Evaluation of Established Perforation Cleanup Models on Dynamic Underbalanced Perforating
- Dennis Haggerty (Halliburton Co.) | Gerald G. Craddock (Halliburton) | Clinton C. Quattlebaum (Halliburton Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 8-10 October, San Antonio, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 2.2.2 Perforating, 1.6.9 Coring, Fishing, 3.2.4 Acidising, 5.3.4 Integration of geomechanics in models, 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology
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The process of perforating in an underbalanced condition has, for many years, been a widely accepted method for ensuring open, clean, and clear perforation tunnels that are conducive to reservoir flow. With the increased popularity of tubing-conveyed perforating (TCP) during the last several decades, this method and the ability to maximize the amount of pressure differential has become even more popular and is often the preferred completion technique. In the last few years, an enhancement to underbalanced perforating, commonly known as dynamic underbalanced perforating, has been examined both through experiments and models. Dynamic underbalanced perforating is a process that creates a negative pressure differential or underbalance, causing fluid to move toward the wellbore even in an initial overbalanced static condition. A dynamic underbalanced condition can be controlled by understanding and carefully managing the temporal pressure transients, using multiple methods within the wellbore during and after gun system detonation. However, fundamental questions remain: What dynamic underbalanced behavior and pressures are required to remove the perforation-crushed zone, and are existing cleanup models sufficient for predicting perforation cleanup given the reservoir condition?
Recently, a series of instrumented perforation experiments using an advanced perforation flow laboratory demonstrated that existing cleanup models do not accurately predict perforation cleanup when perforating in a dynamic underbalanced condition. This work presents initial data and analysis, and suggests a superior method for quantifying perforation cleanup for a given dynamic underbalanced behavior and reservoir condition.
Underbalanced perforating methods have been applied successfully since the 1950s, shooting both wireline and TCP guns (Oliphant and Farris 1946; Allen and Worzel 1956). As shaped charge jet perforator systems became more advanced, using powdered metal liners, performance steadily improved. The art of minimizing the compacted and damaged area surrounding the perforation tunnel, commonly known as the crushed zone (Fig. 1), began shortly afterward. Much of the initial work involved shooting charges into prepared Berea sandstone cores while documenting the effect of a differential pressure toward the wellbore (underbalanced) upon perforation efficiencies (Bell et al. 1972). As the benefits of an underbalanced pressure differential were observed, extensive testing established criteria for sufficient flow volumes and differential pressures required to remove or minimize the crushed zone created during the perforating event. Specifically documented was the relationship of trapped atmospheric pressure inside a perforating gun surrounding the shaped charge and components, known as free gun volume (FGV), shown in Fig. 2, which enabled the formation pressure to act as a differential and expel charge and crushed formation into the gun (Walker et al. 1969).
As perforating research and field observations continued, a series of widely used and accepted formulas were established to document the magnitude of differential pressure required to ensure cleaned perforation tunnels. One notable work by King et al. (1986) evaluated the performance of 90 wells that were perforated with various magnitudes of underbalance, tested, acidized, and retested, and established a correlation between underbalance differential pressure and formation permeability. More work followed, some focusing on more field validated results, such as Crawford (1989), and others solely upon laboratory experimentation and derivation, such as Tariq (1990) and Behrman (1996). Bell (1984) established a typical range of underbalance for cleanup relative to a reservoir type of liquid or gas, and high (>100mD) and low (<100mD) permeability.
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