Application of Seismic Entropy to Increase Frac Efficiency
S.Akopyan, Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences, RF, Moscow
The seismic entropy method is currently used for earthquake forecasting. It enables us to show the seismic ranges of the geological environment which cause violent earthquakes and the formation of faults in an environment of diverse energy levels. This article describes the possibility of applying such methods to monitor micro earthquakes, both natural and manmade, in order to develop a control system over technogenic deformations and faults that can help in oil and gas production. This method may be applied to monitor the dynamics of hydraulic fracturing.
Introduction
The method of monitoring and forecasting earthquakes based on seismic entropy has been practically applied in different regions of the world since 2007. In order to describe the seismic processes that are happening in the earth, seismic entropy was introduced in 1993, where basis microearthquakes were facilitated. (Akopyan, 1998, Akopyan, 2013). It was shown that earthquakes develop within the specific volumes of the lithosphere (rock sphere), which are called seismic systems (SS). In order to detect SS, the value of the total seismic energy released within the geological environment is calculated. This value is called entropy. By introducing small earthquakes in to the calculations, it is possible to create accurate models and diagrams. Currently, over 130 SSs have been detected in subsystems ranging from 20 to 3000km, with magnitudes from 5.0 to 8.5 points.
The development of the entropy method from large to small scale systems, and the lowering of the earthquake magnitudes to microscopic levels (nano-magnitudes from -3 to 0, or only tens or hundreds meters in size), may make it possible to apply this method to solve technical problems in the oil and gas sector. The monitoring of micro seismic activity and the forecasting of tremors within the geologic structures may make it possible to control negative seismic impacts to critical manmade facilities (such as tunnels, bridges, mine workings, dams, gas and oil pipelines, throughways etc.) Indeed, this system may allow us to predict the origins of smaller geological deformations at their early stages and therefore take corresponding measures to reinforce facilities and prevent potential damage.
Applying the method to control and stimulate natural and technogenic earthquakes in oilfields
Conventional energy, which is determined by the recording of seismic waves, may contain the impact of fluids, artificial and technogenic factors in the geologic environment, which may or may not increase the strength of an earthquake. Comparison of the micro seismic energy using seismic entropy with traditional characteristics, which are registered by seismological monitoring networks, makes it possible to recognize that fluid and technogenics are taking part in the process of earthquake development.
It was illustrated (Akopyan, Popov, 2010) that the catastrophic earthquake at Spitak, Armenia in 1988, might have been a triggered the construction of the nearby water storage facility in Akhuryan, which was constructed in 1983, as the epicentre was nearby.
During the course of the seasons, small water storage facilities have significant fluctuations of water level and pressure, unlike larger storage facilities. Bearing in mind these factors, and taking into account the natural fault line in the area, we can assume that these factors were a large part of the natural disaster that took place here. Based on this method, we can reveal a natural trigger mechanism for the earthquake of the Southern California on April the 4th, 2010 that measured 7.2. We can hypothesise that the earthquake measuring 5.8 that took place in Mexicali on the December 30th 2009, touched an unstable zone of the faultline at Laguna Salada, causing a strong earthquake to take place 3 months later. (Akopyan, Popov, 2010). The method was tested at the Regional Geological-and-Physical Research Center “GEON” in 1997 (Akopyan, 1997). A joint analysis of seismicity and earthquake zones for the facilities of the fuel and energy complex in the Caspian area was carried out. As a result, we have maps of seismic hazards in different range of frequencies for velocities and accelerated velocities of the expected seismic forces, and an estimation of when they are likely to occur. The report forecasted earthquakes measuring from 6.2 for a period from 1998 to 2005 for the Caspian aquatic area and the neighboring countries. The earthquakes in the NW of Iran (1998.07.09, measuring 6.2) and in the western part of Turkmenistan (2000.12.06, measuring 7.5) were forecasted as well. Proof in the form of a letter, signed by L.Solodilov, D.Fyodorov, N.Kondorskay, is available.
As an example, an energy diagram of the SS in Sakhalin is presented here in Fig.1, including offshore mining in the SAKHALIN I-V zone (Akopyan, 1998, Akopyan, Kocharyan, 2013, Tsifra, 2008).
Fig.1a displays linear regression equations for the period before and after the earthquake at Uglegorsk in 2000 – see lines (1) and (2), correspondingly. The magnitudes of the Neftegorsk earthquake in 1995 as well as those of the Uglegorsk one in 2000 indicate that the seismic system of Sakhalin had certain variations in its values (Fig.1a). We can see that the bottom values of these earthquake magnitudes within the seismic system of Sakhalin match the equations (1,2). This means that in certain places the strength of the earthquake will be greater, even within the one seismic system of Sakhalin. This could have happened due to the different natural hydrocarbon fluids within the oil and gas fields (Akopyan, Popov, 2010). Offshore oil and gas production north of Sakhalin began in 1971 and coincided with the start of the Neftegorsk earthquake. Indeed, work on the offshore fields might have impacted the natural processes and strengthened the magnitude of the Neftegorsk earthquake. In spite of the fact that the Uglegorsk earthquake was located in the central part of the Sakhalin, the whole seismic system was responsible for its origins. Technogenic changes in the volume of the system might have disturbed the natural current of the seismic processes and accelerated the origins of the Uglegorsk earthquake in 2000. If it had happened some year later, its magnitude would have fitted the energy diagram better. The seismic entropy method can provide reliable results when included into the unified system of the state monitoring of the Sakhalin offshore zone (Krasny and others, 1998, Krasny, Khramushin, 2001).
Using Seismic Entropy to Monitor Hydraulic Fracturing Dynamics
Hydraulic fracturing of formations (fracing) is one of the most popular methods of well stimulation in oil and gas fields, (Shmakov, 2012), and microseismic monitoring is applied to control fracs. In the work by Alexandrov and others in 2013 the accent is put on another feature of this technology, namely, on its application to control technological risks and the quality of the frac process.
To increase production efficiency, it has been suggested that we apply the entropy method to control the process of crack formation. This will enable us to visualize the frac process and monitor the injection process for any potential negative effects.
This technology is based on pre-testing, taking into account the location of the seismic survey system and the pattern of the oil and gas fields, and reveals the development dynamics of fractures with certain energy levels. The sources of seismic emission (“microseismic noise”) in the area of the formation that is being stimulated are caused by a changed in the energy balance resulting from the stress and strain of the rocks as fracturing took place. This method offers us the possibility to estimate the volume and control it, therefore preventing risks such as emergency shutdowns, water encroachment, etc. Using the case of the frac monitoring system, illustrated in Fig.2 (Shmakov, 2012) we can demonstrate the application of the seismic entropy method here.
Customary monitoring consists of visualizing the microseismic activity area. The entropy-energy method makes it possible to forecast the development of this process and more efficiently manage the intensity of injecting a fluid into a well, achieving the desired fracture. The Fig.3 illustrates a histogram of registered microseismic events (green color), superposed with the wellhead pressure plot (red color) and the plot of propping agent concentration in the course of the main frac (dark blue color). The left side scale indicates the number of registered events, while the right side scale shows the pressure in ATM and the concentration in kg/m3. The origin of the microseismic activity sources is partially concordant with the plot of injecting in the process of the main frac. The maximum density is observed at the start of the frac, when the fissures are created during the initial period of injecting, at the stage of injecting propping agent, and during injecting the propping agent at the final stage of the process.
Fig.4 illustrates a track diagram depicting the dynamics of this process based on the entropy method. The start of injecting, and the first and second strong activation of microseismisity make it possible to build-up an understanding of how the environment behaves and the dynamics of the micro seismic activity between these events. The wave of microseismic activity balances with its environment during the initial stages of proppant injection, and with that the seismic activity decreases.
Judging by the location of the trajectory it is then possible to predict when it is going to enter a hazardous circle. In about thirty to forty minutes before the event it is possible to predict and manage the process. In the case discussed, the process was managed for an optimal outcome.
Conclusions
We propose the introduction of seismic entropy, which is unique worldwide, to control minor earthquakes with magnitudes between 4.0-5.0, both natural and technogenic in the oil and gas sector. The system will prevent such deformations from forming at their early stage and will allow us to avoid any environmental disasters. The seismic entropy method is founded on the calculation of transient-free integral and cumulative variables, which can essentially increase the reliability of the results, when employed together with conventional methods of seismic surveillance for the development of oil and gas fields.
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