Conventional well interference testing is applied to obtain observational data on reservoir parameters such as fluid conductivity, fluid diffusivity and structural inhomogeneities or boundaries. Test results are usually interpreted on the basis of forward type curvematching methods (Ramey, 1970) .

Field procedures are generally based on the use of standard size wells for both injection and response monitoring. The pressure sensors are placed into the wells that serve as observational ports. Obviously, the monitoring wells consititute capacitive inhomogeneities ties that can perturb the reservoir flow field and thereby distort the pressure readings. In particular, the capacitance of wellbores with two-phase fluids, gas caps or even a free fluid surface is relatively large and the perturbation can then be substantial. Quite erroneous test results may be obtained in such situations. Moreover, analog perturbations can result from the presence of inactive high-capacitance wells and other reservoir "soft spots" in the neighborhood of the test wells. For example, geothermal systems that appear liquid-dominated may actually include local spots with two-phase pore fluids that have a higher compressibility than the pure liquid. In particular, such soft-spots are likely to develop in regions with temperatures close to boiling and/ or high gas content liquids.

As a matter of course, the capacitive effects are well known and are in the petroleum industry usually referred to as wellbore storage effects. A considerable literature exists, mainly relating to such effects in single-well pressure-buildup or drawdown testing (see, for example, Ramey, 1970; Earlougher and Ramey, 1973; Raghavan, 1976: Chen and Brigham, 1978; Miller, 1979). by Earlougher (1977). A number of aspects relating specifically to interference testing have been discussed by Prats and Scott (1975), Jargon (1976) and Sandal et al. (1979).

In passing, it is of interest to remark that sensor capacitance is a matter of extremely general relevance. For example, capacitive effects interfere with the measurement of time-varying temperatures. Just as we refer to temperature gauges as thermometers, we wiIl here apply the term pressometers for the pressure monitoring devices. In interference testing, the pressoineter consists of the entire monitoring well setup.

The purpose of the present short note is to discuss the capacitive effects from a rather general point of view and, in particular, to derive some basic expression; to enable us to correct for pressometer and soft-spot capacitance. assumption of a forward type interpretational procedure. from a rather qeneral point of view and, in particular to derive some basic expressions to enable us to correct for pressometer and soft-spot capacitance. In other words, the development is based on definite field models that lead to a well posed problem setting. By varying the model parameters, the solutions yield the type-curves that are used to interpret field data. Data interpretation on the basis of so-called inverse procedures is usually not feasible and would lead to a practically impossible problem setting.

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