Identification, Quantification And Distribution Of Clay In Thinly Bedded Reservoir Sandstones From The Integration Of Core And Log Data.

In thinly bedded formations, the vertical resolution of the logging devices does not allow for the proper resolution of the reservoir laminae. This paper outlines techniques which utilize the NMR transverse relaxation time distribution to accurately describe reservoir lamina even when the lamina thickness is less than the vertical resolution of the logging data. We present methods to quantify the volume and distribution of clay in reservoir clastics using NMR core analysis data. An adapted Thomas-Stieber method is used to partition laminar and non-laminar clay using core plug NMR measurements. The predicted clay distributions are compared with core images. Samples are categorized according to the clay distribution. NMR permeability models and free-bound fluid index are defined for each sample group. The core plug derived models can be used to calibrate the logs and formulate an enhanced thin bed interpretation. INTRODUCTION The significance of clay volume and distribution on a reservoir rock’s flow and storage properties as long been recognised. The evaluation of the effect of clay distribution on log responses using the Thomas-Stieber method are well documented (Thomas and Stieber, 1975) (Mezzatesta et al. 2006). The application of these techniques in the evaluation of core analysis data is rare. If core analysis data is to be used to calibrate logs and populate reservoir models accurately, a thorough understanding of how clay is distributed within the reservoir is required. We present methods for quantifying clay volume and distribution in core samples using standard special core analysis test data. The technique is based on the grouping of laminated and non-laminated core samples. Log calibration inputs such as permeability models and NMR T2 cutoffs are then derived for the laminated and non-laminated samples. THE THOMAS-STIEBER METHOD The clay particles can be distributed within a sand body in three ways: Laminated – layers of clay in between sand. Dispersed – clay filling the pore-space between sand grains. Structural – clay particles of similar size to the sand particle which occupy load-bearing positions in the rock. Thomas and Stieber (1975) noted that the distribution of the clay particles could be deduced by plotting porosity against shale/clay volume. In the original paper, shale volume was calculated using the gamma log response. A theoretical model envelope is plotted with boundaries defining laminated, dispersed and structural clay/shale models, Figure 1. The end-point values for 100% sand and 100% clay are selected and the point at which the pores are completely filled with dispersed clay calculated. The volumes of laminated, dispersed and structural clay can now be estimated from the plot or calculated using the model equations. In this paper, clay volume is calculated using NMR T2 distributions for core plug samples. The NMR T2 distribution for a fully brine saturated rock is analogous to the pore-size distribution of the sample. Long T2 times correspond to large pore and fast relaxation times to small pores, Figure 2. Using the saturated core sample T2 distributions total porosity, effective porosity and clay bound water volumes can be calculated. RESULTS The data set used within this paper are from a gas reservoir containing friable clastic sediments. The well was extensively cored. Core confirmed that this reservoir contained a mixture of sediments including laminated sands, high clay content areas and clean sand bodies. An extensive special core analysis program was designed. This included NMR T2 distribution for brine saturated samples and capillary pressure drainage experiments. A total of 95 special core analysis plugs were tested. Due to the friable nature of the core, plug JJ SPWLA 49th Annual Logging Symposium, May 25-28, 2008