A Novel Approach to Estimate Rock Strength Parameters from Multistage Triaxial Tests (The A-HAS Method)

Abstract

Rock strength parameters are vital to creating robust geomechanical models for wellbore stability, sand production, and hydraulic fracturing. Static rock strength parameters can be estimated from single stage triaxial tests (SST) or multistage triaxial tests (MST). Although SST tests are the most accurate way to estimate rock strength parameters, SST tests require at least four to five core plugs to estimate rock strength parameters. Thus, MST tests are used as an alternative method to estimate rock strength parameters from only one core plug to overcome the limitation of materials and save time and money. However, the currently used method to estimate rock strength parameters is the yield-based only method which can significantly underestimate rock strength (especially unconfined compressive strength (UCS)) and can hinder creating a robust geomechanical model. In this work, the authors proposed a novel method called the A-HAS method which is a correction to the yield-based method that is currently used. Using core plugs from three different zones (Nahr Umr shale, Zubair shale, and Zubair sandstone in the Rumaila field, Iraq), SST and MST tests were executed. Rock strength parameters were estimated based on SST tests for each zone (Base Case). Then, rock strength parameters were estimated based on four different methods; the currently used yield-based only method (Case 1) from MST, corrected maximum compressive strength (MCS) (Case 2 or the A-HAS method) from MST, and Cases 3 and 4 which are based on corrections from both SST and MST tests. The results have shown that regardless of the method used, coefficient of friction (μi) and internal friction angle (ɸ) were not significantly affected while UCS and cohesive strength (So) were highly affected. Comparing UCS and So results of the yield-based method (Case 1) and the A-HAS method (Case 2) to the Base Case, the A-HAS method has significantly outperformed the yield-based method in all three zones tested. The A-HAS method has resulted in errors of 4%, 3%, and 2% for Nahr Umr shale, Zubair shale, and Zubair sandstone, respectively while the yield-based method resulted in errors of 41%, 31%, 83% for Nahr Umr shale, Zubair shale, and Zubair sandstone, respectively. The error of the yield-based method is very significant and may hinder creating robust geomechanical models for wellbore stability, sand production, hydraulic fracturing, and many other applications. The authors recommend utilizing the novel A-HAS method to obtain rock strength parameters (especially UCS and So) from MST tests and not utilize the currently used yield-based method since the A-HAS has proven to be effective and produced better results in the three tested zones than the yield-based method. By using the A-HAS method, the limitations of the MST tests can be overcome and reliable results from MST tests can be achieved which will contribute to saving time, money, and material.

Department(s)

Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

MST; Rock Strength Parameters; SST; UCS

International Standard Serial Number (ISSN)

0920-4105

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2022 Elsevier, All rights reserved.

Publication Date

01 Aug 2022

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