Gas Well Testing - Storage

The usual back pressure tests or inflow tests during gas injection are needed to understand gas well flow capacities. Such individual well capacities are combined into composite field deliverability curves for predicting the flowing well head pressure during heavy gas withdrawal periods. Some practices for gas storage in both depleted gas fields and aquifers are discussed. Gas wells are tested regularly in connection with storage operations. Two types of reservoirs will be considered; depleted gas fields and aquifer storage fields. Generally, the objective is to know the flow capacity of the gas wells during the high withdrawal periods. Depleted Gas Fields The flow capacity of gas wells is normally determined shortly after drilling. At the time of conversion of depleted gas fields to storage, it is advisable to re-test the wells for deliverability. Such tests usually must wait until a reasonable degree of repressuring has taken place to give enough gas pressure to permit blowing the wells and to assure that the well sand face is dry. A comparison of the re-test performance with the original test indicates the degree of deterioration and whether remedial work is indicated. Fig. 1shows a Stray Sand gas well with severe deterioration during it's productive life. Any new wells drilled are tested to give their flow capacity. The information of interest is the composite performance curve for the field. This is obtained by taking the sum of the flow capacities [Q] at a given difference of squares of the pressures. Such a field curve along with the gathering system pressure drop will give the field delivery pressure for any flow rate and gas reservoir pressure. The reservoir pressure is often observed directly by a pressure observation well to find Pf. Flow tests are most conveniently taken in the summer time in a period corresponding to gas injection rather than withdrawal. It has been shown that inflow testing gives the same results as outflow testing and Fig. 1 shows inflow test results after conversion to gas storage. In well testing it is customary to use a deadweight pressure gage for well head pressure measurements and individual well orifices meters. The relative close spacing of gas wells for storage removes the usual problem of unsteady state. However, before developing a reservoir, initial tests on a gas well involving unsteady state may be made in the manner followed by Robertson. The more likely problem is that a portion of the gas reservoir has high permeability and hence contains most of the deliverability capacity.