Description and Quantification of Vugs and Fractures in Carbonate Reservoirs from Two Wells in Permian Basin, West Texas: Field Testing of a New Wireline Borehole Imager and New Interpretation Software

D. V. (Vivek) Chitale¹, John Quirein¹, Tegwyn Perkins¹, Ron Stamm¹, Lee Sanders²
¹Halliburton Energy Services, Houston, USA
²Halliburton Energy Services, Midland, USA

Borehole imaging technology is well suited to describe and document the fabric of those carbonate reservoirs in which vugs and fractures constitute the predominant pore types available for the storage of gas, oil and water. Understanding the distribution of vugs and fractures in such reservoirs greatly helps achieve accurate fluid flow modeling and improve the reservoir recoverability. Also, accurate estimation of the secondary porosity can greatly enhance the accuracy of reserve estimation in such carbonate reservoirs.

Carbonate reservoirs are usually drilled with saline mud. In such borehole environments, the Rt:Rm ratio can easily reach as high as a million. Under these conditions, the older electrical images often were not of desired quality. Now, however, a new electrical wireline borehole imaging tool has been designed to obtain superior quality images in environments where Rt >> 5000 ohm-m and borehole fluids are extremely salty (Rm << 0.1 ohm-m). The expanded operating range of the new tool (called the X-tended range micro-imager or XRMI ™) over the previous generation electrical imager is due to a greatly increased signal-to-noise ratio and dynamic range. These increases are achieved through a new, 32-bit state-of-the-art architecture for digital signal acquisition combined with a large increase in available power for the excitation current.

Manual analysis of fractures and vugs revealed by the new tool’s borehole images is both subjective and time intensive. Analysts need a more consistent, fast, computer assisted approach. Current state-of-the-art lacks a technique that uses the object attributes to automatically resolve vugs and fractures in the presence of other features of non-interest, such as bedding. The new technique described herein is an interactive image sculpturing in which objects of interest (vugs and fractures with similar resistivity) are defined using simple thresholding. The objects of no interest are then removed using filtering- and thresholding techniques, thereby producing a “sculpture image’ containing only vugs and fractures. Quantitative interpretation is presented as continuous depth plots of aperture, length, and density of fractures and secondary porosity due to vugs and fractures.

This paper shows examples of the new tool-acquired borehole images of carbonate reservoirs from two wells in the Permian Basin — one a test well and the other a working oil well. They demonstrate the ability of the new imager to enable description and quantification of vugs and fractures in limestone and dolomite in boreholes characterized by high Rt:Rm ratios. The theory and workflows used in the new image interpretation technique are also presented. Further, the paper presents the sculptured images of fractured and vugular carbonates and also the results of determinations of the aperture-, length- and density of fractures and total secondary porosity. As shown in the paper, the new tool’s image log and the results of the new interpretation software provide important insight into the unique and vital geological information required for effective and efficient management of the carbonate reservoirs.

Figure 1 - Progressive sculpturing of a borehole image (left to right) to generate a map of fractures and vugs