Predrill Passive Method Images Fracs

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1 JANUARY 2014 The Better Business Publication Serving the Exploration / Drilling / Production Industry Predrill Passive Method Images Fracs Ambient fracture imaging using passive arrays captures detailed images of seismic activity produced by discrete reservoir features such as natural fractures and faults to map hydraulically transmissive zones prior to drilling or any other activity. By Alfred Lacazette, Jan Vermilye, Samuel Fereja and Charles Sicking DENVER Faulting and natural fracturing are critical to horizontal well placement and hydraulic fracturing design in unconventional resource plays. Knowledge of these discrete transmissive features prior to drilling offers substantial benefits for all phases of operations, from exploration to development planning and hydraulic fracturing. Ambient fracture imaging is a relatively new passive seismic method that generates images of seismic activity produced by discrete reservoir features such as faults and fractures before spudding the well. Fracture/fault zones with high resolved shear stress correlate positively with fracture transmissivity. In many cases, discontinuities with high resolved shear stress will be the most microseismically active. Reproduced for Global Geophysical Services Inc. with permission from The American Oil & Gas Reporter

2 Tech Trends The ambient imaging method allows many hydraulically transmissive zones to be mapped with passive seismic arrays prior to drilling or any other activity. It directly images seismically active hydraulic fractures and natural fractures as complex surfaces and networks. Although it has been used primarily to image hydraulic frac treatments, imaging ambient seismic activity both prior to frac monitoring and by quiet-time monitoring using 3-D reflection grids (i.e., recording on a reflection grid or dedicated passive seismic grid while shooting or vibrating is not in progress) reveals seismically active fracture and fault zones. These correlate with features illuminated by hydraulic fracturing or imaged by 3-D reflection seismic attributes. The presence and hydraulic transmissivity of some of these features have been validated by independent measures, including chemical tracers and pressure monitoring. Earth stress studies have established that the brittle crust is a self-organizing critical system in a state of frictional equilibrium, and is therefore constantly on the verge of movement. The earth s brittle crust is continually loaded by a variety of forces, although isostacy and isostacy-related flexural and tectonic movements appear to dominate. Stress or pressure changes of less than 0.01 atmospheres can stimulate seismicity. The results of our work to date on quantitative correlations between earth tides and ambient seismic activity imaged on large grids suggest that earth tides help stimulate the release of stored elastic strain energy in the brittle crust and may themselves be a driver of ambient emissions. Natural Fracture Networks Natural fracture networks strongly influence the movement of hydrocarbons in the subsurface, and hence the productivity of wells and entire fields. Natural fractures are particularly important to the productivity of unconventional reservoirs. Reflection seismic methods can provide bulk measures of velocity and velocity anisotropy. Although these values have proven useful in detecting fractures in advance of drilling, they do not directly reveal the discrete fracture network or indicate fracture systems that are likely to be hydraulically transmissive. Similarly, high-resolution reflection seismic methods can reveal larger fractures, but cannot reveal the detailed network or the likelihood that the fractures will be productive. Other methods for understanding subsurface fracture networks include borehole images, open- and casedhole production logs, chemical and radioactive frac tracers, well tests, interference tests, production data, and other methods that require drilling and completing multiple wells. These datasets cannot be used to plan field development activities, taking the natural fracture network into account. In contrast, ambient imaging reveals subsurface fracture networks that likely are hydraulically transmissive prior to exploration drilling. The data are obtained inexpensively by listening to ambient seismic emissions during standard 3-D reflection surveys while shooting or vibrating is not in progress. The resulting images allow potential sweet spots to be targeted first and allow field development planning to begin before starting a drilling campaign. Ambient images also can aid in planning hydraulic fracture treatments. More complete images are obtained when monitoring a fracture treatment because the fluid pressure pulse and poroelastic stress changes produced by the frac illuminate the transmissive fracture fairways over a large area and provide more energy for imaging. While less complete than images produced by hydraulic fracture monitoring, prefrac ambient images of natural fracture networks show many features that are illuminated by subsequent fracture treatments, making it useful in exploration and predrill development planning. Ambient Imaging Methodology The ambient fracturing technique is based on a proprietary and patented commercial method that uses seismic emission tomography in combination with empirical data on fracture geometry to directly image and map natural fracture/fault networks and those induced by hydraulic fracturing. To acquire these tomographic fracture images (TFIs): Single (vertical) or multicomponent passive seismic data are recorded continuously throughout the period of interest with a surface or shallow buried array. Uniform grid geometries are preferred, and hexagonal closest-packed grids are ideal. The data are processed similar to standard reflection seismic processing to remove noise, make static corrections, etc. A velocity model of the study volume is developed using any available data, such as sonic logs, stacking velocities from seismic data, or vertical seismic profiling surveys. The velocity model is as detailed as possible. If sufficient data are available, then a 3-D spatially varying, anisotropic velocity model is built. If data such as string or perforation shots are available, they can be used to fine tune the initial model. The study volume is divided into voxels using ray-tracing to compute a table of one-way travel times from every voxel to every receiver. The traces are aligned in time for each voxel, and the semblance or other quantity (such as a measure of energy) is computed for discrete time intervals to produce a five-dimensional data volume. The dimensions of the data volume are the x, y and z coordinates of the voxels, the time step, and the semblance or other measure of cumulative seismic activity in each voxel during each time step. The computed volumes are analyzed, edited and combined to produce a single stacked (summed) depth volume. The stacked volume may be a combination of data from minutes to hours, or even days. The volume is clipped to eliminate low-level noise, leaving sinuous clouds of high activity. The TFI is generated by finding the central surfaces of the clouds. These central surfaces represent the large, seismically active fractures. For hydraulic fracture monitoring, two slightly different proprietary workflows are used to produce two types of TFIs. Near-well TFIs show the highest energy features connected to or near the perforations, which are the induced fractures and most heavily stimulated natural fractures. Reservoir-scale TFIs show the total activity throughout the imaged volume. For ambient imaging studies, only the reservoir-scale workflow is used. Other branches to the workflow include analyzing individual time steps to identify and locate microearthquake hypocenters or perforation shots, and producing movies of cumulative activity volumes from moving sums of time steps. The central surfaces of the high-activity clouds locate the main fracture surfaces. Rock mechanics theory, field studies and

3 January 2014 FIGURE 1 Ambient Activity Depth Slices versus TFI from Frac Treatments (at Horizontal Well Level) experiments show that large fractures are embedded in clouds of smaller fractures and seismic emissions that become progressively more intense near the main fracture surface. Therefore, the central surfaces of high-activity clouds are the loci of large fractures. Identifying seismically active fractures identifies fractures that are likely to be hydraulically transmissive. Extensive industry research demonstrates that in most cases, resolved shear stress on a fracture is correlated positively with its hydraulic transmissivity. Higher resolved shear stress is positively correlated with seismic activity so that more microseismically active fractures generally are expected to be more hydraulically transmissive. However, the converse is not necessarily true. Sensitivity To Ambient Seismic As previously described, ambient seismic emissions are pervasive because the earth s brittle crust is in a state of frictional equilibrium and is pervasively fractured. Even slight stress or pressure changes are sufficient to stimulate seismicity. Small earthquakes are many orders of magnitude more abundant than large ones, and the distribution appears to have no lower magnitude limit. However, distinct earthquakes are not the only type of seismic activity. The brittle crust is constantly emitting seismic energy at a low level in the absence of artificial stimuli. Hydraulic fracture treatments stimulate activity both by reducing the normal stress on pre-existing natural fractures, breaking new rock, and altering the present stress state. A fluid pressure pulse can be transmitted through a fracture network without transmitting fluid. The TFI approach is extremely sensitive because it stacks total trace energy, rather than simply looking for large discrete events. Stacking total trace energy captures energy from numerous microearthquakes too small to discriminate as individual events or too small or indistinct to have separable compressional- and shear-wave arrivals. These small events represent much more energy than relatively rare large microearthquakes amenable to standard seismological methods. Perhaps more importantly, stacking total trace energy captures energy from other types of activity, including, perhaps, long-period, FIGURE 2 long-duration events and other as-of-yet unclassified types of seismic activity. Stacking energy from large numbers of traces over an extended period adds tremendous sensitivity and images surfaces by canceling noise and increasing spatially stable signal strength. Surfaces are imaged because activity along large fracture surfaces is episodic and extended periods are required to fully image such surfaces. This method detects only seismically active features. High-resolved shear stress and seismicity do not always correlate positively with hydraulic transmissivity. Poroelastic stress waves induced by a hydraulic fracture treatment can cause fractures to become seismically active without a direct fluid connection to the wellbore. Moreover, weak features such as fault breccia zones may be very transmissive with little resolved shear stress or associated seismic activity because they are unable to support substantial shear stress. Such weak features are detected only sporadically along their lengths, and in some cases, may not be detected at all with this method. Appalachian Examples Figure 1 shows passive seismic results from a detailed reservoir characterization of stacked unconventional reservoirs in the Upper Devonian section in southern TFI Depth Slices at Horizontal Well Level

4 Tech Trends West Virginia. The TFI results were confirmed by chemical and radioactive tracers, wellbore images and other data. The study was blind, so the well and other data were not known to personnel engaged in the study until the results were delivered. The ambient image of four hours of cumulative seismic activity in the Berea Sandstone is displayed at left. At right are the TFIs from all nine frac stages in the horizontal Berea well shown at the center of each diagram. A subsequent horizontal well (shown as the red bar in the southwest) confirmed that this area was unusually heavily fractured and highly permeable. These results show that prefrac ambient seismic activity imaged major features revealed by frac monitoring. The zone of intense fracturing in the southwest corner of the study area is prominent and the highly transmissive area around the vertical well in the southwest corner of the diagram (west southwest of the horizontal wellhead) is identified. Figure 2 shows results from a fracture treatment of a horizontal gas shale well in Pennsylvania. TFI and cumulative activity sums show that the third treatment stage (well A ) is connected immediately to an older vertical well (well B ) through a natural fracture system. Well B was monitored for pressure and chemical tracers during the fracture treatment. At left is a 10-minute semblance stack with the horizontal treatment well and offset vertical producer shown in gray and the stage three perforation depth range in red. The stack shows the time interval shortly after breakdown, when activity between the treatment well and vertical producer was declining. Low cumulative seismic activity is in blue with red indicating high seismic activity. In the center in Figure 2 is the TFI of the third stage. The fracture connecting the wells is imaged clearly, with the colors denoting the relative intensity of cumulative seismic activity. At top right is a detailed view of the combined reservoir-scale TFIs for stages one to five. The same data are show at bottom right, but with ambient TFI overlain in gray scale. The prominent features in the ambient data were later activated during the fracture treatment, especially the large fracture zones (labeled in pink as features 1 and 2). Activity in the natural fracture system began immediately when stage three was pumped because increased fluid pressure in the fracture reduced the normal stress, and accordingly, friction along the feature to allow slip. During stage four, activity was confined primarily to the area around well B. Pressure and chemical tracer anomalies were not detected in well B until the commencement of stage five. Two stages of pumping were required to move sufficient fluid to displace any preexisting fluid and account for leak-off into the rock matrix and natural fracture system. The ambient data at the lower right are heavily contaminated by noise from a nearby factory, but some prominent features of the frac stage results were forecast by the ambient data. They include the east-to-west trending feature that extends east of the wellbore (feature 1) and the prominent northwest-to-southeast trending fracture zone just south of well B (feature 2). Because a fracture treatment provides much more energy than ambient emissions, noise was much less of a problem for fracture monitoring. If less coherent noise had been present, it is likely that more detail of the natural fracture network would have been resolved in the ambient data. Carbonate Anticline Example Figures 3 and 4 present highlights of an ambient imaging study performed on a carbonate anticline in the Colombian Andes. The study was conducted because a well-developed natural fracture system is required for production in this play. It was conducted blind, so that the personnel conducting the imaging did not know these 50 year-old wells existed prior to delivering the passive seismic results. A surface array of standard 10-hertz vertical component geophones was deployed over the study area in a hexagonal, closest-packed grid pattern, and data were recorded continuously for four days. The array density was 87 receiver points per square kilometer. Structure in the study area is complex. An early thrust-fault- FIGURE 3 Four-Day Cumulative Seismic Activity Sum (Stacked Semblance) in Relation to Anticline Geologic Structure

5 FIGURE 4 Depth Slice of Ambient TFI Derived from Four-Day Activity Sum At Reservoir Level that cuts the anticline (at right). Figure 4 displays the reservoir-level depth slice of the ambient TFI derived from the four-day activity sum in Figure 3. The blue symbols indicate well productivity with 0 denoting no production, S some production and E excellent production. As these field applications demonstrate, ambient seismic emissions can be imaged to reveal parts of hydraulically transmissive natural fracture networks that are independently verifiable. In addition, we tentatively conclude that earth tides measurably affect the intensity of ambient seismic emissions. In fact, our analysis raises the possibility that the seismic response to earth tide displacement components could represent a measure of natural fracture orientations and other fracture network properties, and perhaps even aspects of the fracture flow regime. r bend anticline, now largely inactive, is cut by active wrench faults. The tectonic regime appears to be reactivating parts of the older thrust-fault-bend folds. Figure 3 shows the four-day cumulative seismic activity sum (stacked semblance) in relation to the geologic structure of the anticline. At left is the vertical section of interpreted prestack depth migrated reflection seismic data in gray scale with an overlay of cumulative activity in color. At right is a horizon extraction of cumulative activity data overlain on a structure contour map of the horizon with fault cuts (hotter colors again represent higher activity levels). Note the correspondence of activity with specific structural domains, particularly with the active wrench fault Editor s Note: The authors thank the oil and gas company clients for granting permission to publish the passive seismic ambient fracture imaging results, and acknowledge Global Geophysical Services colleagues Gloria Eisenstadt, Sean Boerner and Rohit Singh for their contributions. This article is a summary of material originally presented in Ambient Fracture Imaging: A New Passive Seismic Method (SPE /URTeC ). The authors suggest reviewing the paper for additional details, particularly analysis of the contribution of earth tides to the ambient signal. ALFRED LACAZETTE is a senior geological adviser at Global Geophysical Services Inc., which has developed the Tomographic Fracture Imaging method. With more than 25 years of experience in structural geology, tectonics and fractured reservoir analysis, he began his career as a fractured reservoir specialist at Texaco. He served at Western Atlas and Golder Associates before establishing NaturalFractures.com, an independent consulting company. Lacazette also worked for EQT Production on the structural geology of unconventional reservoirs. He joined Global Geophysical in 2011 as manager of unconventional consulting. Lacazette holds a B.S. and an M.S. in geology from the University of Kentucky, and a Ph.D. in geoscience from Pennsylvania State University. JAN VERMILYE is manager of the microseismic division and the principal geologist for Tomographic Fracture Imaging at Global Geophysical Services. She started her career in academia, investigating the relationship between seismicity and fracture mechanics with the Southern California Earthquake Center. Vermilye was a research geologist with STRM LLC from 2004 to 2011, working on developing TFI technology. She holds a B.A. in geology from the State University College of New York at New Paltz and an M.S. and a Ph.D. in structural geology from Columbia University. SAMUEL FEREJA is a patent examiner in the U.S. Patent and Trademark Office, and previously served as a microseismic data processor at Global Geophysical Services. After immigrating to the United States from his native Ethiopia, Fereja worked as a quality assurance analyst for Texas Instruments for five years and as a seismic processor at Geotrace Inc. for five years before joining Global Geophysical. He holds a bachelor s in chemical engineering from Addis Ababa University, and a bachelor s in computer science and a master s in electrical engineering from The University of Texas at Dallas. CHARLES SICKING is vice president of research and development at Global Geophysical Services, where he has developed seismic processing algorithms, including azimuth migration, HFVS separation, harmonic noise filters, and 3-D SRME. Sicking also led the development of the company s microseismic processing system. Before joining Global Geophysical, he was chief geophysicist at Weinman GeoSciences. He also served as a research geophysicist at ARCO. During his career, Sicking has developed multiple seismic processing algorithms and technologies for 2-D and 3-D velocity model building using time-to-depth and depth imaging applications. He also led the development of the seismic wavelet processing flow and methodology for ARCO s seismic processing system. He holds a B.A. in physics and a Ph.D. in geophysics from the University of Texas at Austin.