Hydraulic fracture stimulation is commonly conducted in tight gas reservoirs to improve the deliverability. The typical sequence of events during the initial completion of a tight gas well is to conduct a hydraulic fracture treatment, flow back on clean-up for 2–4 days, shut-in to run pressure gauges and then conduct an extended flow and buildup test. The extended flow period can vary from a few hours to a few days and the subsequent buildup can vary from a day to several weeks. Fracture treatments in the Western Canadian Sedimentary Basin (WCSB) increasingly utilize CO2 or Nitrogen to assist in the flow-back of these treatments. A CO2 charged frac can flow back for a day or more, before burnable (reservoir) gas is seen at surface and at this point, there is typically 40+% CO2 in the total gas. In many cases, this is the first gas rate reported by the testers. When conducting a pressure transient analysis, current industry practice is to ignore the CO2 volume injected.
WCSB operators and regulators continue to push for shorter test durations due to economic and environmental concerns. The objective of this paper is to investigate the impact of ignoring CO2 injection on post-frac pressure transient analysis, and to provide guidelines on when the pressure transient analyst should take into account the injected CO2 volumes.
The usage of CO2 in fracture stimulation has been studied extensively1-4. The practical use of CO2 in hydraulic fracturing has been available since the 1960's. It was initially pumped with the frac fluids in a ratio adequate to gas lift the liquid back to surface after the treatment. Later, higher CO2 ratios (50-75%) were used. Recently, liquid CO2 has been used as the fracturing fluid for proppant transport. A CO2-based stimulation can reduce much of the damage related to fracturing fluids. The use of CO2 can provide a fracture fluid recovery mechanism that is independent of reservoir pressure. It is unique because CO2 can be pumped as a liquid and then vaporizes to a gas and flows from the reservoir, leaving no liquid or chemical damage.
Pressure transient analysis is typically performed on the postfrac buildup to estimate reservoir parameters, such as initial reservoir pressure, permeability, effective fracture length, and deliverability potential etc, based on bottomhole pressure measurements and concurrent surface measurement information, such as wellhead pressure(s), and gas, oil and water rates, etc.
A typical tight gas reservoir, vertical well completion in the WCSB commences with a hydraulic fracture treatment. Proppant volumes are rarely below 40 tonnes and frequently exceed 100 tonnes. Fluid volumes are proportionate to the sand volumes and can exceed 200 m3. The cleanup flow back times will vary from operator to operator (based on liquid and sand recovery) but are typically 2 to 4 days. Once burnable gas has been reported and liquid and sand volumes (if any) have fallen to acceptable levels, the well is shut-in to run pressure recorders to monitor the extended flow and buildup test.
