The rate of penetration (ROP) plays a major role in drilling optimization, making it an important area of investigation. In this work, sensitivity and statistical analysis were carried out using data from over 100 wells in the Rumaila field, Iraq. The goal is to assess the effect of mud weight (MW) on ROP, to provide a method for estimating the recommended range for operational mud weight based on the hardness of the rock.

Drilling intervals from data collected from drilling reports from over 100 wells in the Rumaila field, Iraq, were categorized as weak, medium, hard, and very hard, based on UCS, depth, and lithology. Statistical and sensitivity analysis was conducted, developing correlation coefficients (CC) to represent relationships between rock hardness, ROP, and MW from field data. This methodology combines data from mud logging, daily drilling reports, and geological information to investigate the relationship between ROP and MW for different rock types and strengths.

It is well known that MW can dramatically impact ROP, and well documented in literature and laboratory studies. Several early studies focused directly on MW, clearly demonstrating the effect of MW on bit conditions, and therefore ROP. However, these studies do not include consideration of the hardness or lithology of the rock. There is also a strong discrepancy between results from different studies. Different literature concludes the relationship between MW and ROP is inverse, and in some it is direct.

For this study, field data have been gathered (more than 100 wells) to further investigate the relationship between MW and ROP, and how it varies with rock hardness. Field data have been categorized based on the hardness of the rock. The results showed that MW has an inverse relationship with ROP for the weak formation with a CC of -0.57. Thus, MW should be maintained as minimum as possible when drilling through the weak formation. On the other hand, MW has a direct relationship with ROP for medium, hard, and very hard formations with CC of 0.31, 0.4, and 0.42 respectively. Hence, MW has to be maintained as high as possible to maximize ROP for medium, hard, and very hard formations.

Large-scale collection and interpretation of field data were collected to demonstrate the effects of MW on ROP with varying rock hardness and lithology. While field data of this scale is inherent of interest, this analysis also investigates relationships previously unexplored and extends understanding of how MW effects ROP.

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