Use of Injection - Falloff Tests To Evaluate Storage Reservoirs

Abstract

Introduction Probably the single most important piece of reservoir Probably the single most important piece of reservoir data required in the conversion of a depleted gas field to gas storage service is effective rock permeability distribution, and this information is usually not available in sufficient detail during the primary depletion of the field. Wells are drilled on relatively wide spacing; and core or buildup data, even where available, provide insufficient points of control for adequate provide insufficient points of control for adequate mapping. Matching history with a reservoir simulator can sometimes fill in the gaps, but it is not uncommon for areal history matches to be somewhat insensitive to relatively wide variations in over-all reservoir permeability and individual well descriptive parameters (such as damage, turbulence, and crossflow) at the producing rates at which the field was depleted. producing rates at which the field was depleted. As the field is being converted to storage mode, infill drilling provides the required additional points of control. Normally, good porosity limits can be obtained through logging programs, but permeability is another matter. Drilling into pressure-depleted reservoirs with gas-or nitrogen-lightened mud is often ruled out from a cost standpoint, and deep invasion is commonplace when nonaerated muds are used. Additionally, costs in general and costs and problems resulting from lost circulation usually preclude extensive coring. Completed wells more often than not have significant damage and insufficient pressure to flow out invaded and produce at rates and pressures that well permit an acceptable evaluation. In addition to filtrate invasion, it is not uncommon for some solids to drop out, completely blocking the sand face and requiring the well to be acidizing before it will even accept gas on injection. Acidizing - even a small job - is at best a necessary evil as it adds more liquids to an already invaded zone. Deep and severe damage can affect pressure transients for a substantial period of time. period of time. A testing method must be designed that can yield usable information even at low rates and in severely damaged environments. The analytical technique must be able to provide the following:Extent of damage;Permeability of the undamaged zone;Effect or significance of geological heterogeneity;Predicted injectivity capability, including theeffects on injectivity resulting from the removalof damage or from the eventual perforation ofa previous unperforated productive interval;Degree of reliability of test analysis and interpretation. If the field data are taken carefully, their trend should be valid, and the results of a test should be accurate within plus or minus a specific and determinable percentage. percentage. Injection-falloff tests provide a relatively inexpensive means of obtaining information from which adequate analyses can be made. JPT p. 494