1. Motivation
One of the most important objectives of the well testing is to assess the drainable reservoir volume around tested well.
This particularly becomes important in appraisal drilling.
Conventional pressure build-up survey in a single-well reservoir is normally providing assessment of:
- average transmissibility in drainage area
\sigma
- drainage area around the well
A_e
- formation pressure
P_i
- skin-factor S
where
| transmissibility | ||
| mobility | ||
k_a | absolute permeability | ||
k_{rw}, \, k_{ro} | relative permeabilities to water and oil | ||
\mu_w, \mu_o | water and oil viscosities |
It is important to notice that drainage area A_e is calculated based on the permeability estimations from core study and compressibility estimation from porosity correlations which may not be representative of the whole drainage area:
(3) | A_e = 4 \, \chi \, t_e |
where t_e time to reach the reservoir boundary from BUS log-log plot and \chi is pressure diffusivity which is used to translate t_e into a drainage area
(4) | \chi = \big< \frac{k}{\mu} \big> \, \frac{1}{\phi \, c_t} |
and c_t is total compressibility:
(5) | c_t = c_r + (1-s_{wi}) \, c_o + s_{wi} \, c_w |
In regular workflow one pulls permeability
k from core studies, then estimates diffusivity
\chi from
(4) and then calculates average reservoir thickness in drainage area as
(6) | h = \phi \, c_t \, \frac{\sigma}{\chi} |
which leads to assessment of drainable oil volume as
(7) | V_o = (1-s_{wi}) \, \phi \, h \, A_e |
This methodology is strongly dependent on core-based permeability which may not be representative of the whole drainage area.
The alternative way of
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 |
ks | Permeability of the skin-zone |
hs | Effective thickness of the skin-zone |
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 0.3 day)
- Test 2 = mid freq pulsations (10 pulses with 1.5 day)
- Test 3 = Low freq pulsations (5 pulses with 5.5 day)
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