This paper (SPE 51324) was revised for publication from paper SPE 28026, first presented at the 1994 SPE/ISRM Rock Mechanics in Petroleum Engineering, Delft, The Netherlands, 29-31 August. Original manuscript received for review 21 November 1994. Revised manuscript received 1 June 1998. Paper peer approved 30 June 1998.
Summary
A testing procedure and analysis method is proposed that gives compaction and compressibility data that are independent of test duration of unconsolidated sands. Compaction data on unconsolidated sand material [including several Gulf of Mexico (GOM) reservoirs] derived from this method are presented. Tests on twin plugs give essentially the same stress-strain data for test durations from one day to three months. This occurs because of a fundamental time-scaling property of these unconsolidated materials. The creep behavior is modeled with a fractional power law dependence with time, which represents the Voigt material model having an extremely broad distribution of relaxation times. This analysis indicates that the characteristic relaxation times of these materials are measured in decades or longer. Creep equilibrated tests are, therefore, impractical and not relevant to a reservoir that will be depleted in one or two decades. Analysis of the power law creep parameters indicates the creep behavior is consistent at higher stresses among the materials tested. The creep parameters scale with stress and provide a means of decoupling the time-dependent behavior from the stress-strain behavior.
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A testing procedure and analysis method is proposed that gives compaction and compressibility data that are independent of test duration for unconsolidated sands. Compaction data on unconsolidated sand material (including several Gulf of Mexico reservoirs) using this method are presented. Tests on twin plugs give essentially the same stress-strain data for test durations from 1 day to three months. This occurs because of a fundamental time-scaling property of these unconsolidated materials. The creep behavior is modeled using a fractional power law dependence with time, which represents the Voigt material model having an extremely broad distribution of relaxation times. This analysis indicates that the characteristic relaxation times of these materials are measured in decades or longer. ‘Creep equilibrated’ tests are therefore impractical and not relevant to a reservoir that will be depleted in one or two decades. Analysis of the power law creep parameters indicates the creep behavior is consistent at higher stresses among the materials tested. The creep parameters scale with stress and provide a means of decoupling the time-dependent behavior from the stress-strain behavior.
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AbstractBecause of shallow water flow concerns in deepwater wells with narrow stability margins, pressure differences of a few tenths of a lbm/gal can make the difference between straightforward drilling and the need for an extra string of casing to protect shallow intervals. Accurate leak off tests (LOTs) / formation integrity tests (FITs) are essential to enable efficient management of the equivalent circulating density (ECD) within the safe pressure window. • Removing the uncertainties in the compressibility of the drilling fluid; this is particularly true of synthetic muds. • Avoiding the need for additional circulation(s) to condition the mud. • Increasing the accuracy of the LOT/FIT, allowing more precise casing point determination.
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