Abstract
Post-perforation surge tests are frequently used in the oil industry. A surge test is normally conducted by flowing fluids into a closed chamber. This category of testing includes backsurge perforation washing, underbalanced perforating with subsequent flow, and closed chamber drillstem testing. Typically, downhole response is measured by a pressure/temperature gauge. This response may be analyzed for reservoir parameters by using existing techniques; however, variable wellbore conditions and rapidly changing rates make finding a solution difficult and sometimes impossible.
This paper presents a numeric simulator which is used to model surge behavior for various wellbore and reservoir conditions. Because variable wellbore conditions are considered, the resulting analysis is more reliable and often possible when other techniques fail.
Reservoir flow into the wellstring is controlled by the wellstorage coefficient and completion and reservoir parameters. At each time step the wellstorage coefficient is explicitly solved by performing a mass balance on the wellstring fluids. This calculation accounts for compression of air in the surge chamber, variable cushion pressure, displacement of rathole fluid into the wellstring, changing inside diameter in the wellstring, and volume of reservoir fluid produced into the wellstring.
Simulated pressure profiles are presented for various reservoir and wellbore conditions. A simulated match of field test data is also given to illustrate the practicality of this work. The matching process yields permeability, skin, static formation pressure, and radius of investigation.
