A critical step toward the optimized well completion efficiency is to minimize perforating debris. Conventionally zinc-case shaped charges have been used in perforating for their perceived benefits of low cost, low-debris, and dissolvability by acid. The low-debris characteristic has been proven to be a misconception. Charge detonation actually pulverizes a zinc case into fine powder with a minor amount of the zinc material melted and oxidized. The powder form zinc has higher tendency to exit the gun during perforating and retrieval of the gun string, which potentially causes formation damage and other operation problems. Shaped charges that truly result in a low amount of debris are therefore needed by the industry. In response to such a demand, low-debris steel-case shaped charges are developed. Laboratory perforating tests under downhole conditions showed several mechanisms by which a zinc-case charge can cause operation failure and formation damage. (1) The fine zinc debris can be tightly compacted in the perforation tunnel and cause complete perforation plugging if the perforating job is not properly designed. The plugged perforation tunnel not only reduces the productivity and injectivity dramatically, but also causes reservoir fluid to bypasses the length of the perforation and converges into the entrance hole, increasing flow velocity and pore pressure gradient, and hence increasing sand production potential. (2) Zinc-case charges generate higher detonation pressure than steel-case charges due to higher reaction heat. Lower shot density has to be used to ensure underbalance for perforation cleanup. (3) When a kill well operation is required during completions, chemical reaction between zinc and CaCl2/CaBr2 based kill pill can cause failure in fluid loss control. Higher fluid loss will cause high completions cost due to loss of expensive completion fluid. In addition, high fluid loss volume leads to severe formation damage. (4) Expelled zinc debris is anodic to steel. Under common pH, temperature, and chemical environment inside a well, the redox reaction liberates atomic hydrogen, causing hydrogen related corrosion or enbrittlement to the downhole hardware. (5) Difference in performance of zinc-case and steel-case charges shows that the latter is a superior candidate particularly for deep penetrators. Given all other conditions are same, the steel-case charge is capable of deeper perforation. To overcome the problems associated with zinc and to result in truly low amount of debris, steel-case shaped charges were developed. The unique shaped charge design combined with selection of special steel materials for charge cases allows all case fragments to be sufficiently large that they remain inside their hollow carriers, which are eventually retrieved back to surface after perforating. As a result, it not only achieves debris minimization during perforating, but also maintains consistency regardless of gun movement during retrieval operation. This paper discusses the laboratory studies that compare the zinc-case and steel-case charge performances, the potential damage mechanism by zinc-case charges, and presents the extensive field history of the low-debris steel-case charges. Introduction When perforating wells with complicated completions, such as extended reach or horizontal wells, and wells with multi-zones, or multi-laterals, in which many downhole flow controls and monitoring devices are installed, minimizing perforating debris, charge case debris in particular, is crucial to prevent the need for intervention. There are two common ways to minimize the debris. The first is to make the shaped charge fragment into very small particles upon detonation so that the debris can pass through downhole devices. The second way is to make the charge case break into large pieces that are retained inside the gun. Shaped charges with zinc cases are designed based on the former philosophy, while high strength performance shaped charges with low-debris steel cases are designed based on the latter.

In hydraulic fracturing, determining the perforation pressure loss is a critical step in the design strategy, on-site troubleshooting diagnostics and post-fracture analysis. Historically, the most widely assumed and thus unknown components in the perforation friction equationare the coefficient of discharge and the holistic perforation diameter. The perforation coefficient of discharge has long been assumed as a dynamic variable dependent on the amount of fluid and proppant pumped through the perforations. This variable becomes increasingly important when clusters are spaced closer together and fewer perforations are shot such as in a limited entry design. Limited entry is a perforating technique used to generate uniform fractures along the wellbore by creating appropriate pressure differentials from cluster to cluster. With the adoption of consistent hole perforating shaped charges, the perforating diameters are more consistent and predictable. While not all consistent hole shaped charges have low diameter variability, the perforating diameters downhole are no longer an unknown, particularly after the introduction of downhole cameras. Therefore, the coefficient of discharge is the only unknown variable remaining. This paper presents an experimental methodology to accurately define the true coefficient of discharge in common completions perforated by a known consistent hole shaped charge. The test setup is illustrated, detailed test steps are discussed, and experimental data with correlations of rate per perforation and discharge coefficient is presented. Completions tested included 4-1/2", 5", and 5-1/2" casings in common weights and grades. Various perforating strategies were examined such as single shot and angled shot. Critical parameters such as entry hole diameters were made by the actual shaped charges and measured before and after the test. Freshwater and slickwater were used as hydraulic fluid and circulated at real-world pump rates through each perforation to simulate the actual field flow conditions. Based on the study, several correlations for the coefficient of discharge of flow through a perforation are created considering casing thickness, entry hole diameter and rate per perforation for the given consistent hole shaped charges. These correlations can improve perforation and fracturing designs where perforation friction are important variables.

No abstract