Rotary Sidewall Cores- A Cost Effective Means of Determining Young's Modulus

Abstract

Abstract The objective of hydraulic fracturing is to design and execute a fracture stimulation treatment that achieves the desired fracture dimensions (length and conductivity) to maximize a wells production rate and reserve recovery. To achieve this objective, there are several critical parameters to the process, and these fall into two distinct categories:parameters over which we have little control, but need to understand, andthose that we control, but have lesser impact on the process. The first category includes fracture height, fluid loss coefficient, tip effects, and Young's modulus. The second category includes pump rate and fluid viscosity. Of the former parameters, Young's Modulus is the only variable that can be measured, in advance, via lab tests. Traditionally, Young's Modulus is measured through stress-strain testing of geologic samples (core plugs) which always demanded an L/d (Length/ diameter) ration of at least 2:1. The reason for this criterion is that the ultimate failure mechanism for most rocks under compression loads is the formation of a shear fracture. For most rock types, this shear fracture will form at an angle of about 30° from the axis of the maximum compression load. Thus, a 2:1 L/d ratio allows a through-going shear fracture to form for a failure angle of 30°. For stress-strain testing NOT concerned with ultimate failure of the sample, this valid criterion has always been followed - arbitrarily and artificially. Unfortunately, this sample criterion generally eliminates the use of sidewall cores. This paper details and documents an evaluation of the Length to diameter criteria through finite element modeling, tri-axial compression testing of aluminum, and compression testing of actual sedimentary rock samples. Through this work, it is evident that core samples of L/d significantly less than 2:1 can provide reliable values of static Young's Modulus. Further, these results indicate that rotary sidewall cores can be utilized to determine Young's Modulus in many applications provided adequate sample quality assurance is undertaken to ensure sample integrity. Introduction Determination of mechanical rock properties is important to the oil and gas industry for reservoir compaction, borehole stability, formation control, and hydraulic fracturing. Measurements of the elastic rock properties have historically been conducted on whole core and via wireline measurements once the wellbore has been drilled. Application of these methods at the well site in real time on drill cuttings and with Measurement While Drilling (MWD) to improve or optimize the drilling process is the focus of much ongoing research1–5. Studies by Ringstad et al2, Zausa et al3, and Santarelli et al4 evaluated the use of drill cuttings for mechanical properties determination. Because of this, these studies focused on sample size sensitivities and determined that the micro indentation measurements correlated well to Uniaxial Compressive Strength of the rock. However, these measurements correlated poorly with Young's Modulus or porosity. Similarly, Nes et al5 investigated sample size sensitivities on both the static and dynamic behavior of the Pierre Shale. This study evaluated the elastic properties of 0.39 inch diameter samples 0.16 inches in length (L/d = 0.4) and found that the static Young's Moduli of the "hard" shale samples was in excellent agreement (i.e. within 3.5 %) with larger sized core plugs. The static Young's Moduli of the soft shale samples tested were in much poorer agreement (i.e. nearly a 20 % error) as compared to larger sized samples tested. Finally, this work showed excellent agreement (i.e. +/− 2 %) between the dynamic moduli determined with a Continuous Wave Technique (CWT used on smaller samples) and a Pulse Transmission Technique (used on larger samples).