Stressed Rock Penetration Depth Correlation
Downhole shaped charge depth of penetration (DoP) prediction has historically been largely determined by API Section I unstressed concrete performance. As previously reported [1, 2], reliance on unstressed concrete performance is flawed for several reasons and a review of industry models motivated a new approach based on stressed rock experiments.
As a result of a multi-year extensive experimental program, shaped charge DoP into stressed rocks is found to decrease exponentially with an empirically determined formation parameter; the ballistic indicator function, FBI. This parameter which combines formation intrinsic properties (UCS, porosity) and extrinsic properties (overburden stress and pore pressure) is described as well as its integration into an improved downhole shaped charge prediction model.
The development of the ballistic indicator function required approximately 600 single-shot experiments spanning 4 shaped charges, 10 different rocks (sandstones and carbonates), overburden stresses from 1 ksi to 20 ksi, and pore pressures from zero to 10 ksi. A recommended test program is presented which currently requires as few as 12 single-shot experiments for each charge to determine two charge-dependent parameters: the reference DoP, DoPref, and the exponential coefficient, α0. These two coefficients are sufficient for FBI<25 ksi which represents the majority of reservoirs currently being perforated world-wide. For reservoirs in excess of 25 ksi, DoP approaches a non-zero limit which is charge-dependent, and a preliminary discussion of this limit is included.
This model was initially developed based on sandstone targets. A preliminary extension of this model to carbonate targets is also presented. In general, carbonates behave differently than sandstones with less stress dependence and shallower penetration depths.