A full-scale differential-pressure simulator was used to evaluate spotting fluids in the laboratory. Tests at elevated temperature and pressure identified a variety of unweighted and weighted, oil and nonoil fluids that are able to free differentially stuck pipe. Some low-toxicity, water-based spotting fluids are effective and acceptable substitutes for the traditional oil-based systems.
Stuck pipe resulting from differential-pressure sticking (DPS) has long been a major contributor to trouble costs in the drilling industry. DPS is believed to result when the area of the pipe in contact with the mud filter cake is isolated from the pressure in the wellbore. The contact area instead is subject to a pressure closer to that of the formation. The differential pressure acting over the contact area results in a force holding the pipe in place, the magnitude of which can easily exceed the designed overpull for the drillstring.
Influential factors that must be overcome to move differentially stuck pipe include the differential pressure, the contact area, and the coefficient of friction between the pipe and the cake. The sticking force increases with time at a fixed differential pressure because the contact area increases with time by both continued filter-cake deposition and compression. The coefficient of friction between the filter cake and the pipe increases with time as well.
The procedure for freeing differentially stuck pipe often includes replacing the drilling fluid in the stuck zone with a spotting fluid, which promotes a reduction in or eliminates the force holding the pipe in place. If use of the spotting fluid is successful, drilling proceeds. If the spot is not successful, the well is sidetracked or is plugged and abandoned.
Spotting fluids traditionally have been formulated with a petroleum oil as the base fluid, with diesel being the base oil of choice for many years. Environmental concerns in the late 1970's prompted introduction of lower-toxicity mineral oils in spotting-fluid formulations. Recently developed low-toxicity, water-based spotting fluids offer a much reduced adverse environmental impact.