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About Micro-seismic Real-time MonitoringOverview 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. 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 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.
Filter processing All original sources or microseismic events signal should have filter function before further 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.
Un-filtered
Filtered
Recognition of perforation signal or detonating cord signal 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 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.
Oriented of Receivers Velocity model Accurate positioning of micro-seismic events requires a suitable velocity model. The initial velocity model usually uses sonic logging or Vertical seismic profile data.
Establishing original velocity models
Velocity model correction When 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. 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.
Miscroseismic events 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.
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.
Magnitude calculation 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.
Perforation positioning 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
Top view
Side view The 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.
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. |
