Detecting Induced Micro-Fractures in Subsurface Rock Samples
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Solution Overview
Problem
Accurate measurement of rock permeability in source rock reservoirs is challenging due to induced micro-fractures that occur during the process of bringing rock core from the subsurface to the surface, leading to unreliable permeability data and potential wellbore instability issues.
Innovation Solution
A method to detect induced micro-fractures by determining the unloading effective stress and fracture closure stress in rock samples, and using this information to inhibit their impact by adjusting drilling fluid density and type, such as using oil-based drilling fluids, to correct permeability measurements and prevent wellbore instability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If rock core is taken from subsurface to surface for laboratory experiments, then permeability measurements can be conducted, but induced micro-fractures occur due to unloading stress changes
Solution Approach 1:
The patent applies preliminary anti-action by determining the unloading effective stress and comparing it to the fracture closure stress before conducting permeability measurements. By calculating σul = σv - σh and comparing it to the fracture closure stress obtained from stress-sensitive permeability tests, the method predicts micro-fracture occurrence in advance and takes corrective actions (such as applying back-pressure or using different measurement procedures) to prevent or account for these fractures during the actual measurement process.
Solution Approach 2:
The patent applies parameter changes by modifying the stress conditions during permeability measurement to account for unloading effects. The method determines stress-sensitive permeability curves at different effective stresses, identifies the fracture closure stress, and then adjusts the measurement approach based on the calculated unloading effective stress. This may involve conducting measurements at elevated back-pressures or using corrected permeability values that account for the stress state changes from subsurface to surface conditions.
2Ease of operation
If unloading effective stress is high, then core sample can be retrieved from subsurface, but micro-fractures are induced that affect permeability measurements
Solution Approach 1:
The patent applies feedback by using the determined unloading effective stress and fracture closure stress relationship to guide the permeability measurement process. The method first establishes the fracture closure stress through stress-sensitive permeability tests, then uses this information to interpret subsequent permeability measurements. If the unloading effective stress exceeds the fracture closure stress, the method applies correction factors or alternative measurement procedures to compensate for the induced micro-fractures, thereby maintaining measurement reliability despite the high unloading stresses encountered during core retrieval.
3Productivity
If conventional drilling fluids are used, then drilling operations can proceed, but wellbore instability occurs due to micro-fractures in susceptible formation rock
Solution Approach 1:
The patent applies preliminary action by determining the susceptibility of formation rock to micro-fractures before drilling operations proceed. The method calculates the unloading effective stress that will occur during core retrieval and compares it to the fracture closure stress to predict micro-fracture occurrence. Based on this prediction, drilling parameters, mud weights, or drilling fluid compositions can be adjusted in advance to prevent wellbore instability, such as using lighter mud weights or adding specific additives to drilling fluids to reduce stress on the wellbore and prevent micro-fracture initiation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for more accurate permeability measurements and minimizes wellbore instability by accounting for micro-fractures, improving the reliability of hydrocarbon well operations and reducing the risk of drilling-induced fractures.
Implementation Method 1
Since unloading occurs during this process, the change in effective stress between the subsurface and surface conditions causes micro fractures (in the order of micro-meters)
Implementation Method 2
The unloading effective stress can be greater than the difference between an overburden pressure and a mud pressure at a coring depth
Data Source
AI summary
Methods and systems for detecting impact of induced micro-fractures in a subsurface formation are disclosed. The method includes determining an unloading effective stress (σul) in a formation sample taken from a wellbore drilled into the subsurface formation, determining a fracture closure stress (σcl) of the formation sample, determining that the unloading effective stress (σul) is greater than or equal to the fracture closure stress (σul), and in response to determining that unloading effective stress (σul) is greater than or equal to the fracture closure stress (σul), operating the well system to inhibit impact of micro-fractures in the wellbore.