1. Motivation
One of the most important objectives of the well testing is to assess the drainable reservoir volume around a given well.
This particularly becomes important in appraisal drilling.
Conventional pressure build-up survey is normally providing assessment of:
- average transmissibility in drainage area
\sigma = \big< \frac{k}{\mu} \big> \, h
- drainage area around the well Ae
- formation pressure Pi
- skin-factor S
It is important to notice that drainage area Ae 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:
(1) | 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
(2) | \chi = \frac{k}{\mu} \, \frac{1}{\phi \, c_t} |
and c_t is total compressibility:
(3) | c_t = c_r + (1-s_{wi}) \, c_o + s_{wi} \, c_w |
Assuming the permeability to oil
k_o from core studies one can estimate the diffusivity
\chi_o from
(2) and average reservoir thickness in drainage area as
(4) | h = \phi \, c_t \, \frac{\sigma}{\chi} |
and then assess drainable reservoir volume as
(5) | V_o = (1-s_{wi}) \, \phi \, h \, A_e |
This methodology contains a big uncertainty as it is based on
The BUS alone can not assess formation thickness directly and only
instead uses
- the ratio of transmissivity (kh) and reservoir permeability (k) where the latter is estimated from core data which may not be representative for the whole drainage area.
The major drawbacks of this approach is that drainable reservoir volume estimation is based on assumption in reservoir thickness which is estimated from OH logs and may not be representative for the whole drainage area.
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