Matrix-Adjusted Shale Porosity Measured in Horizontal Wells

The determination of accurate density porosity requires an accurate matrix density. This determination is challenging in organic-rich shale using downhole logs because of the presence of insoluble sedimentary organic matter ("kerogen") that is part of the solid matrix but has log-response characteristics more similar to those of pore uids. Methods other than logs used to determine shale matrix density from drill core or cuttings, such as gas pycnometry, rely on remote laboratory services and may not be representative due to microcracks. This study describes a novel approach to quantify porosity in organic-rich shale by the integration of fast wellsite measurements of cuttings and a bulk-density log. The cuttings analysis uses diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which provides an accurate estimate of matrix density explicitly including kerogen as part of the matrix. Matrix density is then used to estimate porosity per the well-known density-porosity relationship. The method is enabled because the DRIFTS analysis explicitly solves for both minerals and kerogen as components of the shale matrix, separate from that of pore uids. It is demonstrated that kerogen density is a critical parameter in the determination of matrix density, and its determination is an integral part of the DRIFTS interpretation. Kerogen density is not measurable using traditional logging methods and is otherwise only obtainable using time-consuming laboratory procedures. The DRIFTS technique is advantageous because it requires minimal sample preparation and footprint, can be run at the wellsite, is rapid enough to keep pace with typical drilling rates, and can be performed in oil- and water-based muds. The method can be run in horizontal wells because cuttings are always available, and because the combined depth-resolution of the cuttings and the logs is typically much finer than the lateral interval over which formation properties vary. The integration of cuttings and logs provides a depth-by-depth estimate of shale porosity at the wellsite that is otherwise not obtainable from either basic logs or cuttings individually.