Hybrid Microseismic and Microdeformation Monitoring of a Coal-Seam-Gas Well
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Providing confidence that hydraulic-fracture geometries are relatively confined to target coal seams and do not grow upward into beneficial groundwater aquifers is a primary concern. A hybrid downhole microseismic and microdeformation array was deployed to monitor fracture stimulation of a vertical coal-seam-gas (CSG) exploration well in the Gloucester Basin in New South Wales, Australia, to provide more-accurate insight into overall fracture height. These technologies complement each other by providing unique, far-field determinations of hydraulic-fracture geometry.
Increasing community and regulatory concern regarding exploration and production of CSG and fracture stimulation has placed extreme constraints on titleholders in New South Wales, to the point of the government rescinding some exploration permits and titleholders nearing a halt to further exploration.
The operator is determined to provide evidence that CSG pilot exploration wells and, in particular, fracture stimulation are not affecting beneficial aquifers or interacting with faults. It is important that the operator be able to provide scientific explanations that are understandable and accessible to the community and assist in regulatory approval processes for the industry.
The primary goals of the project were to assess any fracture-height growth toward aquifers and possible interaction with surrounding faults. In addition, the operator wanted to determine fracture geometry (i.e., length, height, and azimuth), pay-zone coverage, and relative degree of induced-fracture complexity to aid in the optimization of future completions, well placement, drilling strategies, and fracture designs. Hybrid downhole microseismic and microdeformation monitoring was used on the pilot project to provide independent geophysical measurements of fracture-height growth and increase confidence in the results.
Hybridized Downhole Diagnostics
The hybrid receiver array used to map this project is a relatively new technology that uses downhole microdeformation tools (tiltmeters) in conjunction with downhole microseismic geophones. The purpose of combining these two technologies, which complement each other, is to improve assessment of fracture-height growth by determining which microseismic events are likely hydraulically connected and to validate and suggest a more-precise analysis of the measured microseismic events.
Microdeformation. When a hydraulic fracture is created at depth, the dilation of the fracture results in deformation that is transmitted great distances. Deformation emanating up to the surface and laterally to observation wells from deep fracture sources are often very small, sometimes on the order of 10–3 mm. One approach to measuring this deformation is through the use of tiltmeters that can detect extremely small changes in displacement gradient (tilt) attributed to rock movement. Tiltmeters are like carpenter levels, except they can measure rotations as small as one nanoradian. Two orthogonal sensors are used to obtain the total tilt vector, and, by applying a geomechanical inversion to the tilt data obtained from an array of tiltmeters in an offset wellbore, the fracture height and depth can be characterized.
Microseismic. Microseismic fracture mapping provides an image of the fracture by detecting microseisms that are triggered by shear slippage on bedding planes or natural fractures that are adjacent to the hydraulic fracture. The events are located with an array of geophones in an offset wellbore that measure compressional and shear-wave arrivals. A velocity model of the formation then is used to locate the event by minimizing the error of the observed wave arrivals. Microseismic events can be used to infer the fracture height, length, azimuth, asymmetry, dip, and complexity.
Benefits of Hybridization. While microseismic is the primary diagnostic tool for far-field mapping of hydraulic fractures, in many cases, interpreting the microseismicity can be challenging and additional information is needed to draw confident conclusions about the fracture geometry. Coals tend to be aseismic and will highly attenuate seismic signals, making microseismic monitoring difficult, which generally leads to lower data quality. This problem is exacerbated by the typically low treatment rates and volumes used in CSG formations.
Hybrid monitoring provides additional insight into fracture-growth behavior because microdeformation responds to a different mechanism than microseismic. Microseismic activity may be the result of a critically stressed zone shearing as a result of changes in stress from the fracture without any associated fluid placement or deformation. Instead of detecting unique shear-slippage events, which might or might not be related to actual fluid placement, microdeformation responds directly to volumetric change, therefore yielding qualifying insight into whether fluid has been placed in zones where microseismicity has (or has not) been observed.
In addition to providing separate measurements of far-field fracture growth, the microseismic results can be used to help constrain the fracture properties used in the geophysical inverse problem in the microdeformation analysis. This increases confidence in the microdeformation fracture-height solution. The physical alignment of the downhole instruments at each level provides information on the tiltmeter orientation, which allows for an additional relative-source-positioning constraint.
Deployment. A hybrid downhole array uses geophones and tiltmeters located at many different levels to map microseismic and microdeformation data during fracture treatments. For this project, two geophones and one tiltmeter were placed at each level in the tool string, with a total of 12 levels. This arrangement allowed the seismic signal from the two geophones at each level to be stacked, which is a well-known method used to improve the signal/noise ratio. The tiltmeter tools do not have their own clamping mechanism and rely on the clamp arms on the geophones for coupling with the wellbore. A geophone was placed above and below each tiltmeter to provide optimal coupling between the tiltmeters and the observation wellbore. The microdeformation instruments are also keyed in with the microseismic tools at each respective level, which allows the orientation of the tiltmeters to be determined from the orientation of the geophones. The length of the array (185.2 m) was selected on the basis of prejob modeling of the likely deformation field created by an assumed fracture geometry in the target well. The array must be able to measure the peaks in the tilt signal created by the fracture to determine fracture height accurately; thus, the length of the array is critical.
The deployment depth of the array is also critical because it must straddle the target perforations to capture both the top and the bottom of the tilt signals. Because the well in question is a vertical well with 10 planned stages, multiple tool-string positions were necessary for successful downhole microdeformation monitoring. Optimal tool-string positions for each stage were determined on the basis of prejob tilt modeling, with six positions being used to monitor the 10 fracture stages. Fig. 1 above illustrates the six monitoring positions relative to the fracture targets.
Although 10 hydraulic-fracture stages were planned in the treatment well, only nine stages were performed and mapped using the downhole hybrid receiver array. The first stage was terminated early because high pressures limited the pump rate necessary to place the designed proppant volume successfully.
Microseismic. Although the target formations were coal seams, good quality microseismic data were obtained, possibly because of the very thin and laminated coal layers and fractures growing across adjacent siliceous layers. Microseismic data indicated geometries in line with expectations from geomechanical modeling and prejob fracture modeling.
Multiple large faults surround the well, and some concern existed before the fracture treatments regarding whether the faults and hydraulic fractures would interact. In general, microseismic interaction with natural features results in large-magnitude events clustering in the vicinity of the feature. This can be identified on a magnitude/distance plot as a cluster of large-magnitude events equidistant from the tool string. Because no large-magnitude events were observed and few events were observed in the vicinity of the faults, any interaction between the hydraulic fractures and faults appears unlikely.
Microdeformation. Microdeformation analysis was performed on all stages by measuring the total tilt differential throughout the entire treatment. The results of the downhole microdeformation analysis indicated relatively confined vertical-fracture-network heights, with fracturing predominantly occurring in the target coals.
Combined Interpretation. In general, good agreement exists between the fracture-network heights derived from the microseismic data and those from the microdeformation data. Overall, no indication of excessive height growth was present within any of the nine mapped stages.
Overall, there is good agreement between the microseismic and microdeformation data, which both show there was no unconstrained fracture-height growth. The microdeformation data indicate the presence of a highly dipping fracture component on all stages, which would not have been evident from the microseismic data alone. The high fracture gradients observed support the presence of horizontal, or highly dipping, fracture components, which are likely the result of the laminated nature of the formation and the stress regime. Neither the microseismic nor the microdeformation data indicated interaction with any of the numerous surrounding faults. The combined data set gives a high degree of confidence that any hydraulic fractures created in the target formations are unlikely to approach the depth of beneficial shallow aquifers. Hybrid microseismic and microdeformation monitoring is a valid method to obtain improved far-field measurements of fracture growth in shallow CSG reservoirs where microseismic-data quality alone is typically poor.
Hybrid Microseismic and Microdeformation Monitoring of a Coal-Seam-Gas Well
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