1. Motivation
One of the most important objectives of the well testing is to assess the drainable oil reserves and reservoir properties around tested well.
This particularly becomes important in appraisal drilling as well testing is the only source of this information.
In conventional well testing which is based on long-term pressure build-up survey the primary hard data deliverables are:
- formation pressure
P_i
- skin-factor S
- average transmissibility in drainage area
\sigma
- time to reach the reservoir boundary t_e
The conditional deliverables from build-up survey would be:
Deliverables | Description | Input Parameters | Key Uncertainties | ||||||
---|---|---|---|---|---|---|---|---|---|
where c_t is total compressibility:
and \{ c_r, \, c_o \, c_w \} are rock, oil and water compressibility. | Drainable oil reserves | The rock compressibility c_r(\phi) is defined from core lab study or empirical porosity correlations Fluid compressibility \{ c_o \, c_w \} is estimated from PVT study Initial water saturation s_{wi} is estimated from SCAL |
| ||||||
where \chi is pressure diffusivity:
where \phi is reservoir porosity, \big< \frac{k}{\mu} \big> is fluid mobility:
k_a is absolute permeability to air, k_{rw}, \, k_{ro} are relative permeabilities to water and oil,
\mu_w, \mu_o are water and oil viscosities | Drainage area | Absolute permeability to air k_a is estimated from core lab study
Fuid viscosities \mu_w, \mu_o are estimated from PVT study | Absolute permeability to air k_a is estimated from core lab study Relative permeabilities k_{rw}, \, k_{ro} are estimated from SCAL | ||||||
| Effective reservoir thickness | Absolute permeability to air k_a is estimated from core lab study
Fuid viscosities \mu_w, \mu_o are estimated from PVT study | Absolute permeability to air k_a is estimated from core lab study
|
As one can see, the drainage area and the reservoir thickness are conditioned by core data which may not be representative of the whole drainage area.
The SPT provides more information than build-up survey (BUS).
The BUS correlates pressure decline to change in production rate (usually from constant production rate to shut-in) and it strongly depends on formation transmissibility \sigma.
The SPT is doing the same but also tracks the time lag between flowrate variation and pressure response which depends on formation diffusivity \chi and together with transmissibility \sigma this allows estimating effective permeability and formation thickness separately:
(7) | h = \frac{\sigma}{\phi \, c_t \, \chi} |
This allows asessing the drainange area
(8) | A_e = \frac{4 \, \sigma \, t_e}{c_t \, h} |
mobility:
(9) | \bigg< \frac{k}{\mu} \bigg> = \chi \, \phi \, c_t |
and maxcroscopic absolute permeability:
(10) | k_a = \frac{\bigg< \frac{k}{\mu} \bigg>}{\bigg[ \frac{k_{rw}}{\mu_w} + \frac{k_{ro}}{\mu_o} \bigg]} |
Running SPT in two different cycling frequences SPT can assess the near and far resevroir zones spearately.
2. Objectives
- Assess reservoir volume around well
- Assess reservoir permeability and thickness variation around well
3. Deliverables
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 |
4. Inputs
Property | Description | Data Source |
---|---|---|
Bo | Oil Formation Volume Factor | PVT samples |
co | Oil compressibility | PVT samples |
cw | Water compressibility | PVT samples |
cr | Rock compressibility | PVT samples |
swi | Initial water saturation | Core samples |
\phi | Porosity | Core samples |
5. Procedure
Test = Test 1 + Test 2 + Test 3
- Test 1 = high freq pulsations (10 pulses with period T)
- Test 2 = mid freq pulsations (10 pulses with with period 5T)
- Test 3 = Low freq pulsations (10 pulses with period 25 T)
So that total duration of the test is 310 T.
Typically T = 3 hrs and total test duration is around 40 days.
6. Interpretation
- Numerical model
- Single well with circle boundary
- High density LGR
- High density time grid (seconds)
- Single well with circle boundary
- Automated pressure match in PolyGon software
References
\sigma