1. Motivation
One of the most important objectives of the well testing is to
- assess the drainable oil reserves around tested well
- assess the reservoir properties
This particularly becomes important in appraisal drilling.
In conventional pressure build-up survey in a single-well reservoir 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 | Assumptions | ||||||
---|---|---|---|---|---|---|---|---|
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 | ||||||
| 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 |
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 is a more informative survey than build-up survey.
It monitors pressure response and time lag with flowrate variation which yields transmissibility \sigma and diffusivity \chi and can estimate effective formaiton 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