Reservoir stimulation are very common, most of the time conducted to improve the deliverability of reservoir. Stimulation can be generally divided in critical-matrix stimulation and deep reservoir stimulation. Acid stimulation in sandstones and carbonate reservoirs is an example of the first type while hydraulic fracturing is an example of the later.
Both of these jobs require pumping of fluid down the wellbore. Naturally because most reservoirs are considerably deeper in the subsurface, pumping pressure is required to force inject the fluid in the reservoir. Furthermore, if we are conducting hydraulic fracturing, than we need to pump at considerably high pressure to break open the target reservoir rock. In either case, we are handling pressurized fluids on surface within surface pipe lines, which is a serious safety concern.
Most of the time, the pumping equipment is placed at a safe distance from the wellhead which often exceeds 60 feets. To connect the pumping equipment and wellhead, we use several joints of treating iron or pipeline which are screwed together. Most common type of connection used is hammer union with o-rings to create seal (see picture below - ref Revata Engineering). When the rig-up is completed, these pipelines and connections are tested using water at operating pressures to identify and rectify any leak. This exercise is called hydrotesting.
Hydrotesting has couple benefits:
1. Any leak is identified before conducting main job
2. If leak is found, and fluid is being jetted out from joints, damage is minimum as water is not harmful. If leak was identified with acid or other hazardous additives inside the pipe, significant damage could be expected
3. Equipment, pipes and whole setup is pressure-tested at design & operating pressure creating assurance for operators that equipment is fit-for-service
