49th U.S. Rock Mechanics/Geomechanics Symposium,
28 June-1 July,
San Francisco, California
2015. American Rock Mechanics Association
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The aim of this study is to present an integrated and analytical workflow which includes the following execution steps: 1) generating a geomechanical model for the wellbore based on input data from different sources; 2) determining the stress tensor around the wellbore based on a transient thermo-poro-elastic model which include internal/external mud cake effects; 3) determining drilling safe mud weight window based on various failure criteria; 4) identifying troublesome zones with narrow mud weight window throughout the well trajectory; 5) performing an integrated wellbore strengthening analysis based on different mechanisms (e.g., induced fracture propagation and plugging, thermal, external and internal mud cake effects); 6) performing an integrated mud loss volume prediction based on different mechanisms (e.g., natural fracture loss, induced fracture loss, formation loss); and 7) quantifying the amount of strengthening and re-generating mud weight window for safe drilling. The integrated tool provides a suitable workflow when drilling through depleted zones and for lost circulation.
Lost circulation is one of the major causes of nonproductive time in drilling operations. If the near wellbore stress state dictates that they should, induced fractures initiate and propagate away from the wellbore and fluid is lost into the formation. This is particularly true for wells being drilled in complex geological settings (such as deep water or highly depleted zones/intervals). Several geomechanical mechanisms (e.g., near wellbore fracture propagation, thermo-poroelastic processes, mud cake formation, etc.) can act together to control near wellbore stress field, induced fracture characteristics and associated lost circulation risk. If induced fracture characteristics can be accurately predicted, loss circulation materials can be added to the system to plug fractures of different size(s) and width(s) to increase fracture re-initiation pressure (FRIP). This phenomena is also referred as “wellbore strengthening” in the literature.
Induced fracture characteristics are predominantly controlled by the near wellbore stress field. Therefore, one of the prerequisites for a successful design is to establish a workflow (that integrates multiple mechanisms within the framework of a geomechanical engine) to calculate a realistic stress field around the wellbore, to asses induced fracture risk and to come up with an effective wellbore strengthening strategy.
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