Maximum hydrocarbon column height before shale breach: 24-Pet-B4 Well Testing, December 2014, Q8

Section 3, Q3-2: Maximum hydrocarbon column height before shale breach (4 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given.

Quantity	Symbol	Value
Interfacial tension	γ	20 dynes/cm = 0.02 N/m
Shale (seal) pore throat diameter	D_shale	5 μm
Sandstone (reservoir) pore throat diameter	D_sand	30 μm
Hydrocarbon density	ρ_HC	800 kg/m³
Freshwater density	ρ_w	1000 kg/m³
Wettability	θ	0° (water-wet, cosθ=1)

Find. The maximum hydrocarbon column height z that the sandstone/shale seal pair can hold before the shale's own capillary entry (displacement) pressure is exceeded and the seal is breached.

Approach. The seal that limits trapped column height is always the rock with the smaller pore throats – here the shale, whose narrower pore throats give a far higher capillary displacement pressure than the sandstone reservoir. Compute the shale's displacement pressure from P_d = 2γcosθ/R, then convert that pressure into an equivalent hydrocarbon column height via the buoyancy/hydrostatic balance z = 4γ/D × 1/[(ρ_w−ρ_HC)g] – algebraically the same statement, since R = D/2.

Confirm which rock governs. Displacement pressure P_d = 2γcosθ/R is inversely proportional to pore-throat radius, so the smaller-pore-throat shale (R=2.5 μm) has the higher entry pressure and is the seal that must be breached; the larger-pore-throat sandstone (R=15 μm) offers negligible resistance by comparison and does not limit the column.
Shale displacement (seal capillary entry) pressure. $$P_d=\frac{2\gamma\cos\theta}{R}=\frac{2(0.02\,\text{N/m})(1)}{2.5\times10^{-6}\,\text{m}}=16{,}000\ \text{Pa}$$
Convert to column height using the buoyancy balance. The trapped hydrocarbon column exerts a buoyancy pressure at the seal of (ρ_w−ρ_HC)gz; setting this equal to the shale's displacement pressure and solving for z: $$z=\frac{4\gamma}{D_{shale}}\cdot\frac{1}{(\rho_w-\rho_{HC})g}=\frac{4(0.02)}{5\times10^{-6}}\cdot\frac{1}{(1000-800)(9.81)}=\frac{16{,}000}{1{,}962}$$ $$\boxed{z \approx 8.15\ \text{m}}$$

Result	Value
Shale (seal) displacement pressure, P_d	16,000 Pa
Sandstone (reservoir) displacement pressure (for comparison, does not govern)	2,667 Pa
Maximum hydrocarbon column height, z	≈ 8.15 m

Topic. Capillary seal capacity (Berg/Schowalter method).

Key relations. P_d = 2γcosθ/R (Young–Laplace, radial pore-throat capillary tube); z = P_d/[(ρ_w−ρ_HC)g] (hydrostatic/buoyancy conversion of a threshold pressure into an equivalent fluid column).

Why this works. A hydrocarbon phase cannot enter a water-wet, water-saturated pore throat until its buoyant pressure exceeds the capillary resistance of that throat's narrowest constriction; because a caprock seal is defined precisely by having much smaller pore throats than the reservoir it seals, the seal's own (largest) pore throat – not the reservoir's – sets the column height the trap can hold before hydrocarbons leak (breach) through the seal.

Common pitfall. Using the reservoir's (sandstone's) pore throat diameter in the formula instead of the seal's (shale's) – this is backwards: it is always the finer-grained, lower-permeability caprock that limits trap capacity, and using the coarser reservoir value here would understate the true seal capacity by a factor of 6 (30/5).

Source. Schowalter, T.T. (1979), "Mechanics of Secondary Hydrocarbon Migration and Entrapment," AAPG Bulletin 63; Selley & Sonnenberg, ch.6 (capillary seal, Berg method).

Question 8 of 18: Maximum hydrocarbon column height before shale breach

Section 3, Q3-2: Maximum hydrocarbon column height before shale breach (4 marks)