Micro-seismic Real-time 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 microseismic real-time monitoring & real-time decision technology, monitoring is more
closer inside well, 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.
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
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
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.
Wellsite micro-seismic monitoring and processing flow
Taking the wellhead of the monitoring well as the
origin of the coordinates, a unified microseismic real-time monitoring & real-time decision 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.
original sources or microseismic events signal should have filter function
before further processing.
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.
Recognition of perforation signal or detonating
According to the different excitation methods,
the excitation signal of perforation and detonating cord is different from the
microseismic events signal. It has the characteristics of strong compression
wave (P wave) and weak shear wave (S wave).
Position the receivers orient of geophones
In order to accurately locate the microseismic
events, the three-component orientation of the geophone in the well needs to be
determined by perforation or detonating cord excitation signals before
fracturing. After the polarization analysis of the positioning signal P wave,
and then use the azimuth correction amount of the geophone for correction, the
true azimuth of the seismic source can be determined.
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 orient of the micro-seismic event.
positioning of micro-seismic events requires a suitable velocity model. The
initial velocity model usually uses sonic logging or Vertical seismic profile data.
Velocity model correction
the perforation or detonating cord position and original velocity model are
known, set velocity ratio of P wave and S wave according to Poisson ratio or
shear wave logging value (or regional experience value). Combined with the
divided geological layers to adjust the velocity value of each layer and adjust
multiple times to pick the first arrive of S wave so that make Inverted event to
position at perforation point or detonating cord point.
effective fracturing event points, and provides accurate phase expiration
recognition based on the first arrival and peak values of P-Wave and S-Waves.
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.
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.
Calculate the microseismic magnitude based on the
waveform frequency and amplitude, the static gain of the geophone and the
velocity of the formation shear wave etc.
resultsThe case was a shale gas well in Sichuan Province. The
microseismic monitor jobs were completed in two wells on one platform. The
depth of the geophones in the observation well were between 3360-3580m, the maximum
temperature recorded by geophones was 136℃. The fracture stages
covered 1400m in horizontal section.
The downhole microseismic monitoring technology provided
the fracture trend and shape in real time on site which were guided the
optimization of pre-fluid parameters, perforation plan, temporary plugging
time, etc., improve the effect of fracturing reconstruction and performed
effective formation reconstruction.
The evaluation results of post-processing showed the
influence of natural fractures on the fracturing effect, evaluated the well
spacing and the distribution of perforation clusters, and calculated the area
of reservoir reconstruction after fracturing, which provides a basis for the
next step of well development.
Perforation positioning results
Well 35-6, Stage 16
Well 35-5, Stage 16
Well 5, Stage 14
summary of Well 5，stage14
Totally 222 microseismic events were monitored
in this stage, 189 before the pump was stopped, and 33 after the pump was
stopped. The east wing has a slightly larger area than the west wing, which is
presumably related to the monitoring well on the east side of the fracturing
well. It is easier for the geophones to detect the microseismic events of east
side. The monitoring results also show that the complexity of the fracture
network in the east wing of the wellbore is higher than in the west.
The animation shows that after the pump was
stopped at 11:21, the microseismic events were mainly distributed inside the
reconstructed area during the fracturing period, and there was no obvious sign
of fracture network extension.
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
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.
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
monitoring results show that the east and west sides are basically
calculating the b value, there are certain natural fractures in the fracturing
section, and the development of the fracture network is affected by certain
(5) The multi-segment
joint display segment is relatively clear, and the perforation cluster spacing
is more reasonable.