Spiral-Hole Moderation Enhances Borehole-Image Interpretation in Carbonates
Authors: Carpenter Chris 
Published: December 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23355, “Spiral-Hole Moderation Enhances High-Resolution Borehole-Image Interpretation Across Fractured Carbonate Reservoirs,” by Alawi Altarouti, Abdullah Alotaibi, SPE, and Prudencio Guerreiro, Saudi Aramco, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference.

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Dynamic phenomena while drilling such as borehole spiraling or corkscrewing can create a physical imprint on the borehole surface, which masks fine features of imaging. During a drilling campaign in a carbonate reservoir using logging-while-drilling (LWD) technology, corkscrewing affected ultrahigh-resolution borehole image quality severely. The complete paper presents the methodology and results of the bit-design test to mitigate the presence of borehole spiraling, resulting in a significantly improved ultrahigh-resolution borehole image.

Introduction

A recent drilling campaign in an offshore carbonate reservoir in horizontal wellbores required the use of resistivity-based LWD ultrahigh-resolution images (UHRI) with rotary steerable systems (RSS) to evaluate the presence of natural fractures. During LWD logging of the targeted carbonate reservoirs using water-based mud, borehole artifacts were observed as spirals around the borehole. These artifacts had an adverse effect on the borehole-log analysis and interpretation and evaluation of natural fractures, particularly partial fractures that could be masked. In some cases, it was estimated that more than 30% of the fractures could not be identified. Comparing log results of LWD with historical wireline logs run in the same formation and similar well profiles across the field indicated that the artificial spirals only were present when using LWD tools to image the wells.

More commonly, borehole spiraling has been observed on a macro scale, wherein the spiraling can be measured in feet. The spiraling in this well was detected only with high-resolution LWD laterolog imaging and affected fracture interpretation. The spiraling remained undetected, below resolution, on the conventional LWD measurements.

Methodology

Technical Analysis.

Spiraling occurred primarily while drilling across relatively soft formations with bulk density values of approximately 2.15 to 2.20 g/cm3. Intervals with higher densities were affected to a lesser extent or not at all. The spiraling occurred in a consistent manner.

While downhole shocks and vibration were considered to be a possible cause of spiraling, the periodic occurrence of the traces did not align with the chaotic nature of shocks and vibration. The shape of the traces did not match with possible lateral or axial motions of the bottomhole assemblies (BHAs); however, these spirals may have been caused and aggravated by low-frequency torsional vibrations. Although the level of the stick and slips was found to be medium to high throughout the runs, no evidence supported this theory. The wide range of downhole rotational speed fluctuations could not explain the perfect shape of the spirals.

It also was observed that the distance between each trace or spiral correlated with the rate of penetration (ROP) and had constant values in similar ROP ranges. It could have been suspected that the spiraling did not occur across the hard formations because of lower ROP and the overlapping between them as the result of slow drilling. However, the ROP reduced marginally compared with the drilling in the soft formations (approximately only 10%). Thus, ROP was considered one of the lowest-affecting parameters to mitigate spiraling effects.