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Predictions of well damage that results from invasion of a reservoir sand by fresh water are compared with actual well damage observed in the field. The "sand" studied is the Almond sandstone of Cretaceous age in the Patrick Draw field of Southwestern Wyoming. Actual damage, or well response, is estimated from the interpretation of drill stem tests and observation of the rates of injection into two pilot waterflood wells where fresh water was injected into one well and brine into the other.
Laboratory tests generally indicate that the Almond sand should have a medium sensitivity to fresh water and somewhat less sensitivity to brine. Damage ratios interpreted from drill stem tests are high for gas sands but the damage is rapidly removed by production. Damage ratios are less in oil sands than in gas, and are even less in water sands. Injection rates for fresh water were about the same as those for brine, which does not agree with laboratory prediction but the results are not conclusive. The rates for brine injection are close to those calculated from laboratory relative-permeability curves.
The possibility that the productive capacity of many oil an a gas wells is seriously reduced if the producing formation is exposed to fresh water has been recognized for many years. Despite years of field observations and considerable laboratory research, methods of predicting quantitatively the susceptibility of formations to well damage have not been developed to a point of complete reliability and the extent of actual well damage is open to question in many cases. The need for developing better methods for predicting well damage and for assessing actual damage is self-evident.
A simple solution to this problem has not been found because of its complexity. The interplay of the many factors affecting well damage makes the determination of the relative effect of each variable almost impossible; the effect of each variable must be intuitively estimated from observation of gross behavior. All these problems are compounded by the simple economics of producing oil for profit.
The prediction of degree of susceptibility to well damage is based on the results of several interrelated laboratory tests.
