Conventional flow and buildup testing of gas wells in Alberta is common practice for determination of reservoir pressure, permeability and wellbore skin. Post frac welltesting is still regarded as the preferred welltesting method, due in part to current government regulations for initial testing requirements. With the maturing of the Western Canadian Sedimentary Basin in recent years, the development of unconventional gas reservoirs has significantly increased. Deep tight gas reservoirs have become an important part of the unconventional gas resource and the paper will address several issues and pitfalls related to welltesting of tight gas. We are now seeing a large number of disciplines involved with completions. This diverse group is necessary due to the complexity surrounding tight gas reservoirs. Initial reservoir pressure is probably one of the most important "data points" for a completion and yet one of the most abused and assumed parameters. Field examples will highlight major errors that are likely to occur when post frac flow and buildup tests are analyzed for tight gas reservoirs without precise knowledge of the initial reservoir pressure. Furthermore, the paper will demonstrate that errors of reservoir pressure as small as 2% can lead to significant overestimates of the well's recoverable reserves.
The record high commodity prices in the energy sector experienced in recent months has resulted in an increased activity level for the development of unconventional tight gas reservoirs. Natural gas price is the major factor that will determine the economic viability of tight gas reservoirs; however, there are additional factors that should be taken into consideration, such as: original-gas-in-place, completion efficiency, well spacing and the ultimate recoverable reserves. Unlike the conventional gas reservoirs where proven reservoir engineering techniques are easily applied, the predictability of deep tight gas reservoirs is much more difficult to achieve. Too often the post frac welltest results from wells completed in tight gas reservoirs are used to establish initial pressure, completion effectiveness and reservoir permeability for production forecasting for economical decisions. The hydrodynamics of tight gas, hydraulically fractured reservoirs are very complex and significant errors in matrix permeability and fracture parameters evaluation are often occurring. These errors have a much greater impact on predictions for long term gas production and total cumulative gas recovery.
With the maturing of the WCSB and high natural gas prices, the activity levels in the oil and gas industry has increased to record levels. So has increased the number of tight gas well completions. Generally speaking, performance evaluation of tight gas wells reveals a trend where the production performance of the wells is much less than what initially was predicted. These errors are considered the result of over estimating reservoir parameters such as the reservoir flow capacity (kh).
Short references will be made to some of the factors determining the success of developing tight gas resource:
Original-gas-in-place (OGIP) usually is determined by the volumetric method and requires knowledge of reservoir pressure, mapping and petrophysical parameters. One equation used for estimating OGIP is: (equation (1)) (Available in full paper)
