A Laboratory Study of Perforations in Stressed Formation Rocks
Authors: Saucier R.J. , Lands J.F. 
Published: December 2007

Laboratory experiments are described in which the penetration and flow performance of a shaped-charge perforator are evaluated in three rocks performance of a shaped-charge perforator are evaluated in three rocks subjected to simulated overburden and pore pressure stresses. This paper shows that jet perforator penetration in two rocks decreases as stress increases. Penetration into the various rocks is not related directly to their compressive strengths, as formerly was believed.

Introduction
API RP-43 outlines a widely used test method for comparing the performance of perforating guns. These tests involve firing the perforators into Berea sandstone cores under controlled conditions and measuring the resulting flow characteristics and perforation geometry. Some effort is made to simulate wellbore conditions; however, rock stress is not controlled.
Previous experimental and theoretical studies attempted to predict actual perforator performance in a wellbore, but again no measurements were made with target cores subjected to realistic stresses. As well depths have increased, interest has grown in the performance of perforations made down hole under increasing perforations made down hole under increasing overburden stress.
This study documents the effects of formation stresses on shaped-charge penetration and perforated-target flow performance. We describe test conditions and physical performance. We describe test conditions and physical models used and present photographs of typical results. Influence of the test variables on jet penetration and target flow are evaluated and compared with typical API data. We discuss the phenomenon of rock damage around a perforation and in effect on permeability. Finally, we perforation and in effect on permeability. Finally, we outline a procedure for applying these results and suggest directions for further testing.

Test Equipment
All tests were performed in a 14-in.-ID pressure vessel capable of 25,000 psi and temperatures up to 400 deg. F, equipped to allow all the necessary controls of pressures and fluid flows. This was the largest available vessel with the needed capability, but it was not large enough to permit testing standard-size API targets. Consequently, a permit testing standard-size API targets. Consequently, a new "down-hole model" target was designed, similar to but smaller than the standard API RP-43 target, and capable of being stressed by hydraulic pressure (Fig. 1). A hardened steel faceplate (1/2-in. thick) simulated N-80 casing strength and thickness. A 1 1/2-in. plug of high-strength Class G cement simulated wellbore cement. The formation was represented by a cylindrical target of formation material (Fig. 1).
Overburden pressures were simulated by hydrostatic pressure in the annulus between the core and canister. pressure in the annulus between the core and canister. Pore fluids and pressures were supplied through the rear Pore fluids and pressures were supplied through the rear of the core and were separated from overburden fluid by a thin layer of Teflon shrink tubing surrounding the core and its end cap. A clamshell fixture enclosed the target and served as a mount for a single-shot perforating gun (Fig. 1).
To provide a basis for normalizing the down-hole model (Fig. 1) test results with those of API test targets (Fig. 2a), two additional target configurations (shown in Figs. 2b and 2c) were designed and incorporated in the test program.
JPT
P. 1347