An Open Air Museum. Success breeds Success. Depth Imaging; Microseismics; Dip analysis. The King of Giant Fields WESTERN NEWFOUNDLAND:

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1 VOL. 7, NO GEOSCIENCE & TECHNOLOGY EXPLAINED GEO EXPRO VOL. 7, NO Success breeds Success geoexpro.com Country Profile: Senegal Ocean Bottom Node Seismic WESTERN NEWFOUNDLAND: An Open Air Museum GHAWAR, SAUDI ARABIA: The King of Giant Fields Technology, Experience and Performance TECHNOLOGY EXPLAINED: MEXICO Depth Imaging; Microseismics; Dip analysis ANGOLA GoM NIGERIA NW EUROPE Seabird_ad_180 x 250 mm.indd 1 08/07/ :35 GEOLOGY GEOPHYSICS RESERVOIR MANAGEMENT

2 topic TECHNOLOGY The Application of Microseismics in the Oil and Gas Industry Until now, microseismic monitoring as a commercial business has been limited mainly to short term monitoring of hydraulic fracture operations and long-term monitoring of steam injections. However, microseismics have other applications within the oil industry. Real-time microseismic processing workflow and decision making. 58 geo expro september 2010

3 kim gunn maver, shawn maxwell, nick koutsabeloulis, and robert greenaway, schlumberger voir. The induced seismicity may be used to map seismically active faults, fractures, fluid fronts and fluid paths. In this article we give an overview of microseismic usage and the enhancement in technology and workflows, including the future integration of microseismicity with other real time measurements from the well and interwell space, which will further improve the usage of microseismicity. microseismic basics Induced microseismicity can be a consequence of oilfield operations from either production or injection. Microseismic events are induced in the reservoir rock matrix due to pore pressure changes and geomechanical stress field relaxation as reservoir fluids are produced or injected. Stress change slippage, which is predominantly in shear, can occur along zones of weakness like new or pre-existing fractures and faults, and emit microseismicity. In hydraulic fracture monitoring the direct aim is to fracture the rock, which can be monitored through microseismic emissions. To provide useful and meaningful reservoir information it is necessary to detect and accurately locate a population of microseismic events. Since there are many more small events than large ones, the resolution of mi- Recent developments and improvements have significantly increased the usage of microseismics as a reservoir monitoring tool. In particular, they are used in reservoir injection and integrity monitoring (for example, in hydraulic fracture operations, injection of drilling cutting waste, gas storage, injection of CO 2 and H 2 S and steam injection operations) and in production monitoring (oil production, gas production, compaction and fault activation). Microseismic monitoring provides a direct method for monitoring stress changes and geomechanical deformation within a resercroseismic monitoring is related to the size of the detected event population. It is important to detect as many events as possible, which makes the placement of the sensors and use of a monitoring system with a low noise floor important. Microseismic events can be located in space and time and their distribution patterns interpreted in terms of geomechanical deformation associated with injected or produced reservoir fluids. Applications include monitoring of fluid fronts, fluid barriers or leakage paths due to faulting or breached cap rock. This information can be used to improve reservoir management and allow better planning of future wells. microseismic monitoring system Successful microseismic monitoring projects require the use of sensors that are well-coupled to the rock and decoupled from man-made noise sources. Such sensors need to form part of a system that is designed to record low energy signals at close distances and thereby deliver both quality and quantity in terms of microseismic signal. In addition to these microseismic requirements, any installed system also needs to meet the specifications required of all oilfield equipment. Deployment Pros Cons Applications Surface geophones n No drilling cost n More freedom to spatially optimize array n Traditional configuration for surface seismic reflection n Potentially monitor large area n Increased noise at surface n Signal attenuated by caprock n Signal further attenuated by weathering layer n Monitor large magnitude microseismic events n Earthquake monitoring Shallow well n Simple drilling n More freedom to optimize the array n Potentially monitor large area n Less noise than surface but more than deep wells n Signal attenuated in caprock n Monitor large magnitude microseismic events n Earthquake monitoring Cemented observation well n Quiet location n Good sensor coupling n Permanent deployment n Drilling expensive n No option to repair n Limited option to use well for other purposes n Existing wells ready for abandonment Wireline observation well n Quiet location n Data quality controlled by sensor and well completion n May need to suspend a well for temporary monitoring n Temporary deployment n Limited access based on well availability n Preparation costs n Hydraulic frac monitoring n Short term reservoir monitoring Outside casing n Well can be used for production/ injection n Good sensor coupling n Requires careful installation and preparation n No option to repair n Noise will change with well operations n Size of wellbore and upper casings will increase n Long term life-of-field monitoring applications from hydraulic fracturing to reservoir monitoring Tubing deployed in dedicated monitor well n Good sensor coupling n Can be retrieved n Limited access based on well availability n Preparation costs n Longer term for reservoir monitoring Tubing deployed in flowing well (annular space between casing and tubing) n Well can be used for production/ injection n Can be retrieved n Well completion may constrain location n Noise will change with well operations n Longer term life-of-field monitoring applications from hydraulic fracs to reservoir monitoring Microseismicity can be recorded with sensors deployed a number of different ways. geo expro september

4 topic TECHNOLOGY The right deployment solution depends on monitoring objectives, well availability, proximity to microseismic events, timing, oilfield access and cost. With the recent developments in microseismic sensing systems for live wells, technical solutions are available for nearly all microseismic project scenarios. Microseismics have three main applications within the energy industry: short-term hydraulic fracture monitoring, long-term reservoir monitoring, and environmental monitoring. The last is not associated with oilfield activity, although it may influence oilfield operations, and is therefore not discussed here. hydraulic fracture monitoring Hydraulic fracture monitoring is when microseismicity is induced as a consequence of the fracturing of the reservoir formation. This is now a common technique, especially in unconventional reservoirs, which are often naturally fractured and require specific stimulation techniques to economically produce. One particular example is the Barnett Shale in Texas, where the combination of horizontal drilling, large volume water fracs and microseismic imaging of the fracture network has transformed the Barnett Shale from an economically marginal play to one of the largest natural gas fields in the United States by imaging the far-field fracture geometry. Microseismic fracture images have been fundamental in terms of current understanding of the importance of complex fracture networks in unconventional reservoirs. Hydraulic fracturing operations are typically of short duration and ideally suited to a wireline deployment in an offset well. With the sensor array restricted to one or possibly two or more observation wells, a system with good vector fidelity is required to provide accurate microseismic locations. The sensor spacing and position in the well are critical to optimize the location accuracy. The example shows the result of a case study where engineers used real-time microseismic to decide on the timing and effectiveness of a fiber diversion technique. Like most wells in the Barnett Shale, production from this particular well had significantly declined since it was originally fraced. Microseismic data from the original well completion showed that the heel portion of the well was not stimulated. The objective of the re-fracing was an attempt to extend the fracture network, and in particular to attempt to better stimulate the heel. During the initial stages of the stimulation, the microseismicity was primarily clustering in the same areas as from the original stimulation. Based on this observation, progressively more aggressive fiber diversion stages were incorporated, which ultimately steered the stimulation towards the heel and increased the effective stimulated volume. As a result, production increased. long-term reservoir monitoring Where microseismicity may be induced as a consequence of oilfield operations from production or injection or with geothermal projects involving simultaneously injecting and producing water it can be utilized for long-term reservoir monitoring. To ensure close proximity to the reservoir to improve sensitivity during the recording of microseismicity, a dedicated monitoring well has to be available or a live well The Omega-Lok clamping system: (left) Tetrahedral geophone mounted in the clamp and used for microseismic data acquisition is on the left. (right) The clamp mounted on tubing before it is deployed. Live processing in real time during a hydraulic fracture treatment in the Barnett Shale. Each stage is represented by different colored estimated stimulated volumes. The final processed microseismic events located in relation to the well trajectory. Red dots were the first phase of the well stimulation, green dots the second phase of the stimulation and finally the yellow dots were the third and final phase. 60 geo expro september 2010

5 topic TECHNOLOGY Located microseismic events for the Karachaganak Field. The microseismic sensing system is deployed in well K125. Extract of a 3D Geomechanical Earth Model from the VISAGE software showing comparison between detected microseismic events (coloured spheres) and estimated moment magnitude of induced shear deformation. has to be shut-in, due to the noisy live well environment, so shut-in costs or monitoring well availability may limit the application of microseismics for reservoir monitoring. This limitation can be overcome by recording microseismic events with tools properly installed inside live and highly deviated wells with no production/injection stops during recording. Such a tool must be insensitive to the flow noise of the well to detect the microseismic signals of very low energy. The Omega-Lok clamping system, which is part of PS 3 -FW, has these attributes. It is a system of sensors, which are decoupled from the tubing and therefore providing background noise levels largely independent of flow rate. During a test of the tubing clamping device with tetrahedral geophones, microseismicity was recorded during a three-step acid stimulation. A single well was used for the monitoring, which at the reservoir depth of around 15,000ft was highly deviated. A 2-level Omega-Lok system clamping the sensors to the casing was used to record microseismicity during periods of flow in the tubing. Good quality microseismic events were detected and localized, indicating the sensor coupling to the casing was effective and the noise floor low providing reliable results and interpretable microseismic events. Water Injection Reservoir Seal Failure Injector Pressure Shut in Time Producer Injector pressure Producer pressure Depth Depth Temperature Geothermal DTS temperature Microseismic event Temperature Geothermal DTS temperature Flowrate In connection with water injection to stimulate production, microseismicity may be used to detect a failure in the reservoir seal and explain the PT and DTS measurements. 62 geo expro september 2010

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