API REPORT 87-36
Further Effects of Stress on Penetration and Flow Performance of Jet Perforators
|Publication Date:||7 July 1988|
The subject program has been pursued intently for the past two years through a contract with Terra Tek Geosciences Services using laboratory test targets developed, in part, by Schlumberger Well Services. Program objectives and major observations to date are as follows:
1. To develop relationships between rock penetrations in stressed reservoir rock and the proposed API Berea Sandstone "slab" (ambient stress) targets.
2. To develop relationships between flow in stressed targets and flow in the API RP 43 (fourth edition), Section II (flow test) targets.
3. To develop simple transfer functions for different formation rocks, relating perforator penetration to an independent rock measure, such as speed of sound.
Major Observations to Date
The following observations are based on 300 charges fired in 120 separate tests in both the current and previous programs. Five different charges, ranging from 3.2g explosive to 22g, have been tested in four rock types (Berea Sandstone, Bedford Limestone, Wasson Dolomite, Austin Chalk). Penetration and flow data are summarized in Figures A1 and A2.
These program observations demonstrate the importance of developing an improved understanding of perforator performance in the downhole environment. Such development will require close cooperation and interaction between the current program, API RP testing, and industry quality control developments. Failure to do this may mislead development of perforators and completion techniques and thus fail to provide optimum perforator performance for industry operators.
1. Based on 270 shots in 93 tests, mean applied effective stresses as low as 3,000 psi reduce jet perforator penetration into the rock (exclusive of casing and cement) by 10 to 40%. Such penetration reductions can significantly reduce well productivity in natural completions, can adversely affect formation evaluation testing, and may affect stimulation treatments as well.
2. Penetration reduction with stress varies with charge size and design. In particular, smaller charges appear to be more affected than larger ones. Charges selected as the best performers based on proposed RP-43 "slab" tests may not be the best performers under downhole conditions.
3. Based on 12 shots in 3 tests conducted in Berea Sandstone at the same effective stress, the combined effects of increasing well and formation pore pressures to 7,000 psi is to reduce penetration an additional 13%.
4. Based on two 7-shot tests conducted at the same mean stress, an increase in stress perpendicular to the perforation of 38% caused a 5.5% reduction in penetration. Compressive circumferential ("hoop") stresses near a wellbore may thus magnify penetration reductions, especially for smaller charges or large well diameters. This effect only partially explains the large fractional reduction seen in smaller charges.
5. Changes in ultrasonic acoustic velocity and tri-axial compressive strength with confining pressure correlate with penetration reductions. Thus, it may be possible to predict penetration based on acoustic logs or recovered well core.
6. Eight tests, using kerosene-saturated sandstone and limestone targets, show that flow efficiency of perforations decreases with increasing applied stress. In Berea Sandstone, an effective stress of 5,000 psi reduces flow to 25% of that expected (taking stress-induced matrix permeability reduction in to account), in spite of perforation cleanup by 500 psi underbalance.