Research and Application of Rock Mechanics Evaluation Method for Drill Cuttings in Complex Ultra-Deep Well
Authors: Chang Long , Wang Haige , Zhuo Lubin , Hao Chen 
Published: August 2024

Based on the automatic mineral analysis technology of drill cuttings, the evaluation of rock mechanical parameters of drill cuttings in complex ultra-deep wells is carried out, a rock physical model considering formation composition and structural characteristics is established, forming a new method for evaluating rock mechanical parameters based on cuttings measurement while drilling. The evaluation of rock mechanical parameters of complex ultra-deep drill cuttings has been completed for 8 wells. Through parameter calibration, the rock physical model has been improved, and the coincidence rate of rock mechanical parameters interpretation is over 90%. Through the profile construction of key rock mechanics parameters of cuttings while drilling in the well site, the formation pressure analysis while drilling was carried out, and realized dynamic optimization of complex ultra deep well engineering schemes based on rock cuttings measurement while drilling, thus ensuring the safe drilling of complex and ultra-deep wells.

1. INTRODUCTION

The evaluation of rock mechanics parameters is the basis of drilling rock fragmentation, formation pressure monitoring, wellbore stability evaluation and reservoir fracturing reconstruction. The conventional method is to determine the formation mechanical parameters such as Young's modulus, Poisson's ratio and compressive strength from laboratory testing of standard core samples (25 mm diameter and 50 mm length) (Brown, 1981; Jaeger et al., 2009). In order to solve the contradiction of low core data and high cost, acoustic logging data and other logging data are used to determine the formation mechanical properties indirectly, and the mechanical parameters of rock are evaluated by establishing the correlation formula between core test and acoustic data. However, in the new exploration area with less well logging data, this method is also limited.

Drill cuttings represent a large source of information which is available without any additional cost. If used on the rig, information will be available close to real time (Fjaer et al., 2008). There are quite a number of research papers aimed at using cuttings to characterize reservoir rocks. Santarelli et al. (1998) reviewed most of them and demonstrated the feasibility of obtaining representative values of P- and S-wave velocities, rock strength and deformability, permeability, porosity, density, residual fluid content, and saturation. Khodaei et al. (2022) divide all methods into two parts: direct measurements on cuttings and indirect computations based on digital images of the cuttings. Nes et al. (1998) presented a continuous wave technique for measurement of acoustic phase velocities on cuttings. The equipment is particularly well suited for testing of small samples, even on sub-mm-thick shale cuttings. Zausa and Santarelli (1995) devised the new method of indentation technique to determine the characterization of the mechanical properties of measurements on cuttings. Excellent correlation between uniaxial compressive strength and indentation number has been established (Santarelli et al., 1998). Sohail et al. (2023) investigated techniques for estimating hardness and Young's modulus using drill cuttings. The microhardness was measured on shale cuttings of ∼1 mm in size using a micro indenter. Cuttings provide a source of compositional and textural rock data that can be quantitatively measured to train standard petrophysical analysis in order to implement rock physics equations (Dvorkin et al., 2003; Mavko et al., 2020; Kadyrov et al., 2022). Ashton et al. (2013), Oliver et al. (2013) and Swami et al. (2017) measured mineral composition data and high resolution textural information through the use of automated mineralogical instruments (RoqSCAN) to predict elastic properties throughout lateral wells and thus improving frac placements.