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

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 20

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 microseismic

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.

Micro-earthquake and microseismic event are considered as synonyms in this paper

Docsity.com

Partial preview of the text

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 reflection and earthquake seismology

1.1 Introduction Ever since the fi rst oilfield experiments at the start of the 20th^ century, the use of seismology in the oil industry has been focussed on the use of reflection seismology with a minor interest in the refraction seismic technique and limi- ted interest in earthquake seismology. The difference between the oilfield seismologist and the earthquake seismologist lies with the nature of the source being used and the different objectives of each. For the oilfield 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 oilfield 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 oilfield, both earthquake and reflection seismology have always been seen as part of the broader subject of seismo- logy. Within the oilfield 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 oilfields 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 oilfield 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 oilfield microseismic^1 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 oilfield 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

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 fluids 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 oilfield 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

57 ESC2010 6-10 September 2010, Montpellier, France - Keynotes 1.3 Reflection seismology in the oil industry The objective of this short section is not to provide a summary of this discipline but more to identify the major di- rections that this subject is taking and some of the technical advances that are likely to increase the value of this technique within the oil industry. The industry has delivered:

Lower noise through better equipment, better confi gurations, better fi ltering, and better experiment design (P.Christie et al., 2001).
Better sampling through improvements in array geometry (e.g. the use of wide azimuth acquisition: R. Sambell.,

Algorithms that better approximate the reality of seismic energy propagation in the subsurface and or analyse a larger portion of the seismic wavefield (e.g. the use of reverse time migration: Baysal et al., 1983). The result has been:
Imaging of interfaces with complex and/or steep topography.
The use of more realistic velocity models incorporating 3D velocity heterogeneity and anisotropy
Better resolution/sharper images, increased signal to noise and more accurate amplitude representation. Thus going beyond simple reflector location in order to analyse the variation in impedance contrast across boundaries. These improvements now provide the oil industry with a tool capable of imaging sub-surface structure previously outside the reach of seismic technology. In the context of 4D seismic, travel time changes of a few milliseconds or less are detectable and actionable (Yu et al., 2009). The obvious question is what next? The answer to this, in the author’s opinion, probably lies within the following ultimate developments:
The application of HPC technology to the problem of seismic inversion and the proximate application of computa- tionally intensive methods previously considered unworkable
The improved reliability and lower price of permanent systems
Synergistic development and integration with other technologies, in particular that of earthquake seismology There will be many proximate effects of these changes that rely upon the above developments for them to become realisable. 1.4 The value proposition for the integration of both refl ection and earthquake seismology Both reflection seismic and earthquake seismic technologies rely upon the measurement of vibration. Different ap- plications may focus on different bandwidths and amplitude levels but in essence each technology is based on the measurement of the same phenomenon. The man-made sources used in reflection seismology generate energy in a measurement bandwidth of a few Hz to a few hundred Hz. If we defi ne earthquake seismology as a discipline that relates to seismic slip at any scale then the bandwidth varies between a few thousandths of a Hz for long period teleseismic waves to tens of kHz for acoustic emissions emitted by stressed rock. Fortunately the prime bandwidth of microseismic energy recorded in consequence of oilfield operations lies between a few tens of Hz to around 1 kHz when measured downhole. At the surface the top end of this spectrum drops to a few hundred Hz. This means that for most purposes the requisite bandwidth for both reflection seismic and microseismic operations are coincident and the same sensing systems can be used for both. This is an important fact to consider since it offers an opportunity to use the same measurement systems to deter- mine two very different attributes of the subsurface (i) impedance and its change (ii) geomechanical change through the detection, location and characterisation of microseismic events. Recent work using OBC sensing systems deployed for 4D seismic purposes, has shown that these devices can also be used to detect and locate microseismic activity (Chambers et al., 2010). The installation of shallow buried arrays in parts of the Middle East and the USA for steam injection and hydraulic fracture monitoring purposes represent the start of systematic installation of permanent equipment used for microseismic monitoring. These multi-level, multi- component arrays with channel counts generally between 20 and 100, are installed within multiple wells of a few hundred metres depth over an area of 10 to 20 km^2.

58 ESC2010 6-10 September 2010, Montpellier, France - Keynotes Over the past 10 years we have seen the rise of the microseismic technique within the oilfield seismic community. The author believes that we are now starting to see a search for the synergy between the two disciplines as the digital oilfield begins to take shape. The reasons and potential avenues of development of this synergy are listed below:

Both disciplines measure the same phenomenon of earth vibration
Within the oilfield the measurement bandwidths overlap
Oilfield seismology is widening its scope to include both permanent as well as temporary wireline installations
The automated determination of source location and source character is important to both disciplines.
Many of the techniques developed for exploration seismology can be adapted to the analysis of microseismic sour- ces.
Reservoir imaging may be possible based on the use of microseismic events as sources
The joint inversion of synthetic source and natural source data to invert for reservoir properties 1.5 Conclusions The value of microseismic monitoring to the reservoir stimulation business has demonstrated the potential of ear- thquake seismology to the reservoir engineers whose role is to understand and try to predict reservoir response to oilfield operations. The widening scope of reflection seismology from an exploration tool to one for production monitoring brings it into alignment with the reservoir monitoring role played by microseismic technology. New acquisition hardware in the form of permanent OBC and permanent downhole arrays together with parallel com- puting technology provide opportunities to develop the potential synergy between the two disciplines. The outlook for seismology is an exciting mix of how best to apply the funding and knowledge from each side of the seismology fence to the intriguing problems and imaging concerns of our colleagues on the other. 1.6 References E. Baysal et al., 1983. Reverse time migration. Geophysics. 48. 1514-1524. C. Chambers et al., 2010. The Leading Edge. March Issue. P. Christie et al., 2001. Raising the standard of seismic data quality. Schlumberger Oilfield Review. P. Duncan. 2007. Microseismic monitoring - coming of age. Harts E&P. Sept Issue. Rayleigh et al., 1976. An Experiment in Earthquake Control at Rangely, Colorado. Science. 191. 1230-1236. Rutledge et al., 2004. Faulting Induced by Forced Fluid Injection and Fluid Flow Forced by Faulting: An Interpretation of Hydraulic- Fracture Microseismicity, Carthage Cotton Valley Gas Field, Texas. BSSA, 94, 1817–1830. R. Sambell et al.., 2010. Land seismic super-crew unlocks the Ara carbonate play of the Southern Oman Salt Basin with wide azi- muth survey. First Break. 28. 61-68. J.H.Y. Yu & J.Thiebaud. How small is a small 4D signal – a model study for 4D resolution. 71st^ EAGE Conference and Exhibition

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

Related documents

Partial preview of the text

Download Oilfield and earthquake seismology: a new era of imaging. and more Study notes Geology in PDF only on Docsity!

Oilfi eld seismology in the 21st century: the marriage of reflection and earthquake seismology