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Estimating Hydrocarbon Column Heights Based on Seal Capacity


Industry practice demands review of potential hydrocarbon column height limitations when the seal capacity is small, i.e. when the prospect is shallow and near-hydrostatically pressured or when the reservoir pressures are expected to be well above hydrostatic and close to the fracture strength of the top seal. Traditionally, seal analysis is conducted at top reservoir/base seal, assuming that hydrocarbon buoyancy (hydrocarbon pressure minus reservoir fluid pressure) cannot exceed the aquifer seal capacity (ASC), which is fracture pressure minus reservoir fluid pressure. The maximum hydrocarbon column height is determined by the magnitude of the ASC and the difference between the aquifer fluid density and the hydrocarbon fluid density, with gas columns shorter than oil column for the same ASC. This paper will review a basin dataset where there are many well penetrations with a mixture of dry holes and hydrocarbon discoveries sharing low ASC values.

The determination of hydrocarbon column height from seal capacity creates three areas of concern:

1. an implicit assumption that hydrocarbon buoyancy pressure can induce hydraulic failure,

2. uncertainty about the proportion of hydrocarbons lost when top seals fail due to hydraulic fracturing, as well as early history of failure and refilling,

3. sensitivity of the method to the input data (reservoir pressure and fracture pressure), relative to maximum closure, which can render the approach of little value in most areas. Examples will be used to illustrate this sensitivity. We suggest that fracture pressures (based on LOTs) may systematically overestimate the fluid pressure required to re-open existing faults/fracture and that fracture pressure over geologic time may be lower than that recorded in LOT.

The supposition that determination of hydrocarbon column height from seal capacity data is the optimum way to risk filling of large structures in high pressure plays will be challenged.