Hydraulic stimulation technologies, which are vital in maximizing the production of unconventional reservoirs, have typically focused on pumping capacities and rates, hydraulic fluid viscosities, and proppant materials. A technology that historically has been overlooked, but is critical to an efficient hydraulic stimulation, is the actual perforations through which the treatment is pumped. Fundamental rock and fracture mechanics demonstrate that perforation shot density, phasing, and hole size affect breakdown and treating pressures and injection rates, and can be a cause of early screenout. Although some of these perforation parameters are considered in the best practices, perforation hole size is often misunderstood. Complex computer fracture models, used to plan stimulation and completion programs, often incorporate average holediameter values with little consideration of the actual hole sizes or the variance from shot to shot. New research documents how this inconsistency can be considerable when using standard shaped-charge perforators. Nevertheless, perforating charges designed for natural completions, which focus primarily on the depth of penetration, are continuously used before hydraulic stimulation, ignoring the importance of consistent hole size. Advanced simulations using finite element analysis (FEA) have confirmed that fracture placement in a reservoir requiring stimulation would benefit significantly from maintaining consistency in the size of the perforation exit-hole. This work reviews the analysis performed and the subsequent successful field results for a new class of shaped-charge. The new fracture charge is engineered to maximize hole-diameter while maintaining a consistent exit-hole diameter independent of well profile and/or gun eccentricity. Designed for perforating before a hydraulic stimulation, a fracture charge has been shown to optimize fracturing efficiency and placement by ensuring that each perforation tunnel contributes equally during the fracture treatment, which contributes to providing cost effective hydraulic stimulation and maximizing subsequent asset value.
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.
The selection of the appropriate perforating gun system provides an essential element of maximizing the potential recovery of an oil or gas field. Perforating guns are often selected with minimal knowledge of the likely downhole charge performance, which commonly includes API 19B Section I test data that evaluates only cement penetration among competing systems. Unfortunately, cement penetration does not always correlate to the penetration in a downhole environment or inflow potential.A study was recently conducted to evaluate the charge performance in downhole conditions for a gas reservoir in a perforation flow lab (PFL) based on API 19B Section IV 1 methods to identify the best performing charge design for a 3-3/8-in. 6 spf gun system. The study evaluated the effects of cement sheath thickness and casing wall material (hard stainless steel vs. carbon steel). The magnitude of pressure in the simulated wellbore was varied to compare slight underbalance with high underbalance to determine the effect of offline perforating limitations. In addition, the separate testing variable of target core diameter was investigated and resulted in a dramatic effect on the perforator performance.These testing results identified the optimum gun system for use in the Talisman Bunga Orkid gas field. The productivity estimates derived during the charge evaluation were used to validate inflow models, which to date closely match actual production.An advanced evaluation of perforating systems can provide valuable insight into what is not always intuitive from available published cement penetration data and can be used to further advance production models. The parameters used in the models should be well understood to confidently predict the perforation hole size, penetration, and flow performance. IntroductionThe quest to identify an optimum perforating solution for a given field condition can always be influenced by operational efficiency, as seen in the Talisman Bunga Orkid gas field, whereby the need to maintain continuous drilling operation was imperative to improve rig use. In this case, offline perforating, a unique method of perforating during continuous drilling operations on adjacent wells, was developed. This method used limited vertical height below the rig floor to perforate, while drilling operations continued in another well slot, providing a simultaneous operation (Fig. A1). Unfortunately, establishing a significant static underbalance in the monobore completions was cost and time prohibitive. The required flow area, gun volume, and short gun lengths, because of the limited rig-up height, minimized the dynamic underbalance cleanup. Therefore, a new approach was needed to optimize the perforating design for this field. The approach taken was to evaluate various available perforating gun systems and to identify the best performing system at field conditions.Evaluating shaped charge performance is conventionally conducted by correcting API 19B Section I penetration data to downhole conditions. Numerous calculations are used...
Propellant-based well stimulation is an accepted technology in the oil industry; a number of papers have been published regarding its advantages and advancements (Folse et al. 2001; Gilliat et al. 1999; Yang et al. 1992). The additions of propellant stimulation modeling software and high-speed pressure recorders have greatly assisted in the development and maturing of the technology (Schatz et al. 1999). The most commonly used propellant for near wellbore stimulation is the propellant sleeve. Propellant sleeve is simply a hollow tube of propellant that is positioned outside of a standard perforating gun and held into place by retaining rings. The detonation of the perforating charges ignites the propellant. The internal ballistic train of the perforating gun is not affected by the propellant because of the sleeve's external placement. This system is both reliable and effective. A second and less frequently used technique places the propellant inside a vented hollow steel carrier, allowing propellant stimulation without the need of introducing new perforations. Unlike the propellant sleeve system, this system uses detonating cord to ignite the propellant. This method has the ability to stimulate virtually an unlimited amount of borehole by simply interconnecting the vented propellant carriers. However, the ballistic train is exposed to wellbore fluids and pressures. Therefore, when deploying longer intervals (greater than 7 m), the propellant system must be able to reliably continue the ballistics train between interconnected propellant carriers. In the past, the wet-connect system used was unreliable and limited the system to low wellbore pressure applications. Consequently, this method has struggled to gain acceptance primarily due to a lack of a reliable ignition system combined with limited stimulation pressure data. This paper discusses a new propellant stimulation system that addresses reliability issues that have plagued propellant stimulation systems in the past. It also offers case studies that demonstrate the effectiveness of the new system, performance of the optimized propellant formula as well as the value created from the system's performance and flexibility. Introduction It is widely accepted that the processes involved in drilling, completing, and producing / injecting a well have potential to create a zone of reduced effective permeability surrounding the wellbore (Klotz et al. 1974). This is commonly referred to as the damaged zone (Tariq 1987). There are multiple remediation processes available today to re-establish a conductive path through the damage zone to the unaltered reservoir rock. Using pressure generated by the combustion of propellants to initiate and extend short fractures from the wellbore has proved to be a cost-efficient and convenient method of near-wellbore stimulation (Wieland et al. 2006; Schatz et al. 1989). A selection of propellant tools is available today. The potassium perchlorate propellant sleeve is probably the most widely accepted product. This system consists of a perforating gun with an external sleeve of propellant. When conveyed on tubing or wireline to the correct depth, the gun is fired. The well is both perforated and stimulated in one operation. There are multiple other systems consisting of a protective (vented) carrier, a propellant tube, and a separate ignition system - typically utilizing detonating cord. This is a single trip-operation process used to stimulate either perforated or open-hole intervals. Some systems incorporate conventional interconnect tandem subs allowing multiple propellant carriers to be assembled to stimulate very long intervals. Unlike the propellant sleeve assembly, which has been popularized by its ease of use, reliability, and its dual function, the carrier conveyed system has only gained limited industry acceptance due to the inconsistent ignition of propellant.
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