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Reports: AC8 47463-AC8: Primary Migration of Oil Through Self-Propagating Fractures
In the 2009 – 2010 period, we continue to focus on primary migration of hydrocarbon from the source rock with extremely low permeability and porosity to the overlying reservoir by fracture propagation. Besides finishing the work on oil migration through subcritical growth of collinear blade-shaped microcracks due to volume expansion of kerogen-oil transformation, we have also investigated oil migration through subcritical growth of a penny-shaped microcrack during kerogen-oil transformation as field observations indicate that microcracks containing hydrocarbon residue are more penny-shaped than blade-shaped. We have conducted a parametric study on the propagation of an oil-filled, penny-shaped crack and studied the difference in the propagation behavior between the blade and penny-shaped cracks. Moreover, we are investigating hydrocarbon migration by subcritical propagation of both blade and penny shaped microcracks due to the volume expansion associated with the thermal cracking of oil to gas. Two journal papers have been published as a result of this research. A third paper is being submitted for publication. Next we describe the approach for solving the problem and present the results obtained.
1. Primary oil migration through subcritical propagation of blade-shaped collinear microcracks due to volume expansion resulting from kerogen-oil conversion
This part is a continuation from last year’s study. We use a coupled model of fracture mechanics and kerogen-oil transformation kinetics to study the effects of crack spacing on the subcritical crack propagation distance and duration. Our results show that the propagation duration is mainly governed by the transformation kinetics for a single oil-filled crack. All kerogen has been converted to oil when the crack propagation is arrested. For periodically spaced collinear cracks the crack propagation duration may be reduced by two orders of magnitude due to crack interactions. The oil pressure on the crack surfaces decreases monotonically with time during crack propagation, but decreases precipitously when the collinear cracks are about to coalesce.
2. Oil migration through subcritical propagation of a penny-shaped crack during the transformation of kerogen to oil
We have investigated the effects of physical and material parameters on the subcritical propagation behavior of a single penny-shaped crack. The parameters include Young’s modulus and fracture toughness of the source rock, volume expansion rate associated with the kerogen-oil conversion, initial kerogen particle size and source rock temperature. We assume that kerogen converts to oil only, which may be applied to the first stage of kerogen to oil/gas transformation.
In our parametric sensitivity analysis, Young’s modulus varies from 2.0 to 4.0 GPa, fracture toughness varies from 0.1 to 0.5 MPa-m1/2, initial thickness of the kerogen particle varies from 5 to 10 μm, volume expansion rate of kerogen-oil conversion varies from 0.1 to 0.2, and source rock temperature varies from 120 to 150 oC. These ranges for physical and mechanical properties are consistent with the available petroleum source rock data. Our study shows that higher Young’s modulus, lower threshold stress intensity factor (hence lower fracture toughness), and higher initial kerogen particle volume all yield significantly larger crack propagation distance, and therefore would increase the fracture permeability of the source rock. Higher source rock temperature significantly reduces the crack propagation duration but does not affect the final propagation distance. The crack propagation behavior (propagation distance and duration) is relatively insensitive to the volume expansion rate of kerogen-oil conversion.
To study the difference in the oil migration behavior by the propagation of the blade and penny-shaped cracks, we have considered a blade crack 100 μm long and a penny crack with a diameter equal to the blade crack length. For the blade crack in the source rock with a modulus of E = 2 GPa, it takes about 6.5 m.y. (which includes a period of 26 kyr from the beginning of conversion to the start of crack growth) for the crack to propagate from an initial length of 100 μm to the final length of 1390 μm, which is more than twice the final diameter (519 μm) for the penny-shaped crack. Hence, blade cracks can more readily form interconnected fracture networks than penny-shaped cracks. Accompanying the longer propagation distance, lower excess oil pressure is needed to propagate the blade crack.
3. Subcritical crack propagation during thermal cracking of oil to gas
It is known that the oil window is commonly found in the temperature interval of 65-150 oC, and the gas window in the interval of 100-200 oC. Hence, at higher temperatures, both oil and kerogen residue may be decomposed into gas. The effect of gas generation on subcritical propagation of oil/gas cracks must be considered to more completely understand primary migration behavior of hydrocarbons. We are investigating hydrocarbon migration through propagation of both blade and penny-shaped microcracks during thermal cracking of oil to gas. We have formulated the gas migration/microcrack propagation model by using fracture mechanics, oil to gas transformation kinetics, and the state equation of the gas. We are analyzing the effects of burial depth of the hydrocarbon (which influences the gas pressure in the crack) and source rock temperature on the behavior of gas migration and microcrack propagation. Our preliminary numerical results show that first, for a given crack shape and source rock temperature, the excess pressure on the crack surface increases with the increasing depth. Second, final crack propagation distance (horizontal distance of gas migration) decreases with increasing burial depth. Finally, crack propagation duration becomes much longer for host rocks at lower temperatures.
4. Impact
The research has enhanced our fundamental understanding of primary migration of hydrocarbon through fracture propagation during conversion of kerogen to oil and gas. The grant allows Scott Johnson, an expert of structural geology and tectonics, to expand his research into petroleum geology, and allows Zhihe Jin, a junior faculty member in mechanical engineering, to develop a new research program in geophysics. A doctoral student is deeply involved in the project and will alleviate the shortage of scientists/engineers in the interdisciplinary area of geology and engineering mechanics.
