Thick Wall Cylinder Strength and Critical Strain Limit From Core Tests and Well Logs, Implications for Sand Control Decisions

Thick Wall Cylinder (TWC) or Hollow Cylinder (HC) core tests are routinely used in analytical and numerical sanding evaluation required for sand control and well completion decisions through the identification of sand production risks, quantification of sanding rate and also as a scale model of wellbore or perforation stabilities. In these tests a hollow cylinder is loaded under increasing hydrostatic stress until collapse occurs in the walls of the cylinder. The hydrostatic stress at which failure initiates in the internal wall is reported as the TWC-Internal and the stress that causes external wall failure is called TWC External or TWC collapse. Numerical modeling of rock failure and in particular some sand production prediction methodologies require the stress condition corresponding to the initiation of internal wall failure to define the critical strain limit (CSL) beyond which rock failure and sanding is expected under the wellbore flowing condition. Unlike the uniaxial compressive strength (UCS), there are very few publically available empirical equations to estimate TWC from well logs and there is no published relationship between CSL and other petrophysical and rock mechanical properties. This paper shows a series of new empirical equations between core measured TWC strength and other rock properties including porosity, UCS, and log-derived parameters such as acoustic transit times and dynamic elastic moduli for a range of reservoir rocks including both sandstones and carbonates from different lithofabrics, geological age and sedimentary basins around the globe. Several strong correlations between the numerically modelled CSL from TWC tests and measured UCS for sandstones are also presented. The applicability of these empirical equations is discussed and examples of the misuse of equations with erroneous sand control and well completion decisions are shown when typical TWC equations are used beyond their range of applicability.