Wellbore Storage Effects in Geothermal Wells

The early time response in the well testing of an homogeneous reservoir is expected to give a unit slope when log (pressure) is plotted versus log (time). It is shown that this response is a special case, and that in general another non-dimensional parameter must be defined to describe the set of curves that could take place for each value of C/sub D/. This parameter, t/sub Rw/, is the time response of the reservoir divided by the time response of the well. When t/sub Rw/ is large, a unit slope results (as in oil and gas fields where kh/..mu.. is relatively low) while no unit slope should be measured for small values of t/sub Rw/ (as in geothermal fields where kh/..mu.. is much larger than in an oil field). Using a numerical model of transient two-phase flow, the predicted early time behavior in well testing has been plotted for C/sub D/ = 25, 100, and for values of t/sub Rw/ = 10, 1, 0.1 in each case. It is shown that both C/sub D/ and t/sub Rw/ are needed to correlate the behavior. In addition, the effect of heat transfer on the downhole pressure change with time has been calculatedmore » using the assumption of thermodynamic equilibrium. If the well test data is analyzed without taking into account the heat loss, the calculations for kh/..mu.. are wrong. It is also postulated that non-equilibrium effects in the well many lead to abrupt changes in the pressure versus time curve. Such changes have been observed in field tests of flashed geothermal systems.« less