ABSTRACT:

Minimum stress and fracture gradient are critical parameters used in geomechanics modeling, such as borehole stability and wellbore strengthening analyses, to optimize operational plans and reduce drilling risks. They can be inferred through interpretation of initiation and closure pressures of induced tensile fractures formed during extended leak-off tests. Since mud loss events while drilling are more common than the extended leak-off tests, and if the loss zone can be identified by using resistivity logging after drilling (MAD PASS), these events can be used to estimate the minimum stress and fracture gradient utilizing timebased pressure while drilling data (APWD). Using MAD PASS logs and APWD data integrating a petrophysical assessment of the lithology of the loss zone, the downhole initiation and closure pressures can be determined which can be used to constrain fracture gradient and minimum stress models.

1. Introduction

Interpretation of minimum stress and fracture gradient from drilling data can be challenging due to a variety of factors that impact the pressure at which tensile fractures initiate in the borehole wall, and the ambiguity inherent in distinguishing far field minimum stress in the presence of near-field stress perturbations. Minimum stress can be estimated from extended leak-off (XLOT) tests, a method involving intentionally raising borehole pressure via slowly pumping until annular fluid pressure exceeds fracture leak-off pressure plus the tensile strength of the rock (Fig. 1). In an XLOT, pumping continues until the induced tensile fracture propagates far enough (length is inferred from the volume pumped) to clear the perturbed stress field around the wellbore. Then, when pumping stops, fracture closure pressure can be determined. The minimum stress (σmin) is typically aligned with fracture closure pressure (Pc), as shown in Eq. (1), which can be observed on the decline curve of a XLOT following the formation breakdown pressure (Pb in Fig. 1).

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