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How Accurate Can We Compute Hydrocarbon Column Heights in Overpressured Prospects?
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 79th EAGE Conference and Exhibition 2017, Jun 2017, Volume 2017, p.1 - 5
Abstract
Computation of maximum column height is common during evaluation of overpressured traps. The main idea behind such computations is to assume tensile fracturing, and that the hydrocarbon buoyancy cannot exceed the difference between the fracturing pressure and the reservoir pore pressure. The uncertainties of such computations are rarely addressed and have to date not been quantified.
We perform such quantification by using pre-production reservoir pore pressure variations to estimate the accuracy that can realistically be attained in evaluating pore pressures in undrilled prospects. We have also analysed uncertainties in fracture pressure determination from leak off tests. This was performed by comparing leak off pressures and fracture closing pressure for a large number of extended leak off tests from the Norwegian Continental Shelf. The uncertainties of the fracturing pressure is frequently 5 MPa or more, which is comparable to the buoyancy of a 1 km gas column. We therefore conclude that computation of maximum column heights should not be computed from individual wells, and that computations based on regional fracture pressure trends need further verification.