ABSTRACT:

All subsurface stress measurement methods available for deep subsurface applications are indirect and result in significant remaining uncertainty in the state of stress. Quantifying these uncertainties is critical for applications where the state of stress is a component of a risk analysis with significant potential safety, environmental, and economic impacts. A publicly available and open source tool called the State of Stress Analysis Tool (SOSAT) has been developed to aid in performing a Bayesian uncertainty quantification and geomechanical risk analysis using many types of commonly available data. A new feature of this tool is demonstrated using data from the FutureGen 2.0 site. The new feature of the tool uses information about the absence or presence of borehole breakouts to constrain the state of stress. This new feature allows the user to specify probability distributions for the relevant parameters such as the drilling mud pressure, temperature, and the formation strength properties. The tool computes the resulting posterior joint probability distribution of the horizontal principal stresses. Using random samples from this posterior distribution the tool then computes the probability of activating a critically oriented fault at a range of pore pressures. This latter feature can be used to aid in the assessment of the risk of induced seismicity and leakage in geologic carbon sequestration and enhanced geothermal systems.

1. Introduction

Characterizing the subsurface state of stress is important for many applications. For some of these applications, such as hydraulic fracturing for petroleum production, it is mainly of interest for completion and stimulation design to optimize production rate, recovery, and costs. For other applications, such as mining and dam construction, in addition to economic factors, an accurate understanding of the state of stress is crucial to the safety of project personnel and the public.

Geologic carbon sequestration (GCS) and enhanced geothermal systems (EGS) are two applications of emerging public interest that have somewhat unique stress measurement requirements compared to the previously cited applications. Among the current barriers to commercialization of these technologies are a set of risks related to the subsurface state of stress state: the risk of leakage of CO2 or brine through the sealing formations for GCS, and induced seismicity for both technologies.

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