Density Measurement Sensitivity To Varying Borehole Angle And Tool Azimuth In Medium To Thin Beds (2007)

ABSTRACT We report on a Monte Carlo radiation transport simulation study that assesses the effect on the estimated bulk density due to variation of three key parameters in horizontal/ high-angle (HA/HZ) wells. These parameters are 1) the hole-angle, 2) bed thickness and 3) tool azimuth as the tool traverses beds of varying thickness. The major findings were, 1) significant errors can result from using standard compensation in thin beds, 2) density correction decreases at higher well-bore inclination, 3) benefit from enhanced or so-called alpha processing decreases as bore-hole angle increases and may even add to errors, 4) the tool azimuth angle has a significant effect in bed boundary detection, but a lesser effect on resolution, and 5) the density log response can see a non-symmetric volume based on whether one exits or enters lower density bed. Comparison with field data from Africa indicated a clear presence of the effects seen from modeling. INTRODUCTION As the number of high-angle/horizontal (HA/HZ) wells drilled increase, accurate estimation of porosity using nuclear techniques has been of considerable concern. Recent field examples (Renderio et al., 2005, Radtke et al., 2006, Badruzzaman et al., 2007) have shown that the estimated porosity in typical reservoir-quality thin-bedded formations may be 3-4 porosity units (p.u.) higher than would be from vertical wells. Such uncertainty can impact reserves estimates and the quality of geosteering. Badruzzaman et al., 2007 also reported an example where the density obtained in the presence of medium to thin beds through HA/HZ wells agreed well with core data while density through vertical wells did not. Badruzzaman et. al., 2007, also discuss a number of issues that may affect the accuracy of porosity estimated though HA/HZ well in medium to thin beds. These include highlights of the effect of hole-angle variation on estimated density described in detail in the present paper, LWD tool placement relative to interfaces of thin beds, variable standoff in LWD, and gas absorption by certain muds which can result in a dynamic variation of the properties of the invasion fluid and hence of the tool response. Density measurements through HA/HZ wells, especially in reservoirs with thin beds have been of particular interest recently, as we log more wells with LWD density only to estimate the bulk density and construct images. In vertical wells, source-to-detector (S-D) spacing governs the limits of a tool to resolve thin (horizontal) beds. This has led to use of the short-detector response to compensate or “alpha-process” in order to improve the accuracy of the long-spaced detector density (in a dual-detector tool) to obtain the final density. In horizontal wells, the resolution of such beds will be governed by the depth of investigation (DOI) of the detector. At angles in-between it will be a combination of the two, thus raising issues about the adequacy of usual density compensation/correction procedures in HA/HZ wells. Previous analysis in such wells included, 1) a study of hole-angle effects in 16” laminations (Mendoza et al., 2006), 2) effect of density contrast across interfaces of contrasting beds (Yin et al., 2006), and 3) effects in thin beds at two end points defined by a vertical and a horizontal well (Radtke et al., 2006). In this paper we study the effect of three key parameters in HA/HZ wells, 1) hole-inclination, 2) bed thickness and 3) tool azimuth, on measured density. The bed thicknesses studied were 8, 6, 4 and 2 inches. We utilize three-dimensional Monte Carlo radiation transport code, MCNP, from Los Alamos (Edited by Briesmeister, 2003) to compute detector fluxes of a model dual-detector density tool in these conditions. We construct long-spaced (LS) and short-spaced (SS) densities from the fluxes utilizing usual procedures. We then construct a compensated bulk density by using the spine-and-ribs approach. We also compute the so-called enhanced or alpha-processed density. We examine two alternative density correction procedures, based on using the SS density. We then compare the conclusions of the modeling study with field data from a high-angle/horizontal well with thin beds.