Correlation Between Drilling Parameters and Geomechanical Properties - The Hidden Geomechanical Information

Abstract

The drilling process is fundamentally controlled by the interaction of the bit with the rock which is characterized by its mechanical properties. The focus of various studies is directed mainly on the optimal design of drill bits and drilling operations related to the (expected) geological situation for a safe and efficient drilling process. The question arises: "Is it possible to extract any rock properties information from drilling data?". In a previous paper the correlation of drilling properties with the lithology of penetrated formation was evaluated. A clear correlation could be demonstrated by applying different statistical methods. Based on these results a simplified model for the rock destruction at the bit is developed and a "formation strength parameter S" is defined. As a cut-force parameter in the equation, weight on bit (WOB) is used primarily for vertical sections. For deviated or horizontal sections the differential pressure Delta-p is a more relevant parameter. A new parameter S is calculated as a function of drilling parameters only (RPM, WOB, Delta-p, Bit-size). This new parameter S is compared with existing parameters for rock characterization (Mechanical Specific Energy) and discussed herein. For four wells this parameter S is plotted as function of the depth representing a first geomechanical model. The main results are: Well A: Geomechanical characterization of different clastic and carbonate sections of a vertical well. Clastic (sandstone, shale) and carbonate (limestone, dolomite) are clearly separated. The carbonate section is detailed and subdivided in dense-hard and fissured-porous parts. Well B: Calculations are made for vertical (based on WOB) and deviated/horizontal section (based on Delta-p) and compared; for the deviated/horizontal section the Delta-p version gives more reliable results. Well C: For a specific section of this well after drilling a profile of Unconfined Compression Strength (UCS) with a standard well-logging algorithm was calculated. A comparison of the "during-drilling" derived geomechanical model in terms of S and the "post-drilling" derived in terms of UCS demonstrates similar tendencies and confirms the character of S as a geomechanical measure. Well D: Data (WOB, ROP, RPM) for this example are logged as part of a test of the new PDS Digital Drilling technology (Powerline Drillstring Technology) in an igneous formation (phyllite). It demonstrates the ability to discriminate different geomechanical sections of one formation.

Exposure Time Impact on the Geomechanical Characteristics of Sandstone Formation during Horizontal Drilling

“…The experiments were run for different exposure times between the mud and the rock samples (30 min, 1 day, and 2 days) under 300 psi differential pressure and 250 • F. The results showed that UCS decreased as the exposure time increased for limestone samples. Lamik et al [35] presented a new rock strength parameter that can be derived from the drilling parameters while drilling or from the sonic slowness log. The parameter is very sensitive to the lithology and helped to identify the rock type and formations' boundary.…”