Shale and shaly formations are the most common lithologies encountered in drilling for oil and gas in the Gulf of Mexico. These formations often cause bit performance problems. This paper introduces a method relating bit performance to the cation exchange capacity of the shaly formation being drilled.

The relationship between drilling parameters, such as normalized rate of penetration and specific energy, and cation exchange capacity is investigated statistically using actual field data. The correlations have shown potential for diagnosis of ineffective drilling of PDC bits in overpressured shaly formations. Template charts are developed based on the correlations to be used for the diagnosis of ineffective drilling for PDC bits while drilling overpressured Miocene shales with water-based mud in offset wells.


Global bit balling was identified as the primary cause of ineffective PDC bit performance when drilling shales with water-based Muds(1). Global bit balling results from cohesion between shale cuttings. Agglomeration of cuttings creates a ball, which jams the space between the bit body and the bottom of the hole reducing bit efficiency. It was theorized that the origin of this phenomenon and its severity are related to shale electrochemical properties. Shale electrochemical properties can be represented by its cation exchange capacity, CEC. Ineffective drilling may be diagnosed if a correlation between drilling parameters and cation exchange capacity is determined. The correlation is based on real field data and is used to create template charts, which can be used in future wells drilled under similar conditions.

The field data used in the correlation is from a Matagorda Island (MI) area field, located in the Gulf of Mexico offshore of Texas, and operated, at the time of drilling, by Amoco Production Company. The geological section incorporated in this study is the Lower Miocene, which is overpressured, and predominantly shale and shaly sand. The measured CEC in this study is also obtained from sidewall core and cuttings from the Matagorda Island (MI) field.

Validation of Concept

This study is based on the conclusion of a previous study by Demircan, Smith, and Bassiouni(2) that the rate of penetration and normalized rate of penetration correlate adequately with the formation's CEC, which reflects the electrochemical properties of clay. In addition to the normalized rate of penetration, ROPn, the following parameters are also used to represent drilling performance:

  • specific energy; and,

  • depth of cut.

In this study, the chronological order of data points is considered.

Normalized Rate of Penetration

In order to compare and combine data from different drilled intervals and different wells, ROP has to be normalized. The normalization is performed using the following model(2).

Figure 1 shows the plot of measured CEC vs. average ROPn for Well MI622 #6 bit Run #8. Normalized rate of penetration in Figure 1 represents an average of over 30 ft corresponding to each of the 20 samples of cuttings that were used for CEC measurements.

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