Mechanical Specific Energy (MSE) in Coring: A Tool to Understand the Drilling Mechanism and Coring Parameters Optimization for Improved Core Recovery

Abstract

The Mechanical Specific Energy (MSE) is not so well-known in coring as compared to conventional drilling. MSE for the full face-bit was first defined by Teale (1965) as an amount to energy spent to remove unit volume of rock. Pessier and Fear (1992) introduced MSE in its expanded form in O&G industry. At present MSE has been used widely to understand the mechanism of drilling, evaluate efficient drilling, and diagnose the root cause of in-efficiency. MSE is also used real-time for drilling performance evaluation (Dupriest et al., 2005; Pessier et al., 2016). These processes have saved Billions of dollars in the O&G industry. However, the MSE concept has not been transferred to coring operations. Current work examines the use of MSE and its adaptation in coring processes. Limited data published were reviewed, re-analyzed, and finally compared with field example of MSE in coring thereby explaining the mechanism of coring, its usefulness in getting a better recovery, and the best bore-hole quality. The MSE for coring can be expressed as: MSE = (W/A) + 2pi N.T / (A.R). Where, W, the weight-on-bit, and T, the torque are available from drilling rig through some mechanical loss. The rate-of-penetration is R, number of core-bit revolutions per minute is N and the core-bit kerf area (A) is given by coring diameter (OD-ID). The unit of MSE in metric unit is MPa or psi in imperial unit. The limited published data obtained from laboratory based coring do not give the clear picture of coring operation. The re-processed data and a careful analysis shows (Fig. 1) that the depth-of-cut, DOC, is a better indicator of R and N, higher DOC results in lower MSE; stronger rocks ends up having higher MSE, and efficient coring zones could easily be identified. A similar and consistent result is obtained in the present work. The coring operation was conducted in a test well in Oklahoma, the rock types encounterd were sand, shale and the basement granite, Fig 2. This paper will discuss the coring operations results in details. The role of axial and rotational energy will be analyzed and their influence on rock properties will be discussed. The efficient coring zone of linear weight-on-bit with DOC and torque with DOC will be presented and anomaly due to balling or undue vibrations will be discussed. Finally, the preliminary results shows that the axial energy is proportional to hardness and rotational energy (nearly equal to MSE) is proportional to confined compressive strength.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018