Anisotropic Mechanical and Acoustic Characteristics of Longmaxi Shale Formation

Guangqing Zhang¹, Manika Prasad², and Henglin Yang^3
1China University of Petroleum, Beijing, China
²Colorado School of Mines, Golden, Colorado, USA
³CNPC Drilling Research Institute, Beijing, China

As the main means to improve productions of natural gas from gas formation, multi-stage fracturing of horizontal wells is becoming increasingly important. These horizontal wells are usually oriented in the direction of the minimum horizontal stress to generate multiple parallel fractures. Anisotropic characteristics of shale properties and relation between horizontal wellbore and underground stresses can influence wellbore stability and have an effect on the resulting change in productivity and fracture containment.

In this paper, outcrops from black shale of Longmaxi Group, the typical shale reservoir in China, are taken to prepare test samples to evaluate anisotropic characteristics of their mechanical properties, such as uniaxial compression, tensile strength, tri-axial compression and acoustic wave measurement.

We find that the elastic modulus and compressive strength perpendicular and parallel to the stratification are greater than that in the 45 degrees direction. And Possion’s ratios are just the opposite. The tensile strength is lowest perpendicular to the stratification, greatest parallel to the stratification, and that in the 45 degrees is in the middle. Acoustic wave travels fastest perpendicular to the stratification, slower in the 45 degrees direction and slowest parallel to the stratification. And dynamic elastic properties are in the similar trends as for the results from compressive tests. This is strange - normally moduli of the 45 degree direction lie between 0 and 90 degrees!

Shales are strongly layered rocks, with the bedding often being weak planes. And normally, measurements of shale properties show E_parallel > E_45 > E_perpendicular, Vp_parallel > Vp_45 > Vp_perpendicular. For our shale samples, there are visible calcite-filled natural fractures, almost perpendicular to the bedding, which also are weak planes(see Fig1). And analysis indicates these calcite-filled natural fractures make the mechanical and acoustic measurements different from those normally measured.

These measurements have important implications for hydraulic fracture stimulation and fracture containment, as well as for wellbore stability. The anisotropic mechanical properties will alter horizontal stresses on a layer - by - layer basis. We will present a comparison of the horizontal stresses for isotropic and anisotropic mechanical properties.

It is concluded that both stratification and natural fractures of shale compete to contribute to the anisotropic characteristics, and natural fractures of shale seem to dominate in the Longmaxi shale formation.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013