振荡脉冲压力法渗透率测量

Accepted 28 October 1996
Abstract
Permeability of Indiana Limestone samples undergoing deformation in a triaxial cell along several stress paths such as triaxial compression, hydrostatic compression and uniaxial strain was measured using the oscillating pulse technique. This technique consists of applying a sinusoidal pressure wave at the upstream end of the sample and recording the pressure-time behavior at the downstream end. The solution to the general one-dimensional diffusivity equation can be obtained by applying the appropriate boundary conditions to give a method of evaluating diffusivity and permeability in a relatively short time. The process of sending a pulse to evaluate permeability has been programmed in Visual Basic which can be incorporated into any regular triaxial testing automated routines to measure permeability at different stages of loading until failure. Stored waveforms of the upstream and downstream pressure responses with time are analyzed to evaluate the attenuation and phase shift in the waveform. For comparison purposes, permeability was also measured using steady-state methods for samples subjected to a hydrostatic compression stress path and good correlations were observed. During triaxial compression failure tests, in most tests the permeability decreased continuously until the end of the test. Permeability also decreased with increasing confining pressure, while under uniaxial strain (i.e. one-dimensional compaction (k 0) tests), there was a sudden reduction of permeability at a particular stress level.
P. Suri a, M. Azeemuddin b, M. Zaman ~~, A.R. Kukreti a, J.-C. Roegiers b
a School of Ciril Engineering and Encironmental Science, The Um) ersitv ~/'Oklahoma, Norman, OK 73019. USA h School of Petroleum and Geological Engineering, The Unicersity ~f Oklahoma, Norman, OK 73019. USA
* Corresponding author.
0920-4105/97/$17.(X1 Copyright ~5 1997 Elsevier Science B.V. All rights reserved. PII S0920-4105(96)00073-3
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P. Suri et al. / Journal of Petroleum Science and Engineering 17 ~1997) 247 264
ity formations, when subjected to effective stress increases due to productions, are often found to undergo sudden and drastic volume reductions through a micro-level mechanism called pore collapse. Reductions in permeability can result in a decrease of ultimate recovery, reservoir compaction, subsidence, etc. Consequently, the measurement of permeability in the laboratory under simulated conditions of pressure and temperature is important for the petroleum industry. Permeability measurements in a triaxial test setup have traditionally been carried out in one of two ways: (1) steady-state methods; and (2) transient methods (e.g., Brace et al., 1968). Rocks with high permeability can easily be measured by the former technique while it is more convenient to use transient methods for low-permeability rocks having permeability values of the order of < 1 - 10 14 c m 2 (l ~.LD) (Brace et al., 1968). For tight rocks, either due to the fact that the flow rate is too small to be measured accurately, or that unrealistic long times are required to reach stabilized flow conditions, the steady-state measurement methods become inconvenient and impractical. Moreover, pressure can usually be measured more accurately than flow rate. In such cases, transient techniques, originally pioneered by Brace et al. (1968) are resorted to. The method consists of applying a sudden pressure pulse upstream of a jacketed sample subjected to confining pressure and pore pressure and attached to upstream and downstream reservoirs of known volumes; and recording the pressure-time histories in both the reservoirs. The decay characteristics depend upon the permeability, the dimensions of the sample and the reservoirs, and on the physical characteristics of the fluid. The permeability can be evaluated by comparing the observed pressure decay in the upstream reservoir with the behavior predicted analytically. A more general approach was used by Trimmer et al. (1980) and Lin (1982), in which no experimental restrictions on the reservoir volumes were imposed. The measurements were performed in the same manner as described by Brace et al. (1968), and permeabilities were evaluated numerically using a one-dimensional finite-difference model. Hsieh et al. (1981) presented a general analytical solution which accounts for the compressive storage in the rock sample, and is applicable to all sizes of the upstream and downstream reservoirs. Based on the techniques developed by Hsieh et al. (1981) and Neuzil et al. (1981), one can graphically estimate the permeability and storage capacity of the rock samples. The transient step method requires measurements for long periods of time and utilizes graphical curve-matching procedures. Kranz et al. (1990), based on the literature on thermal diffusivity measurements (Cowan, 1961; Cerceo and Childers, 1963), applied oscillating boundary conditions to study the hydraulic properties of tight rocks. The method was also used by Fischer (1992) and Fischer and Paterson (1992) to determine the permeability and storage capacity of rocks under high temperature and pressure. The method consists of applying a sinusoidal pore pressure oscillation at the upstream end of a sample and recording the pressure-time response at the downstream end. In this paper, the oscillating pulse technique has been modified to be applicable to a wider range of rocks. The technique has been programmed in Visual Basic language and is used to determine the permeability changes in Indiana Limestone subjected to increasing effective stresses under different stress paths in a triaxial cell. The results are found to be consistent with those obtained using other techniques (steady-state methods). Drastic changes in permeability were observed during uniaxial consolidation at the time of pore his study was to develop a fast and efficient means of measuring stress-dependent permeability in rocks undergoing given loading paths, and to determine the applicability of the oscillating pulse technique. It was also intended to evaluate the sensitivity of the frequency of the oscillating pulse on the values determined using this technique.