the geophone position and the fracturing section are established.

Speed model

Accurate positioning of micro-seismic events requires a suitable velocity model. The initial velocity model usually uses sonic logging and VSP.

Establishing logging speed & original speed models

Corrected velocity model


When the perforation position and original velocity model are known, the velocity value of each horizon is adjusted according to the detonating cord or perforation signal P-wave signal, combined with the divided geological horizon, and the velocity model is adjusted multiple times to make the rupture event Positioned at the detonating cord point.

The main purpose of the detonating cord or perforation signal includes two aspects. One is to obtain the direction of the detector and correct the original setting of the detector. The other is to verify the rationality of the speed model, and make the necessary optimization and correction.

Geophone orientation

In order to accurately locate the micro-earthquake, the three-component orientation of the geophone in the well needs to be determined by perforation recording or detonating cord excitation before fracturing.


Filter processing

Strong noise has caused interference to the signal, the difference of characteristics in apparent speed and spectrum suppresses the noise, which highlights the signal energy and improves the positioning accuracy of the fracture event.

First pick

Screens effective fracturing event points, and provides accurate phase expiration recognition based on the first arrival and peak values of P-Wave and S-Waves.

Polarization analysis

Polarization analysis rotates the micro-seismic signal into P, SH, and SV phases. The analysis signal source is a P-Wave source, a SH-based source, or an SV-based source. The P-Wave is used to calculate the location of the micro-seismic event.


Schematic diagram of polarization analysis

Inversion processing

The difference method can identify P-waves and S-waves for the monitoring records with relatively high signal-to-noise, and use P-wave time difference method to retrieve the location of the fracturing event point.

For the monitoring records with low signal-to-noise ratio, only P-Wave or S-Wave can be identified, and the location of the fracturing event point can be retrieved using the same wave type time difference method.

Case
Perforation positioning results

Well 35-6, Stage 16

Well 35-5, Stage 16

Well 5, Stage 14

Top view

Side view

b value calculation



Well 5, Stage 16

Well 6, Stage 16

The b value represents the extent to which an area is subjected to the average stress and the receiving strength limit, and the stress state of the medium under the action of the tectonic stress field. When the regional average stress increases, the b value decreases, and conversely the b value increases. Calculating the b values of this section in the two wells are 0.45 and 0.47 respectively, indicating that there are certain natural fractures in the fracturing section. It is speculated that due to the good brittleness of the formation, the early fracturing stimulated the formation activity. The degree is affected by natural fractures.

Well 5, Stage 14-17, Joint Display

Well 6, Stage 15-18, Joint Display

Dual-Well Joint Display

Top View

Dual-Well Joint Display

Side View

Dual-Well Joint Display

3D View

Well 5

Well 6


Results

(1) The monitoring results of eight sections in two wells show that the fracture length of Well-5 is 282-327 meters, the fracture width is 59-64 meters, the fracture height is 40-60 meters, and the direction is 63-76 ° from north to east. The fracture length of Well-6 is 287 -338 meters, fracture width is 45-66 meters, fracture height is 43-51 meters, direction is 66-86 ° from north to east.

(2) The micro-seismic events are mainly distributed in the formation stage of the main fracture network. During this period, the micro-seismic events are intensive and the energy is strong. The fracturing has obvious effect on the formation transformation.

(3) The monitoring results show that the east and west sides are basically symmetrically distributed.

(4) By calculating the b value, there are certain natural fractures in the fracturing section, and the development of the fracture network is affected by certain natural fractures.

(5) The multi-segment joint display segment is relatively clear, and the perforation cluster spacing is more reasonable.