This paper presents the findings of a joint research project studying the relationships between rock acoustic properties and shaped charge penetration. The project was based on results from API project 89-36 in which a relationship between compressional wave velocity and penetration depth was found to be valid regardless of overburden stress. Two new charges were used in this study and the scope was expanded to include testing in three sandstones (dominantly quartz), three limestones (dominantly calcite), and a quartzite. The target properties studies were also expanded by measuring shear velocity (Vs) as well as compressional velocity (Vp). Penetration was correlated with a range of calculated parameters available from sonic and density logs such as bulk modulus, compressional impedance, and shear impedance. Well logs from a number of actual wells were reviewed to determine the values of these parameters for typical reservoir rocks.
Shaped charge penetration can be correlated and predicted in situ using certain acoustic rock properties. The effects of the target stress can be accounted for by the corresponding change in the rock’s acoustic properties. Penetration decreases with increasing values of all parameters studied, but on to a plateau level. Further increases in the acoustic parameters above these critical values, due either to using different target materials or to increasing stress, cause little further penetration decrease. The best correlation appears to be with bulk modulus. Many of the actual wells studied have values of bulk modulus above this critical level, and thus penetration depths for the studied charges should be equal to the plateau value in many reservoirs. It is speculated that this minimum penetration corresponds to the hydrodynamic penetration by the supersonic portion of the jet.
These findings will allow more accurate estimates of the downhole performance by using the acoustic and density logs from the well. Downhole penetrations can be estimated without regard for rock compressive strength, stress, or overburden. Given only a curve for penetration of the specified charge vs. an acoustic parameter, in situ penetration can be estimated in the actual formation. It is anticipated that the theoretical models of shaped charge penetration can be improved using these observations.