The Miqrat and Barik deep sand reservoirs of the Saih Rawl gas field in Oman are currently undergoing further development, with the addition of high angle and horizontal wells. The field has already experienced 4–11 MPa depletion, and geomechanics-related problems already encountered include wellbore instabilities, poor hole quality and casing deformation. With a further 15 MPa depletion being anticipated over the remaining field life, and with the need for deviated wells, geomechanics effects will become more pronounced and the associated technical and economic challenges facing the field may increase.
To assist in well planning and field development, and to understand the problems already encountered in the existing vertical wells, mechanical earth models were generated for three well locations in the crestal region of the field. These covered not only the two main sandstone reservoirs (Miqrat and Barik), but also two shaly formations through which high angle drilling might also take place (i.e., Al Bashair shale above the Miqrat, and the Mabrouk shale between the two reservoirs). This integration of data from drilling, openhole logs, core tests, borehole images, formation pressure measurements, leak off tests and hydraulic fracturing provided a calibrated and consistent description of the in situ state of stress, pore pressures and rock mechanics properties.
These model were further improved by accounting for cooling effects of the circulating mud when history matching breakouts and instabilities in the existing wells to the in situ stresses, and also by accounting for historical depletion in the field. The depletion modeling was performed analytically and also using a numerical simulator performing finite difference coupled reservoir/stress calculations. The two approaches provided very similar estimates of current and expected formation displacements and estimates of stress changes in the reservoir intervals and their adjacent shales. These results were then considered in the analyses of wellbore stability of the future deviated and horizontal wells.