"Wave" Hello to What’s Beneath Texas

Using Seismic methods to monitor the subsurface and carbon plume under Farnsworth Field

Geologic carbon sequestration is a critical technology in the fight against climate change. It involves capturing potentially harmful carbon dioxide (CO2) emissions and storing them underground to prevent them from entering the atmosphere. Enhanced Oil Recovery (EOR) is a process where this supercritical CO2 is injected into depleted oil wells to help extract more oil. These sustainable practices have both economic and environmental benefits. The Farnsworth Field in Texas is a site where these two processes are combined. Seismic methods play a crucial role in monitoring and characterizing both the subsurface geology and the behaviour of the sequestered carbon plume, to ensure the success and safety of these operations.

To carry out the EOR/Carbon Capture, Utilization, and Sequestration (CCUS) project, 3 seismic methods were used:

• 3D Seismic Survey
• Time-Lapse Vertical Seismic Profiles
• Cross-well Seismic Tomography

3D Surface Seismic Survey

This is a geophysical survey method used to create a 3D image of the subsurface by sending sound waves into the ground and recording their reflections.

Data Acquisition

At Farnsworth Field, the survey covered an area of approximately 108.8 km², and aimed to better interpret the field’s structure, stratigraphy and geomechanical properties and also estimate its storage capacity. Firstly, a Survey Evaluation and Design (SED) was conducted. Field tests were carried out to fine-tune parameters from the Vibroseis (the wave source instrument). This produced geometry readings that prescribed the optimal locations of the source and receiver.

Data Processing

A thin sandstone layer called the Morrow B formation was the target reservoir for CO2 injection. In some cases, if data is processed in only one way, certain key elements of the subsurface may be overlooked. Numerous processing techniques were employed, enabling the operators to discern different characteristics about the site from the same raw waveforms. The first of the two main processing methods was traditional time domain processing, recording the time it takes for seismic waves to travel through the subsurface, reflect off various geological layers, and return to the surface. The second method was the conversion of the seismic volume into the depth domain by means of a velocity model, which improves the ability to image the Morrow B reservoir interval.

Results

The survey provided a detailed understanding of the subsurface structure, resulting in a 3D model that was essential for subsequent seismic techniques. It helped identify the extent and behaviour of the CO2 plume, aiding in effective monitoring and management.

Vertical Seismic Profile (VSP)

This is a type of seismic survey where seismic sources are placed at the surface and receivers are positioned down a borehole, providing detailed information about fluid flow within the borehole.

Data Acquisition

VSP surveys were conducted at wells designated for CO2 injection. First researchers conducted a baseline VSP of the well before CO2 injection. Around the same time each year, for two years following CO2 injection, more surveys were undergone to observe any potential changes in the reservoir.

Data Processing

To improve the quality of the raw data from the Vertical Seismic Profile (VSP) surveys, several processing techniques were applied:

• Noise Attenuation: Reducing unwanted background noise to make the important seismic signals clearer.
• Consistent Amplitude Compensation: Adjusting for differences in how well the seismic source is coupled to the ground at different locations, ensuring the signal strength is uniform.
• Waveform Separation: Using special filters to isolate the downward-traveling seismic waves, enhancing the clarity of the data.

Additionally, thorough quality control checks were performed to ensure that the data from the baseline and monitor surveys were consistent and reliable, showing strong agreement between the two.

Results

The VSP results revealed significant fluid flow changes in the reservoir, indicating the migration of the CO2 plume. The primary geo-body detected showed asymmetry, suggesting preferential migration paths for the CO2, essential information for optimizing injection strategies.

Cross-Well Seismic Tomography

This involves measuring seismic signals between the seismic signal transmitted from a source, located in one well, to a receiver array in a neighbouring well, to create a reflection image or map other rock properties.

Acquisition and Processing

This method was used to characterize the thin Morrow B formation and monitor CO2 movement between wells, providing a detailed view of the subsurface.

Results

The tomography results showed good agreement with well logs, confirming the reliability of this method. However, there were no substantial changes in the velocity field between baseline and monitor surveys. Theoretically, and as confirmed by the VSP, it is known that there exists a plume between wells. The supercritical CO2 should have expectedly created a disturbance in the velocity field. From the tomography though, this was not observed.

The study at Farnsworth Field utilized three seismic methods: a 3D surface seismic survey, time-lapse VSP, and cross-well seismic tomography. The 3D surface survey provided essential geologic structure and velocity models necessary for the other methods. The time-lapse VSP tracked changes in seismic properties over time, revealing the carbon dioxide plume's movement and identifying the geo-body. The cross-well tomography, although aligning with well logs, had limitations due to certain data absence and was less effective in detecting velocity changes. Overall, the study highlights the importance of each technique, but while cross-well tomography was a precise seismic method, it was less useful under the given conditions. This suggests that the seismic method used depends a lot on the situation.

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