This paper discusses the merits of using wellhead (surface) pressure data for pressure transient testing on a variety of oil and gas wells.
The case for using wellhead pressures for pre-frac completion procedures on shallow gas wells is examined in the context of obtaining sufficient well and reservoir data within a cost-effective test program. A discussion is included regarding pressure buildup data that is collected acoustically, with illustrations of how surface pressures can measure sensitive reservoir pressure changes, even with liquid level movement above the formation. The case for considering wellhead pressures when wellbore configuration and liquid production obscures valid down-hole gauge data is presented to suggest that a ‘quiet’ side annulus may provide more accurate pressure communication with the reservoir.
With recent improvements to thermal transducer compensation, a lot can now be done with surface pressure transient data. A realm that was once thought to be solely the domain of subsurface pressures data.
The concept of well test analysis using surface data is not new. One of the industry's most recognized methods is the acoustic fluid level instrument that offers valuable subsurface pressure information from depth-to-liquid measurements. The sum of the casing pressure plus the hydrostatic head of the gas column pressure, plus the liquid column hydrostatic pressures are used to calculate the subsurface pressure.1
Another recognized use of surface data for well testing is with shallow gas wells. Within Alberta, the AEUB Guide G-402 states that surface pressures are acceptable for dry gas wells less than 1500 m. This acceptance has proven to be very economical for the development of shallow gas reservoirs throughout Western Canada. Examples #1 and #2 show the pressure derivative response from a 325 m low density sweet gas well showing both the surface and subsurface pressure derivative response. Analytical results from both data sets are shown in Table 1 and demonstrate the validity of the technique for shallow gas wells.
With the expected improvements in software program algorithms for calculations of subsurface pressures from surface pressures, it is expected that deeper wells and high flow- rate gas wells can be tested at surface3. At the time of writing this paper the authors have participated in over one thousand surface well tests ranging from shallow gas wells to surface pressure interference testing on both oil and gas wells. Although the title of the paper suggests that 76 examples will be shown, in actual fact, we will limit this paper to the best of 76.
Perhaps the most significant development in recent years has been the proliferation of digital pressure gauges designed specifically for surface pressure transient testing, in both Canada and in the United States. Electronic surface pressure recorders are now available on the market ranging from single channel stainless steel strain gauges, to multiple channel readings and large data sampling options using silicon crystal transducers and the shear-mode, quartz crystal resonator transducers.