Oilfield and earthquake seismology: a new era of imaging., Study notes of Geology

Download Oilfield and earthquake seismology: a new era of imaging. and more Study notes Geology in PDF only on Docsity! 54 ESC2010 6-10 September 2010, Montpellier, France - Keynotes Oilfi eld seismology in the 21st century: the marriage of refl ection and earthquake seismology 1.1 Introduction Ever since the fi rst oilfi eld experiments at the start of the 20th century, the use of seismology in the oil industry has been focussed on the use of refl ection seismology with a minor interest in the refraction seismic technique and limi- ted interest in earthquake seismology. The difference between the oilfi eld seismologist and the earthquake seismologist lies with the nature of the source being used and the different objectives of each. For the oilfi eld seismologist the nature and location of the source is known and engineered for its purpose. Although the source is clearly important, the focus of the work is in processing and interpreting what this source is able to say about the sub-surface. The objective of oilfi eld seismology is to better image hydrocarbon bearing and overlying formations in order to fi nd such hydrocarbons and also to reduce the cost of producing such a resource or reduce the risk of exploitation. For the earthquake seismologist the nature and location of the source is unknown. The objective of earthquake seismology has been to locate and defi ne the nature of any unknown natural source, to attempt to understand the relationship between source generation and the local or regional stress regimes, to image the structure of the earth and to use this knowledge to mitigate against future potential earthquake hazard (The NTBT monitoring programme does rather extend this defi nition to include the location and defi nition of an unknown artifi cial source). Outside the oilfi eld, both earthquake and refl ection seismology have always been seen as part of the broader subject of seismo- logy. Within the oilfi eld the subject of earthquake seismology has been of limited interest since it has offered no value in terms of understanding reservoir geometry or process. However over the past ten years this has been changing (Duncan, 2007). The seismic community has known about the occurrence of induced seismic events in mines, under reservoirs and within oil reservoirs for decades (Rayleigh et al., 1976). Although seismic moments measured within oilfi elds tend to be many orders of magnitude smaller than those measured in the context of global seismic studies, the source me- chanics of any macro-seismic and microseismic events are effectively the same, as are the seismic body waves that are generated. In order for the application of earthquake seismology in the oil industry to really take off the coming together of several factors was required. These factors included (i) the improvement of downhole seismic tools (ii) the arrival of cheaper acquisition hardware and (iii) the rising importance of hydraulic fracturing and unconventional natural gas to the domestic gas supply in the USA. In 1995 unconventional gas supplied around 5% of US production. In 2009 it reached around 60%. Microseismic monitoring has been one of the key technologies in helping to optimise the stimu- lation of these unconventional reservoirs. Hydraulic stimulation is expensive and by mapping the extent of the hydraulically stimulated zone corresponding to the locus of microseismic events, one can reduce the risk of bypassing reserves or unnecessarily stimulating the same volume twice. The oilfi eld microseismic business now represents a substantial undertaking. From a limited pilot market worth a few hundred thousand dollars per year in the late 1990’s, the oilfi eld microseismic1 business has grown into a market worth over 100 M$ annually. This market continues to grow as improved methods are applied to better locate and defi ne the nature of the natural seismic sources that are induced by oilfi eld operations. The delivery of microseismic event locations is now possible within a few seconds of their occurrence enabling the engineer to make decisions with an added value of several hundreds of thousands of dollars. 1 Micro-earthquake and microseismic event are considered as synonyms in this paper Docsity.com 55 ESC2010 6-10 September 2010, Montpellier, France - Keynotes 1.2 Earthquake seismology in the oil industry 1.2.1 The relevancy of earthquake seismology As soon as a well is drilled the geomechanical state of the sub-surface is altered. If fl uids are then injected or produ- ced it is possible that some part of the sub-surface may exceed its failure criterion and in consequence seismic slip may occur. The slip vector of such an event may be only a few microns in dimension and the moment magnitude only minus 3. However the occurrence of this event tells the seismologist something about the state of the sub-surface that was not known before. The understanding of the inter-well region within an oil reservoir is very limited. Any technique that can provide in- formation about this region has potential value to a reservoir engineer. Microseismic monitoring and location is such a technology. It can be used to map any stress changing process in the sub-surface caused by oilfi eld operations. Its most common use is to map the processes of hydraulic fracturing and steam injection for unconventional gas and heavy oil produc- tion. Figure 2 shows a location plot for a population of microseismic events that took place during a series of hydraulic fracture stages. Such a plot provides the engineer with (i) the direction of the frac (ii) an estimate of the spatial extent of the frac A movie of microseismic cloud development provides the engineer with an understanding of how the frac design is affecting the sub-surface during pumping and the ability to change the frac if the microseismic events show undesi- rable frac growth. The acquisition geometry of the receivers used to locate the events shown in Figure 2 can be seen to the right of the fi gure. In this experiment two receiver arrays were used. Conventionally only a single, multi-level, 3C array is used to monitoring hydraulic frac operations. Such a confi guration can result in uncertainty estimates of several hundred metres. The availability of suitable monitor wells is a very signifi cant constraint on the number of fracs that can be monitored. Globally several tens of thousands of frac operations take place each year. Of these perhaps only 1% are monitored microseismically. In consequence alternative, surface-based techniques have recently been developed that use the power of large surface arrays to stack potential energy sources in the sub-surface (Chambers et al., 2010). The prin- ciple behind the surface technique is a simple one whereby the recorded amplitudes corresponding to the modelled moveout of a notional event position, are stacked (Figure 3). If the signal energy across the array is coherent the summed amplitudes will create a high stack amplitude. If the signal energy across the array is incoherent the summed amplitudes will destructively interfere producing a low stack amplitude. This migration procedure is then repeated for all notional positions and all times. Figure 1: Rising proportion of unconventional gas production in USA Docsity.com