Appraisal of deep tight gas reservoirs offers many challenges, including production rate predictions when wells are drilled overbalanced. Overbalance leads to near wellbore porosity / permeability damage to the rock matrix and fractures. Furthermore, poro-elastic effects due to invasion contribute to difficulties initiating and propagating hydraulic fractures. Damage to natural fractures intersecting the well can prevent their detection leading to missed productive intervals.

Alternatively, under-balanced drilling (UBD) can avoid these effects and thereby better indicate the gas potential of these reservoirs. However, not all reservoirs are suitable for UBD as there can be a greater risk of mechanical wellbore instability. Hence, geomechanical analysis prior to drilling can help evaluate the feasibility of UBD operations.

Formation evaluation in UBD conditions also poses many challenges due to technical (temperature and pressure) limitations of tools and higher uncertainties with petrophysical calculations. Conventional core to log and porosity/permeability relationships are often inappropriate where matrix permeability is extremely low and fractures contribute to flow, therefore alternative ways of understanding and evaluating the reservoir are essential.

In this paper, we demonstrate how geomechanics was used in formulating a strategy for appraisal of a well placed in a tight gas reservoir and also as a decision support tool while drilling.

We present here an analysis of data from the Amin reservoir in the Fahud Salt Basin, Sultanate of Oman, where mechanical borehole failure due to in-situ stresses, pore pressure and rock properties was successfully simulated to understand the mechanical behaviour of the rock matrix and natural fractures which played an important role in controlling the gas flow profile.

Planning of this Amin reservoir well involved geomechanical analysis prior to well spud to evaluate UBD feasibility based on offset well data, followed by updating the models as the well progressed. Evaluation of natural fractures from image logs and identifying sets of critically-stressed fractures (hydraulically-conductive fractures) was an important component of the geomechanical analysis that played a key role in supporting the appraisal strategy.

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