Wellbore stability analysis has some instability problems. The methodologies are legion, data requirements are murky and the answers are often questionable. A new joint industry project (JIP) focused on global wellbore stability methodologies aims to clear up many of these difficulties.
Drilling engineers are asked to design stable well bores based on the industry's uncertain grip on wellbore stability. Casing and fluid plans, trajectories and drilling practices rely on doubtful projections of wellbore stability.
The results are less than impressive. Billions of dollars are spent annually dealing with wellbore stability-related issues. Left unplanned and unaccounted for, these geomechanical issues lead to many wellbore instability problems including wellbore collapse, lost circulation, stuck pipe, casing collapse, reservoir damage, inability to acquire logging data and complete loss of the well bore.
Many modern well bores present an increased risk of wellbore instability when drilling. Deviated well bores involve greater risk than vertical wells. Near salt, stress-related rubble zones are a stability problem. In tectonically stressed regions, variable stratigraphy leads to variable rock strengths, contrasts and stress.
It is clear that maintaining wellbore stability is a key factor in improving safety and drilling efficiency while minimizing problem costs associated with construction and production operations.
Despite this need to understand the conditions that create instability, there is no industry consensus on stability analysis methodologies. Several failure models are widely used to predict wellbore stability, and there is a diverse range of experience and opinion as to which model is more accurate and which more applicable under what conditions.
Collaboration is needed to identify and develop best practices for practical wellbore stability analysis as well as develop guidelines to assess the relative priority of data types and the minimum data needed for effective modeling.
JIP objectives
The purpose of the wellbore stability JIP being conducted by Knowledge Systems is to identify and develop best practices for practical wellbore stability prediction.
Analysis will be performed by an experienced multidisciplinary team of geomechanical engineers, drilling engineers, geologists, geophysicists and petrophysicists, and guided by a technical advisory committee from participating operating and service companies. The resulting database and methodologies will focus on practicality and ready implementation by drilling engineers.
JIP research will:
• Survey existing failure criteria models and the development of improved methods;
• Survey existing rock strength correlations/predictions and develop improved methods;
• Survey existing methods and development of improved methods for derivation of S2 intermediate stress;
• Examine the impact of time and wellbore fluid chemistry on rock strength;
• Study the use of stochastic modeling to build constrained predictive models in the absence of copious data; and
• Examine analytical versus finite element modeling.
The project will examine about 250 well bores from five regions around the world. Studies are planned for Western Canada, US shelf and deepwater Gulf of Mexico, Australian Northwest Shelf and the North Sea. These geologic areas will be examined in detail using the most promising approaches. Stability models will be developed and a database of publicly available digital well data will be built and maintained.
Improved methodologies may include new or combined models as well as guidelines for applying existing models. New tools for defining and constraining rock strength may include temperature effects, streamlined workflow using stochastic modeling and rock strength correlations.
JIP history
This project follows on a series of successful joint-industry projects on predicting earth pressure and in-situ stress. The Drilling Engineering Association's DEA 59 project, initiated in 1989 by Knowledge Systems, pioneered the development of new software for pore pressure and fracture gradient prediction and has led to real-time drilling capabilities. The effort was followed by DEA 119 (completed in 2003) sponsored by Chevron USA to improve the methodologies used in deepwater pore pressure and fracture gradient prediction. Subsalt pore pressure prediction is the focus of a current joint-industry project examining several salt mini-basins using 3-D geologic earth modeling.
These projects significantly improved the industry's ability to maintain hole integrity. By reducing casing and mud-related costs and reliance on contingency casing designs, well construction time and costs have been lowered. In addition, the quality of formation evaluation data has been improved.
As a result of this experience, significant improvements in wellbore stability prediction may be achieved through the identification and development of best practices and methodologies.

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