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Dependence of sonic velocity on effective stress in North Sea Mesozoic mudstones [An article from: Marine and Petroleum Geology]
Book Details
PublisherElsevier
ISBN / ASINB000P6XLNC
ISBN-13978B000P6XLN6
AvailabilityAvailable for download now
Sales Rank99,999,999
MarketplaceUnited States 🇺🇸
Description
This digital document is a journal article from Marine and Petroleum Geology, published by Elsevier in 2006. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.
Description:
Overpressure estimation methods that use sonic velocity as a proxy for porosity only account for excess pressure due to undercompaction, so the influence of unloading processes in generating overpressure is ignored. We have used wireline log data and pore pressure measurements in mudstones of the Cromer Knoll Group and the Heather Formation in the North Sea to investigate whether velocity is sensitive to the contribution of unloading processes to observed overpressures. Our approach was to fit the data by four alternative linear relationships with sonic velocity as the dependent variable. The independent variables necessarily included porosity and vertical effective stress, and optionally the natural @c log response and depth. Our preferred results are V"p=3746-3744@f+11.1@s"v-4.7@c in the Cromer Knoll mudstones and V"p=4035-4269@f+8.6@s"v-5.5@c in the Heather mudstones, for vertical effective stress over the range 10-30MPa. The sensitivity of sonic velocity to vertical effective stress is, therefore, around 10ms^-^1MPa^-^1. This sensitivity should be sufficient to improve estimates of pore pressure while drilling by using sonic, density and natural @c logs in combination. To make use of this sensitivity for pore pressure estimation, robust empirical relationships with vertical effective stress as the dependent variable would have to be established for the formations of interest.
Description:
Overpressure estimation methods that use sonic velocity as a proxy for porosity only account for excess pressure due to undercompaction, so the influence of unloading processes in generating overpressure is ignored. We have used wireline log data and pore pressure measurements in mudstones of the Cromer Knoll Group and the Heather Formation in the North Sea to investigate whether velocity is sensitive to the contribution of unloading processes to observed overpressures. Our approach was to fit the data by four alternative linear relationships with sonic velocity as the dependent variable. The independent variables necessarily included porosity and vertical effective stress, and optionally the natural @c log response and depth. Our preferred results are V"p=3746-3744@f+11.1@s"v-4.7@c in the Cromer Knoll mudstones and V"p=4035-4269@f+8.6@s"v-5.5@c in the Heather mudstones, for vertical effective stress over the range 10-30MPa. The sensitivity of sonic velocity to vertical effective stress is, therefore, around 10ms^-^1MPa^-^1. This sensitivity should be sufficient to improve estimates of pore pressure while drilling by using sonic, density and natural @c logs in combination. To make use of this sensitivity for pore pressure estimation, robust empirical relationships with vertical effective stress as the dependent variable would have to be established for the formations of interest.
