Examination of the Relationship between Rate of Penetration and Mud Weight Based on Unconfined Compressive Strength of the Rock


The rate of penetration (ROP) plays a major role in drilling optimization, which makes it an important area of investigation. In this work, sensitivity and statistical analysis were carried out using a big set of data gathered from over 800 wells in the Basra oil fields, Iraq. The main objective of this investigation is to assess the effect of mud weight (MW) on ROP and to provide a method for estimating the recommended range for operational mud weight based on the rock hardness.

The collected data for the drilling intervals from the drilling reports for over 800 wells in the Basra oil fields, Iraq, were categorized as weak, medium, hard, and very hard, based on the unconfined compressive strength (UCS), depth, and lithology. Statistical and sensitivity analyses were conducted and correlation coefficients (CC) were developed to describe 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.

Several previous studies focused directly on MW, where the effect of MW on bit conditions, and thus the ROP was clearly demonstrated. However, these studies did not take into consideration the rock hardness or lithology. There is also a strong discrepancy between results from different studies, where some studies showed an inverse relationship between MW and ROP, while other studies showed it was a direct relationship

In this study, field data have been gathered (more than 800 wells) to further investigate the relationship between MW and ROP, and how it varies with rock hardness. The results showed that MW has an inverse relationship with ROP for the weak formation with a CC of -0.57, suggesting that the MW should be maintained as minimum as possible when drilling through the weak formation. On the other hand, MW showed 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.


Geosciences and Geological and Petroleum Engineering


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Keywords and Phrases

Basra oil fields; Iraq; Mud weight; Rate of penetration; UCS

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Article - Journal

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Publication Date

01 Oct 2019