Review and Improvement of Brittleness Evaluation Methods in Shale Gas Reservoir
Li, Qinghui; Chen, Mian; Jin, Yan; Hou, Bing; Wang, Mingyuan
Brittleness, a key parameter to evaluating the mechanical properties of gas shale, affects the drilling and hydraulic fracturing work. It's of vital significance to testing and evaluating the brittleness of shale.
In view of this, this paper investigated and summarized 20 basic methods for testing brittleness, including methods based on strength, hardness and ruggedness, focusing on testing theory and steps based on stress-strain tests. Testing system MTS 815 is used to carry out triaxial compression tests for specimens from Barnett, Haynesville, Eagle Ford in North American, and Longmaxi shale in South China. Their failure modes and mechanical behaviors under different confining pressures are obtained correspondingly. Considering fracture mechanism, brittleness of gas shale was defined as a synthetic characteristic, affecting by its heterogeneity and external testing environment. To identify brittleness of shale, overall stress-strain curve consisted of pre-peak and post-peak parts should both be considered. To improve the accuracy, stress-strain tests should be conducted simulating the underground environment. Capacity for resisting elastic deformation before rupture and losing rate of bearing capacity after rupture were the main mechanical performance of brittleness.
To improve accuracy of evaluating, modified schemes were also presented in this paper. This improved method using an integrated index combing the pre-peak stain and the drop speed of post-peak stress to evaluated the brittleness. Meanwhile, brittleness of black shale from South China was tested through true triaxial tests and then evaluated using improved methods. The new method for evaluating brittleness is based on brittle failure of standard specimens and the results of this paper can be used in rock property evaluation and stimulation style designing directly.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013