Single-well Pressure Pulsation survey
One of the most important objectives of the well testing is to assess the drainable hydrocarbon reserves and reservoir properties around tested well.
This particularly becomes important in appraisal drilling as well testing is the only source of this information.
The Self-Pulse Test (SPT) is a single-well pressure test with periodic changes in flow rate and pressure (see Fig. 1).
Fig. 1. Typical record of pressure and rate variation during SPT |
When flow rate is being intentionally varied in harmonic cycles with sandface amplitude and cycling frequency :
then bottom-hole pressure will follow the same variation pattern:
with a bottom-hole pressure amplitude and the time delay . It takes some time (3-5 cycles ) for pressure to develop a stabilized response to rate variations. The pressure delay and associated dimensionless phase shift represent the inertia effects from the adjoined reservoir and characterized by formation pressure diffusivity:
The diffusion nature of pressure dictates that amplitude of pressure variation is proportional to amplitude of sandface flowerate variation and the ratio is related to formation transmissibility:
The exact solution of differential diffusion equation for vertical well with negligible well storage and infinite boundary homogeneous reservoir can be represented by a system of non-linear algebraic equations, relating field-measured parameters to formation properties :
The above equations assume that diffusivity and dimensionless radius are found from – and then is substituted to to calculate transmissibility . In case of a low frequency pulsations:
the equations – can be explicitly resolved in terms of transmissibility and diffusivity:
where . The above analytical approach (either – or – ) is rarely helpful in practise. The field operations are very finnicky and difficult to follow the pre-desgined sequence of clean harmonic pulsations. As result, the flowrate variation becomes a complex sum of harmonics:
and the pressure response becomes complex as well:
The use of analytical formulas requires fourier transformation to identify the key harmonics from the raw data with a manual control from analyst and a certain amount of subjectivism on which harmonics to pick up for calculating the transmissibility and diffusivity. Once the harmonics are identified one need to search for the best fit to a complex system of non-linear algebraic equations:
Still, formulas and play important academic role and provide fast track estimations in SPT engineering. |
The effect of the pressure response delay to flow rate variation in a single well test is so small (usually seconds) that conventional build-up can not capture it reliably due to a high pressure contamination and wellbore instability at early build-up times and hence pressure diffusivity normally can not be assessed. |
Objectives
The advantages of SPT deliverables over conventional single-well test is illustrated below.
Vhc | Potential hydrocarbon reserves |
Ve | Drainage volume |
Ae | Drainage area |
knear | Permeability of the near-reservoir zone |
hnear | Effective thickness of the near-reservoir zone |
kfar | Permeability of the far-reservoir zone |
hfar | Effective thickness of the far-reservoir zone |
S | Skin-factor |
Pu(t) | Deconvolution of the long-term unit-rate response |
Property | Description | Data Source |
---|---|---|
Bo | Oil Formation Volume Factor | PVT samples / Correlations |
Bg | Gas Formation Volume Factor | PVT samples / Correlations |
Bw | Water Formation Volume Factor | PVT samples / Correlations |
co | Oil compressibility | PVT samples / Correlations |
cw | Water compressibility | PVT samples / Correlations |
cg | Gas compressibility | PVT samples / Correlations |
cr | Rock compressibility | Core samples / Correlations |
swi | Initial water saturation | Core samples |
Porosity | Core samples |
The typical SPT procedure is brought on Fig. 2.
Fig. 2. Typical SPT procedure |
It normally consists ion three consequent tests with three different cycling frequencies:
The total duration of the test is 155 T.
Typically T = 3 hrs and total test duration is around 20 days.
Every pulse includes one choke-up and one choke-down so that full SPT survey require 60 choke operations during 40 days which is a lot of field activity for a given well.
It would be extremely difficult to perform this manually and usual practice is to arrange a programmable remote-controlled flow variation.
Automated flowrate-pressure fata records match in multiphase 2D pressure simulator with:
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing / Cased-Hole Pressure Transient Test / Pressure Interference Test (PIT) / Pressure Pulsations Survey
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