Logging Tool Enables Fracture Characterization for Enhanced Field Recovery
Abstract: TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractExxonMobil has developed a fracture mapping tool that uses acoustic methods to infer the geometry of induced hydraulic fractures. This tool may be run in the treatment well during fracturing, or it can be used in one or more offset monitoring wells. This tool, "TABS" for Triaxial Borehole Seismic, consists of an array of three triaxial geophone receivers, a telemetry sub to transmit the data to surface in real time on conventional 7-strand wireline, and an in… Show more
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Improving Hydraulic Frac Diagnostics by Joint Inversion of Downhole Microseismic and Tiltmeter Data
Downhole microseismic and downhole tiltmeter mapping are the primary direct diagnostic techniques for monitoring the geometry of hydraulic fractures at depth. Although these techniques have seldom been used together because of a lack of available monitor wells, the advent of new hybrid arrays having both microseismic receivers and downhole tiltmeters has now made application of both technologies from a single well possible. This paper discusses algorithms for joint inversion of the combined data along with the development of hybrid arrays for combined tiltmeter and microseismic. Examples from several data sets show how the combined results can be used to improve velocity models and obtain better constrained-height estimates, as well as improved confidence in other fracture geometry parameters. Introduction Diagnosis of hydraulic fracture behavior and geometry and accurate design of fracture treatments usually require much more information about the geology, stress, permeability, rock moduli, and other parameters than is customarily available. As a result, it has fallen on fracture diagnostics to provide a more detailed understanding of the overall behavior of propagating hydraulic fractures, particularly in complex or unconventional environments. The subsequent incorporation of these direct measurements of fracture dimensions into hydraulic fracture simulators then enables the creation of customized calibrated models that can be used as predictive tool for specific areas. While there are numerous fracture diagnostic techniques available to the industry, high-resolution mapping of hydraulic fractures is primarily performed with either the microseismic technique using downhole receiver arrays or downhole tiltmeter monitoring. These two techniques measure different properties and different characteristics of the hydraulic fracture, potentially yielding alternate perspectives of the results of a treatment. In general, these two types of measurements have not been performed on the same tests because each requires its own observation well. Improvements in borehole technology, telemetry, and computational capabilities have now made it possible to combine both of these technologies in a single diagnostic array, allowing both types of information to be obtained in a single observation well. The microseismic data provides a detailed map of micro-earthquakes that are induced by the hydraulic fracturing process, whereas the tiltmeters provide an integrated measure of the actual rock deformation. Separately, they both give important information about fracture growth process and the resultant geometry. Jointly, however, they can provide a much more complete, or composite, view of the fracture. This is particularly true if the two data sets can be jointly analyzed, or inverted, to give a single overall view of the fracture. The previously described coupling of tiltmeters and microseismic receivers, analyzed by joint inversion techniques, has been the focus of a US DOE funded project on fracture diagnostics. This paper describes some initial approaches taken in obtaining both tiltmeter and microseismic data during a fracture treatment and also some analysis methods that have been used to jointly analyze the data. Downhole Tiltmeter and Microseismic Technologies Downhole tiltmeter mapping and microseismic mapping using downhole receivers are the two fracture monitoring technologies that can provide high-resolution monitoring of hydraulic fracture growth and behavior. A brief overview of each of these technologies is discussed separately below. Downhole Tiltmeter Mapping. Tiltmeters1,2 are extremely accurate measuring devices that use a sophisticated bubble sensor - much like a carpenter's level - to detect changes in the angular position of the sensor. Sensitivities of these sensors are typically in the nanoradian (10-9 radians) range, equivalent to 0.2 inch movement over a 3,000 mile span. A measurement of the angular position, which is equivalent to the gradient of displacement orthogonal to the displacement direction, provides all the information needed to determine how the earth is deforming in response to some process.
Improving Hydraulic Frac Diagnostics by Joint Inversion of Downhole Microseismic and Tiltmeter Data
Downhole microseismic and downhole tiltmeter mapping are the primary direct diagnostic techniques for monitoring the geometry of hydraulic fractures at depth. Although these techniques have seldom been used together because of a lack of available monitor wells, the advent of new hybrid arrays having both microseismic receivers and downhole tiltmeters has now made application of both technologies from a single well possible. This paper discusses algorithms for joint inversion of the combined data along with the development of hybrid arrays for combined tiltmeter and microseismic. Examples from several data sets show how the combined results can be used to improve velocity models and obtain better constrained-height estimates, as well as improved confidence in other fracture geometry parameters. Introduction Diagnosis of hydraulic fracture behavior and geometry and accurate design of fracture treatments usually require much more information about the geology, stress, permeability, rock moduli, and other parameters than is customarily available. As a result, it has fallen on fracture diagnostics to provide a more detailed understanding of the overall behavior of propagating hydraulic fractures, particularly in complex or unconventional environments. The subsequent incorporation of these direct measurements of fracture dimensions into hydraulic fracture simulators then enables the creation of customized calibrated models that can be used as predictive tool for specific areas. While there are numerous fracture diagnostic techniques available to the industry, high-resolution mapping of hydraulic fractures is primarily performed with either the microseismic technique using downhole receiver arrays or downhole tiltmeter monitoring. These two techniques measure different properties and different characteristics of the hydraulic fracture, potentially yielding alternate perspectives of the results of a treatment. In general, these two types of measurements have not been performed on the same tests because each requires its own observation well. Improvements in borehole technology, telemetry, and computational capabilities have now made it possible to combine both of these technologies in a single diagnostic array, allowing both types of information to be obtained in a single observation well. The microseismic data provides a detailed map of micro-earthquakes that are induced by the hydraulic fracturing process, whereas the tiltmeters provide an integrated measure of the actual rock deformation. Separately, they both give important information about fracture growth process and the resultant geometry. Jointly, however, they can provide a much more complete, or composite, view of the fracture. This is particularly true if the two data sets can be jointly analyzed, or inverted, to give a single overall view of the fracture. The previously described coupling of tiltmeters and microseismic receivers, analyzed by joint inversion techniques, has been the focus of a US DOE funded project on fracture diagnostics. This paper describes some initial approaches taken in obtaining both tiltmeter and microseismic data during a fracture treatment and also some analysis methods that have been used to jointly analyze the data. Downhole Tiltmeter and Microseismic Technologies Downhole tiltmeter mapping and microseismic mapping using downhole receivers are the two fracture monitoring technologies that can provide high-resolution monitoring of hydraulic fracture growth and behavior. A brief overview of each of these technologies is discussed separately below. Downhole Tiltmeter Mapping. Tiltmeters1,2 are extremely accurate measuring devices that use a sophisticated bubble sensor - much like a carpenter's level - to detect changes in the angular position of the sensor. Sensitivities of these sensors are typically in the nanoradian (10-9 radians) range, equivalent to 0.2 inch movement over a 3,000 mile span. A measurement of the angular position, which is equivalent to the gradient of displacement orthogonal to the displacement direction, provides all the information needed to determine how the earth is deforming in response to some process.
Cyclic Steam Stimulation (CSS) is a cost-effective means to produce heavy oil at the Cold Lake field in Alberta, Canada. The high viscosity of bitumen is the main obstacle to economic production, but the bitumen viscosity decreases significantly with temperature. Steam is injected at fracturing conditions, resulting in complex interactions of reservoir expansion (dilation) and contraction (recompaction) that propagate stress and strain fields in the overburden. The mechanical loads on wells resulting from this production process are an important design consideration. To enhance operational integrity, a dedicated passive seismic monitoring well is installed on new development pads to provide early detection of casing failures and possible fracturing of the formation overburden. There is now an installed base of almost 90 such acoustic monitoring wells in the operator's field. With data acquisition of 15 to 30 geophones per system, recording continuously at 2000 or 3000 samples per second, the data management issues for this monitoring network are challenging. Several classes of acoustic events have been identified, including those due to casing failure, formation heave, near-wellbore cement cracking, and production rod pump background noise, in addition to "Continuous Microseismic Radiation" (CMR) that resembles harmonic tremors. Most casing failures are detected by observation of singular events. The detection of fracturing of the overburden, which may include the presence of bitumen and/or produced water that has migrated out of zone, is a more complex process that requires distinguishing shear events and CMR events from normal formation heave and other environmental noise. The operator has stewarded the development of a cost-effective system that includes local pad data acquisition, uploading of selected data to a server with data archiving facilities, and downloading data to dedicated analysts. This paper will present a summary of the data management and processing technologies developed to address the challenge of managing this data-intensive problem.
Passively listening to induced fracturing that is related to production or hydraulic stimulation from a downhole setting has become widely accepted in the oil and gas business. Completions planning and infill drilling programs continue to benefit from this technology with the ultimate goal of enhancing recovery whilst reducing overall costs. The mapped microseismic events are driven by localized and regional stress directions and pre-existing fracture networks. The microseismic map influences the subsequent interpreted fracture network. Reservoir performance and hydrocarbon recovery can be optimized by correctly applying the right downhole monitoring technology to a given specific objective. Here we present three different methods for collecting microseismic data from a downhole environment. We demonstrate the variable response of a single reservoir to hydraulic fracture treatment. Standard observation well setup and treatment well deployments are discussed and case histories are shown. We weigh up the benefits and drawbacks of both. With the advanced observation well tool we show better lateral coverage with high variability of formation response. The tool that can be deployed in the treatment well itself shows better accuracy of frac height measurements and highly localized changes in frac orientation but with limitation of lateral coverage. Extending the monitoring window to years or decades requires yet a different approach. The unique system that contains no downhole electronics extends the life expectancy of the monitoring array for true permanent applications. The three distinct techniques of observation well deployment, treatment well deployment and permanent deployment are successfully applied and demonstrated with case studies. The data that is collected can be used for planning and optimization in the short, medium and long term. When applied in difficult and unconventional settings, this leads to increased reservoir performance and reduced overall cost.
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