The Drillstem Test (DST) has been a popular form of well testing since the early 1900's. To aid in the completion process, the pressure response from a DST can be analyzed to determine formation pressure, permeability, and amount of well damage. A downhole valve is used to control flow into the wellstring in most DSTs. Typically, the reservoir is intermittently flowed and shut-in multiple times. A variation of a DST is a closed chamber DST.
Usually, the DST buildup periods are analyzed by using Homer3 time or Odeh and Selig's5 method. Limited analysis capabilities exist for the flow periods. Most DST analysis techniques lack the completeness to obtain accurate reservoir parameters.
This paper presents a numeric simulator which is used to model DST and closed chamber DST behavior for a wide range of reservoir and wellbore conditions. Because variable wellbore conditions are considered, analysis is more reliable and often possible when other techniques fail.
Reservoir flow into the wellstring is controlled by wellbore and reservoir parameters. In this simulator, wellbore conditions are modelled by using a wellstorage coefficient which is solved at each time step by performing a mass balance on the wellstring fluids. Volume of produced reservoir fluid, rathole mud production, variable cushion pressure, closed chamber air compression, inner wellstring diameter changes, hole deviation, and variable wellstring volume are considered in the calculation. By accounting for these parameters, this model is more complete than existing models or analysis techniques. The wellstorage coefficient is linked with reservoir values to solve for rate and pressure.
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 strengths and weaknesses of the simulator are briefly cited.