The problem of reservoir fluid migration through oil- and water- impermeable layers has not been completely resolved yet. A variety of different models is proposed to explain this process of oil migration from oil-source rocks. Migration through the impermeable layers occurs due to two main mechanisms: through the discontinuities of the medium (along fractures and faults) and through the continuous medium itself (multiphase filtration through the pore space). Remarkably enough there are no satisfactory mathematical models that describe these mechanisms of the migration process through the impermeable layers.
The article focuses on the problem of mathematical and numerical description of the fluid migration process through the impermeable layers along the fractures induced by hydraulic fracturing. The mechanism of hydraulic fracture vertical growth due to buoyancy forces is considered as minimum closure stress gradient in the reservoir exceeds pressure gradient of the fluid induced by gravity.
This paper presents the following results. We present a closed integral-differential equation system describing the fluid flow through the impermeable layer (fluid stop) in the presence of fractures. We show that for the typical geological parameters with fracture height over 20 meters, the fluid flow is determined by the buoyancy forces and described by a simple transport equation. We describe the mechanism of fracture break through the permeable layer. It is shown that fracture grows slower in the permeable area. Even if fracture breaks through into an upper impermeable layer, it grows slower than in the absence of a permeable area. This mechanism may describe the transfer of oil from source rock into a series of permeable layers.
The authors believe this mechanism of liquid filtration through impermeable seals along hydraulic fractures may play a major role in oil and gas deposits formation process.
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