Reduction of Jet Perforator Penetration in Rock Under Stress

Abstract

ABSTRACT Tests using multi-shot perforating guns in large 3/4 × 3/4 × 9/10 m blocks of stressed rock have been used to quantify stress-induced penetration reductions for three different charges. All three charges were tested in Berea Sandstone, one charge was tested in Bedford Limestone. The fractional reduction in penetration depends on charge design, size and rock type. In tests with elevated pore pressure, data best fit an effective stress law, P(effective) = P(confining) – P(pore). Larger fractional penetration reductions were observed for a small charge (10-cm rock penetration). This may be partially explained by correlating penetration with hoop stress near the wellbore rather than with mean effective stress. Two methods are considered to predict the observed penetration reductions. First, changes in acoustic velocities with pressure are correlated directly with penetration depths. Thus, log or core data may be useful in predicting downhole penetration. Second, a penetration model that includes the effect of target strength under confining stress is evaluated. The latter theoretical analysis results in a charge-dependent semi-empirical parameter related to impact pressure. The value of this parameter is less for the smaller Charge A than for Charge B. The ability of this simple theory to reproduce observed penetration reductions suggests that a more sophisticated treatment may be able to predict downhole perforator performance directly from the characteristics of a given charge.