The Use of the Probe Permeameter in Carbonates - Addressing the Problems of Permeability Support and Stationarity

Patrick Corbett1, Septi Anggraeni, and David Bowen
 

ABSTRACT
Permeability measurements taken with the probe permeameter in carbonates can be highly variable and very sensitive to small changes of location and/or tip dimensions. These problems are associated with the measurement volume of the probe permeameter, which is very small (1-9x10-7m3) and is illustrated in this paper. The problem of measurement volume (which is also the statistical support volume) is usually avoided by taking larger (whole core) samples. However, with whole core samples, it is not possible to assess the support by measuring an adjacent lateral sample. The variability of adjacent measurements is analogous to the variability of an average at different locations (which is statistical stationarity). This paper shows how the probe permeameter can be used to assess permeability support and stationarity in a variety of carbonate pore types.
A published method is applied here-multisupport probe (MSP) permeametry-to provide speedy, non-destructive sampling at high density (close spacing) and at multiple volumes. The key feature of the MSP is the use of different tip sizes to measure the permeability at different sample support volumes. Through use of the MSP permeameter, the permeability data are screened to assess whether they are appropriate for upscaling or modeling. The use of MSP permeameter as a screening device for petrophysical measurement in carbonates is recommended.
A numerical model of the unsteady state probe permeameter is described. This has been used to understand the tool response in a very heterogeneous carbonate rock sample. The results show that the device is sensitive to the ratio between the size of the vugs and the probe tip dimensions and can be used as a qualitative measure of vug connectivity.
An experiment is also described to demonstrate the ability of this method to quantify the upscaling process. To obtain the required averaging method in carbonate rock, an upscaling experiment has been performed on cubic samples. The averaging method for the probe-scale data is quantified by comparison between the probe and cube data. In a carbonate with isolated vugs, the cube data are best estimated by the harmonic or geometric averages, an outcome that is in accord with the connectivity of the pore system.

Barbara Anderson, Thomas Barber, Vladimir Druskin, Ping Lee and Elizabeth Dussan V.,
Leonid Knizhnerman, and Sofia Davydycheva
 

ABSTRACT
With the rapid expansion of horizontal drilling, the interpretation of logs, especially resistivity logs, has become an increasingly complex problem. The proximity of shale layers or of water legs can seriously affect deep resistivity logs, and invasion can strongly affect shallow resistivity logs. The current state of affairs is that determining Rt in a horizontal or very high angle well is often impossible.
Modeling techniques are now available for solving the full 3D problem necessary for deviated well interpretation. This paper describes a 3D modeling code and applies it to improve the interpretation of multiarray induction tool response. The code uses the Lanczos spectral-decomposition method to solve Maxwell's equations on a staggered finite-difference grid. The finite-difference code has been benchmarked against analytical solutions for subsets of the 3D geometry, and agreement is within three percent. Fifty ft of 3D log can be generated in under half an hour on a  modern workstation or high-end PC. The code takes into account dipping beds and unsymmetrical invasion at the same time, as well as resistivity anisotropy.
Several horizontal well interpretation problems are investigated with the code. One is the case of axisymmetric cylindrical invasion in a permeable zone below a cap shale interface. In this case modeling shows that for shallow invasion, the deepest Array Induction Imager tool (AIT) curves can be used to infer Rt and proximity to the shale cap, while the shallowest curve indicates Rxo. If deeper invasion is modeled, only the deepest induction curve indicates Rt, while several of the shallow curves read Rxo.
The code is also used to analyze non-circular invasion fronts caused by either permeability anisotropy or buoyancy segregation typical in highly deviated wells. Both cases are characterized by a considerable quantity of filtrate shunted away from the well in preferential directions, resulting in less invading fluid near the wellbore. As a consequence, there is an increase of the influence of Rt on the shallow AIT logs. These cases indicate that induction logs in complex formations still have geometrical interpretations, but that they are different than interpretations used in vertical wells.
A log example illustrates the power of 3D modeling in interpreting multiarray induction logs in difficult wells. In a horizontal well with moderately salty invasion, modeling shows that a large separation between the deepest induction curves is caused by a combination of invasion effects and polarization horns near a cap shale. In addition, an annulus is present to complicate
interpretation.