Microseismic monitoring based on MicroseismicCSP technology

Technology description

Passive monitoring is carried out by registering microseismic emission signals on the day surface using a seismic sensors located above the emission sources. Technology uses and specialized data processing, based on cluster calculations.

The MicroseismicCSP technology can solve the following tasks:

Hydraulic fracture monitoring.
Fluid injection monitoring.
Well drainage area m
Fault-block structure detection near the borehole.
Filtration activity monitoring after hydraulic fracture.

Equipment. Field observation is carried out using digital seismic stations «Scout» (Fig. 1) equipped with GS-One seismic sensors. Used equipment allows data aquisition with the following parameters:

synchronous registration with an error less than 100 microseconds;
sampling frequency bigger than 1 kHz;
frequency range 5-200 Hz;
Continuity of registration up to 28 days.

The applied differential GPS – receiver allows to measure the relative coordinates of seismic sensors with high accuracy.

Fig. 1. –Digital multichannel seismic station “Scout” with GS-One sensors

Observation system. We use surface observation system, which has following features:

High accuracy in determining the lateral coordinates of microseismic events – azimuth and length of hydraulic fractures.
A large number of sensors makes it possible to most effectively detect weak signals of microseismic events and determine the location and parameters of their sources;
Determination the seismic moment tensor (and other dynamic parameters of events);
Flexible design of the of the observation system configuration, to meet permitting requirements or geographic constraints.

MicroseismicCSP technology uses software which can modeling the accuracy and parameters of acquisition system before field observation.

Hydraulic Fracture Monitoring

One of the main tasks of hydraulic fracturing monitoring is determining the geometric parameters of the fractured zone formed by hydraulic fracturing in the well. The length of the fractured zone and its azimuth are estimated from the hypocenters of microseismic events, recorded during the monitoring process. An example of estimating the length of the fractured zone and its azimuth is shown in Fig. 2. For a better understanding of the fracking process, the technology makes it possible to visualize the forming of microseismic events in a static and dynamic version – for matching with hydraulic fracture process technical operations (Fig. 3).

Fig. 2 – Map of microseismic events, generated during hydraulic fracturing. Events size are proportional to their energy. Events color corresponding to frac stage: violet – replacing stage, blue – mini frac, red – main frac.

Fig. 3 – Histogram of the main hydraulic fracturing microseismic events by the time, combined with the plots of technological processes (the graph of the tubing pressure is highlighted in red, the pressure in the annulus is in blue, the flow rate is in green, the concentration of proppant by the density meter is in pink, the injection rate is in purple).

Comparative characteristics of multistage hydraulic fracturing stages. The estimation of the injection capacity is carried out according to the graphs of technological processes (Fig. 3). The total values of microseismic energy and their relation to the total injection energy characterize various stages of multi-stage hydraulic fracturing and make it possible to compare them with each other. In fig. 4 shows the imbalance of the deformation of isotropic expansion energies and maximum shear stresses for different stages of multi-stage hydraulic fracturing. In fig. 5 shows the comparative values of the absolute and relative energies of microseismic emission for the stages of multi-stage hydraulic fracturing.

Fig. 4 – Directions of the main stress axes of microseismic events (seismic moment tensor) (top). Uncompensated part of the deformation of isotropic expansion energy (dP) and uncompensated part of the maximum shear stresses energy (dSgN) for various stages of multistage hydraulic fracturing (bottom)

Fig. 5 – Map of microseismic events energy density (top); cumulative values of emitted seismic energy (E) and their relation to injection energies (E/Einj) (bottom) for various stages of multistage hydraulic fracturing

Fluid injection monitoring

Monitoring of the fluid displacement process is carried out by determining the hypocenters of generated microseismic events. The solution of the inverse dynamic problem is the seismic moment tensor. It allows you to determine the energy parameters of microseismic events, which characterize the processes of microseismic activity during the fluid injection into injection wells. Analysis of the areas of microseismic activity in time (Fig. 6) makes it possible to recognize the direction of injected water movement (flood front location).

An example of microseismic events cloud, registrated over 15 days of observation is shown in Fig. 7 a). Changes in injection modes (decrease in pressure and increase in fluid volumes) are reflected in the values of the energy parameters of events. In fig. 7 b) and c) show histograms of the uncompensated part of the deformation energy of isotropic expansion (dP) and maximum shear stresses (dSgN), combined with the plots of daily injection fluid volumes and pressures.Monitoring of the process of displacement of fluid from the borehole environment a working agent is carried out by determining hypocenters of microseismic events. The solution of the inverse dynamic problem is the seismic moment tensor. It allows you to determine energy parameters of the microseismic events, which, in turn, characterize the processes of micro-seismic activity during fluid injection into the injection wells. Analysis of the development of the microseismic activity over time (Fig. 6) allows one to estimate the direction of movement of the injected water (waterflooding front).

Fig. 6 – Expansion of microseismic activity in time, grid cell 100 m. Numbers mark different days of observation

Fig. 7 – a) Map of events energy density over 15 days of monitoring, Grid cell 100 m. b) Uncompensated part of the deformation of isotropic expansion energy histogram (blue) and plot of fluid injection volumes (green), c) histograms of the uncompensated part of the maximum shear stresses energy (red) and tube pressure plot (violet)

Well drainage area mapping

The producing well drainage area is a source of useful and important information about the reservoir. Long-term microseismic monitoring of the production process allows to determine the areas of microseismic activity in the reservoir. In fig. 8 shows the highlighted area – the zone of the highest intensity of microseismic events in the field during oil production in a porous reservoir without pressure maintenance.

Fig. 8 – Production well drainage area mapping, bright colors correspond to higher intensity of microseismic events

Fault–block structure detection

Fig. 9 shows the results of passive microseismic monitoring of oil production on a field with a mixed reservoir type (porous and fractured) after hydraulic fracturing. The configuration of the microseismic events characterizes the fault-block structure in the area near the production well.

Fig. 9 – Microseismic events near the production well (left) and their interpretation – the fault-block structure (right).

Filtration activity monitoring

Traditionally, microseismic monitoring is used to control hydraulic fracturing and determining the geometric parameters of the fractured zones. But our technology includes passive microseismic monitoring carrying out after multistage hydraulic fracturing. It makes possible to record microseismic activity in the area of fracking ports and detect which ports is really working (i.e. successfully fracked). An example of passive monitoring after hydraulic fracturing is shown in Fig. 10. Maximum microseismic activity observed near 3, 4, 5 ports. Production logging confirmed that this ports has maximum flow rate.

To explain a reason why only three ports are working we use scattered wave seismic data (detecting fractures). This makes possible to identify the naturally fractured zones of reservoirs. The working ports are located in such areas.

Fig. 10 – Structural map of producing horizon with faults and microseismic events registered during fracturing and after it (violet)

Fig. 11 – Scattered waves section along well borehole with microseismic events