permeability is indicated, and the well then is acidized selectively, a survey 

 following the acid job often shows that the permeability of the zone has been 

 increased. 



Surveys for locating and evaluating permeable zones play a major role in 

 solving the problems of reservoir control, in the control of individual wells, in 

 well completion, and in acidizing and work-over operations. 



When an attempt is made to produce a field as a unit, it is necessary to 

 know as much as possible about the mechanics of drainage and movement of 

 fluids in the reservoir. For example, consider a limestone reservoir in which 

 the production is from a series of permeable and porous lenses with little or no 

 vertical communication. The most efficient method of producing such a reservoir 

 is to deplete all lenses or zones at such a rate that water or gas does not encroach 

 prematurely on any individual member. 



Knowledge of the positions, thicknesses, and capacities of permeable zones 

 in each well, as it is completed during the development of the field, is valuable. 

 When correlated with other information such as structural position, gas-oil and 

 water-oil contacts, and fluid-content logs, the permeability survey data assist 

 materially in programming optimum perforating, acidizing, shooting, and work- 

 over operations. 



Owing to the variability of the porosity and permeability of limestone 

 reservoirs, any simultaneous acid treatment of two or more zones frequently 

 results in the productivity of one zone being greatly increased and that of the 

 other only slightly increased. Permeability surveys made before and after acid 

 treatments have shown this to be true in most cases. Such surveys also have shown 

 that the zone having the greatest capacity before treatment may not have the 

 greatest capacity after treatment. It is not always possible to predict which of the 

 several zones will be benefited most by a simultaneous acid treatment of all zones. 



SELECTION OF METHOD Selection of the best method of obtaining 



comparative permeability data takes into con- 

 sideration a number of factors such as cost, reliability, time required for con- 

 ducting the survey, and individual well conditions. 



In general, the spinner flowmeter type of survey is run most easily and 

 quickly. This survey is accurate and requires the injection of only one fluid 

 into the formation. It can be run in any type of well fluid, including water, oil, 

 drilling mud, and in some instances, gas. One limitation of this tool is that the 

 delicately balanced impeller blade may become entangled with solid materials 

 such as shale, gravel, paraffin, rope fibers, or other lost circulation agents, 

 thus giving erroneous readings. Its principal limitation, however, is that the 

 accuracy of the measurement decreases sharply at fluid velocities of 3 feet per 

 minute or less. 



406 



