Simple Method Predicts Downhole Shaped-Charge Gun Performance

Abstract

Summary A simple method is proposed for predicting downhole shaped-charge gun performance based on the use of API RP 43, Edition 5, Sec. 1 data. API Sec. 1 has been the preferred method for assessing perforating gun system performance because of the simplicity of the test and its use of standard field guns fired at maximum shot densities and positioned as they would be in an actual well. The validity of the proposed method is demonstrated, allaying past concerns regarding the translation of data from Sec. 1 nonrock, nonstressed concrete targets to downhole conditions. The new method is based on an observed linear relationship between Edition 5, Sec. 1 and Sec. 2 penetration information. The applicability of the well-known Thompson relationship between formation compressive strength and perforator penetration to Edition 5, Sec. 2 and therefore to Sec. 1 data is shown. Incorporating necessary corrections for casing entrance hole size, downhole effective formation stress, and casing configurations different from those in the API test completes the translation of surface data to downhole conditions. Introduction Well flow performance is significantly affected by the extent of perforated hole penetration into the formation and the hole size in the casing, together with other fixed geometric parameters, such as shot density and gun phasing. Values of perforation penetration and hole size commonly available to the completion designer are provided by API RP 43, Edition 5 published data. These data are derived from tests at the surface and provide only limited simulation of subsurface conditions regarding formation physical properties and stress. Surface data can significantly vary from that to be expected downhole and must be converted to in-situ values before proceeding with well flow performance calculations. This conversion of RP 43, Edition 5 surface test performance to downhole involves consideration of the specific downhole formation physical properties, formation in-situ stress, casing properties, and the specific gun-to-casing configuration. This paper reviews the factors affecting downhole penetration and casing entrance hole size for perforating guns, discusses API data as a basis for predicting downhole performance, reviews API test results and results of tests performed specifically for this paper, and proposes a procedure for translating surface API data to downhole conditions. Factors Affecting Downhole Performance of Perforating Guns Gun-to-Casing Clearance. Clearance, the distance between the gun OD and the casing ID along the axis of the shaped-charge jet, can have a significant effect on total penetration, L, and casing entrance hole size, deh. As Fig. 1 shows, the estimated downhole L/deh of a commercial 33/8-in. gun perforating 7-in. casing varies from 14.45 in./0.35 in. to 7.65 in./0.21 in. when operated in the common eccentric running position (Fig. 1a). Values are constant when clearance is controlled (Figs. 1b and 1c), typical methods for positioning guns. The importance of the gun-to-casing arrangement is evident; it is the estimated downhole L/deh used in mathematical models to calculate well flow. Formation Strength. Compressive strength of the formation being penetrated influences perforation penetration depth. Thompson disclosed a semilog relationship between formation wet compressive strength and total penetration depth (casing thickness + cement thickness + formation penetration) for API Edition 4 Section 2 type targets. Penetration performance in unstressed sandstones and limestones of different compressive strengths is available in the literature. The high-strength end of Thompson's relationship (beyond 14,000 psi)was modified by data from Weeks' formula, resulting in the composite representation in Fig. 2. As Fig. 2 indicates, a perforating gun that provides a penetration of 11.8 in. in rock with a wet compressive strength of 7,000 psi (Point A) will penetrate less than 7 in. in a l4,000-psi formation (Point B). On the other hand, it would penetrate 15 in. in a 3,000-psi material (Point C). Use of mean wet uniaxial compressive strength, S, values is suggested in applying the above relationship. S is defined as the average of compressive strength values taken perpendicular and parallel to the bedding plane of saturated rock. It is related to the commonly used dry compressive strength measured perpendicular to the bedding plane, Sd, as follows: (1) When the value of S is unavailable, it may be approximated using formation porosity by means of Fig. 3, which is derived from the results of tests in several sandstones and limestones. These tests were performed in cores taken from surface outcrops, and results might be somewhat different in downhole formations. Data are limited below about 15% porosity for sandstones. Additional work should be done to improve the definition of the porosity/compressive strength relationship over a broader range of porosity and over a larger number of formation rocks. Nevertheless, in the range of 18% to 23% porosity, substantial and consistent data are available, providing a good curve fit. Formation Effective Stress. Formation effective stress is the overburden stress, po, minus the reservoir or pore pressure, pp: (2) where all factors are measured in psi. Stress reduces penetration (Fig. 4). Conceptually, increasing stress makes the formation appear stronger. When predicting downhole penetration performance from API test results, the effect can result in either a reduction or a gain in estimated penetration. The magnitude and nature of the effect will depend on stress in the formation compared with the stress in the RP 43, Edition 5 tests. Specifics are developed later. Hydrostatic Pressure. Although wellbore pressure tends to reduce penetration, the correlation for these hydrostatic pressure effects is included in the formation effective stress correction described above. Casing Strength. Casing grade affects perforation entrance hole diameter, deh, to a significant degree but exerts only a negligible effect on penetration across the typical API Sec. 1 test range of single casing-wall thicknesses. In single-casing completions, deh varies with the midrange Brinell hardness, H, of the particular casing grade, according to the following expression: (3) P. 171^

New Effective Stress Law for Predicting Perforation Depth at Downhole Conditions

“…The perforating community has long recognized an inverse relationship between rock stress and shaped charge penetration depth [6,7,20]. Predictive models have related penetration to the simpler definition of effective stress discussed above (σ eff = σ c -P p ).…”

New Predictive Model of Penetration Depth for Oilwell-Perforating Shaped Charges

“…The fact that alternative penetration models have recently been introduced ([20]) is further testimony to the industry-recognized need for improvements over the conventional models. However, the primary penetration prediction method in [28] is still borrows heavily from the traditional methods of [28].…”

A Survey of Industry Models for Perforator Performance: Suggestions for Improvements

“…the method of ref. [11]) tend to become increasingly optimistic with regard to penetration depth prediction under downhole conditions. A fourth reason is the change in the standard API Section I concrete target from RP 43 [21] to RP 19B [22].…”

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