Stochastic Inversion of Wellbore Stability Models Calibrated With Hard and Soft Data

Wellbore stability models are used in well-planning to determine the safe mud-weight window for drilling. More generally, calibration of wellbore stability models against observations (such as image logs, caliper measurements, and generaldrilling observations) is an essential step in constructing reliable 1D and 3D Mechanical Earth Models (MEMs) which are used to design safe drilling, completion, and production strategies. However, such calibration usually produces non-unique results, partly because most common types of calibration data impose only soft (inequality) constraints on wellbore stability models. Such nonuniqueness can be represented using probability density functions (PDFs). In this paper we show the results of stochastic inversion for stress parameters performed by drawing samples from these PDFs using a Markov Chain Monte Carlo procedure. Most types of calibration data (e.g., breakouts, drilling-induced fractures) produce a wide range of possible solutions for the stress parameters. However, the uncertainty reduces dramatically as data from an increasing number of depth locations is simultaneously inverted. The results also illustrate how including depths where breakouts and drilling-induced fractures are absent produces a powerful constraint on inferred stress parameters.