BENEFITS OF EARTH MODEL CALIBRATION TO MAP HYDRAULIC FRACTURE PROPAGATION: COMBINING GEOLOGIC OBSERVATIONS WITH VERTICAL SEISMIC IMAGING

Today, microseismic monitoring is an accurate way to achieve an image of a hydraulically induced fracture geometry. Critical elements of a microseismic monitoring system include the receivers and the telemetry system. Receivers must be extremely sensitive and well coupled. They also have to exhibit a linear response to capture a wide range of signal strengths. Additionally, they have to be correctly calibrated before and after the monitoring campaign to ensure the quality of the recorded signals. Telemetry cartridges need to capture and transmit the continuous stream of data acquired by the receivers and withstand high-pressure, high-temperature environments often encountered in the intervals of interest.

Nevertheless, technology can help overcome geological complexities in terms of stimulation design and interpretation results. Rock properties govern the types of fluids injected into the formation as well as the pumping schedule. Lithological packaging controls the location of the monitoring device, dictates the depth at which perforations should be located, and controls how hydrocarbons flow into the wellbore. Despite these facts, geological impact on the survey program is often overlooked.

We illustrate with case studies how to generate an accurate representation of the hydraulic fracture system and analyze the induced microseismic events by combining all available information gathered, in both the geological and geophysical domains.

We demonstrate that the accuracy of the velocity model used to process the vast amount of acquired data is a key component to an efficient survey program. Traditional borehole seismic checkshot, vertical seismic profile (VSP), and walkaway VSP surveys have been employed in the construction of calibrated earth models for many years. We show how a borehole seismic survey prior to the hydraulic fracture stimulation enables (i) calibration of sonic velocities, (ii) estimates of energy loss during propagation, (iii) better knowledge of anisotropy, and (iv) qualification of wavefield mode conversions.