By using pressure and fluid indetification data gathered with pump-out wireline formation testers, operators can gain a better understanding of the reservoir, resulting in quality decisions concerning the economic feasibility of stimulating particular zones with hydraulic fractures. Permeabilities and extrapolated reservoir pressures are calculated by performing pressure transient analysis (PTA) on the gathered data. These key parameters can be combined with parameters estimated from petrophysical logs and/or correlations from neighboring wells, resulting in a production forecast for the effectiveness of a potential hydraulic fracture. Input parameters for this hydraulic fracture model can be varied to generate multiple scenario production forecasts. Based on these forecasts, a well-informed decision can be made on whether or not to stimulate a zone with a hydraulic fracture. The pressure tests are performed in cased-hole environments where zones can be isolated with a dual packer tool assembly after perforating. With the pump-out capability of this tool, multiple drawdown and buildup cycles are performed.
This study is focused on low permeability tight-gas reservoirs. Permeabilities to gas are generally less than 1 milliDarcy, but can be as low as 0.1 microDarcy. The typical way to complete these wells is to stimulate the reservoir zones with hydraulic fractures. Although many reservoir rock types may look similar on openhole logs, not all of these reservoirs may produce as anticipated, and will, therefore, not produce enough gas to recover the costs of the stimulation. This paper will describe an effective way to answer the question "Will my tight-gas reservoir produce at economical rates after hydraulic fracture stimulation?" before the decision to stimulate is made.
A general operating process will be outlined for the use of wireline formation testing to successfully predict hydraulic fracture performance. Additionally, several case studies will be presented where forecasts are compared to actual production data. The reservoirs in the studies are located in the Western Canadian Sedimentary Basin.
The stages of completing and hydraulic fracturing of cased wellbores can be broken down into five steps (Fig. 1):
Select possible producing intervals
Determine perforating intervals
Conduct wireline formation testing
Perform pressure transient analysis on gathered data
Decide whether or not to perform a hydraulic fracture.
Step One: Select producing intervals by evaluating the openhole log data from which the input parameters can be determined for the subsequent pressure analysis. Both porosity and formation thickness are obtained from these logs. Other input data, such as average water saturation and gas gravity can be derived from nearby wells.
Step Two: The preferred perforating technique uses a propellant to assist in the perforating process1. This technique provides sufficient stimulation energy to clean up formation damage in the near wellbore region. The application of this technique in tight-gas reservoirs, when combined with wireline formation testers, results in less supercharging and an enhanced inflow of reservoir fluids. More reliable test results can then be acquired, when compared to conventional perforating.