ABSTRACT

This paper presents a conceptual model of Pressuremeter Testing (PMT) in oil and gas wells. The elastic rock deformation was simulated to characterize the mechanical state of the rock using the cavity expansion theory. The results generated from the numerical approach demonstrated the impact of the stress relief during the excavation process on the borehole displacements. Moreover, the impact of the geometrical shape of the borehole on the PMT loading was discussed and analyzed. Several numerical models are conducted to simulate the PMT loading using Eidsvolt Siltstone and Warwick Sandstone properties under anisotropic and isotropic in-situ stress contrasts. Characterizing the mechanics of the geomaterials can be obtained from the core samples. However, the in-situ mechanical state of the rock mass in oil and gas wells is hypothetically described. The results showed apparent difference in the mechanical state of the rock based on the rock type and the surface contact between the PMT and borehole.

1. BACKGROUND

In-situ rock testing in oil and gas wells requires an in-depth analysis of borehole mechanical responses in order to obtain representative measurements of the rock formation. During the drilling process, it is crucially important to minimize wellbore instability to be able to sequentially recover the full mechanical responses during the Pressuremeter testing (PMT). Optimum drilling practices should be implemented to minimize wellbore instability problems such as collapse, breakouts and washouts. The borehole's wall failure is an indication of major stress relief, which may influence the in-situ measurements conducted by the PMT.

The PMT is a standard in-situ testing procedure to investigate the mechanical state of subterranean formations. It is based on cavity expansion theory. The test is typically used to study the soil and rock mechanics (i.e., strength, deformational characteristics and in-situ stresses) for underground applications such as drilling, tunneling, excavation, and mining (Clarke, 1994). The mechanical behavior of the soil and rock may follow the same physics. However, compared to soil applications, the stress relief due to oil and gas well drilling varies significantly based on the regional in-situ stress contrasts, rock type and rock fabric. Moreover, the borehole shapes in the oil and gas wells are usually irregular, which may affect the PMT results. The initial mechanical responses of the wellbore govern the state of stresses exerted by the contraction and expansion mechanism at the borehole circumference. Drilling fluid forms filter cake around the wellbore creating a barrier between the formation rock and borehole. The pressure gradient of drilling fluid is vital to mechanically minimize borehole instability in problematic formations.

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