One of the major fields in the South of the Sultanate of Oman produces oil at 94% average BSW. All produced water is disposed of without treatment into the same aquifer. The disposed water is injected under fracing conditions and takes place more than 200 m below the lower shale (cap rock) in the sandstone (Fig.1). However the failure of the field's cap rock could result in oil deferment and the contamination of the overlying aquifer; therefore rigorous risk management is strongly required.Several in-situ stress testing (mini-frac) stations were conducted in the field focusing on lower shale and sandstone formations to evaluate the risk of breaking the cap rock and above formations associated with the water disposal. However this was very challenging using the normal ways of testing; hence the stress testing application was conducted with an oil based viscous fluid. The viscous fluid allowed accomplishing the objectives in higher permeability and shale zones. The oil based viscous fluid facilitates initiation of a fracture in highly permeable sandstone formations and reduces the osmosis and plastic behavior effects in shale zones.The viscous fluid stress testing provides the results required to update the numerical models for each formation by obtaining minimum stress and fracture initiation, propagation, and closure pressures (frac gradients). Moreover the test results will help to build models that can predict fracture growth while injecting water. This paper will discuss in detail the viscous fluid stress test and how the results can be used to understand the reservoir behavior. Realistic injection forecasts were made possible using commercial fracture simulation software and some analytical and diagnostic methods based on pressure survey and injection volumes history.