Perforating

Perforating is a process used to establish a flow path between the near reservoir and the wellbore. It normally involves initiating a hole from the wellbore through the casing and any cement sheath into the producing zone.

Perforating history

Bullet guns were the first commercial perforating devices.^[1] A hardened steel bullet was fired from a short-barrel gun powered by a gas-producing explosive. These guns first saw commercial use in the early 1930s. The wall thickness and hardness of the casing and the hardness of the formation limit bullet perforating. Bullet guns are still used in some applications, usually in soft formations for deep penetration or brittle formations in which the shattering produced by the bullet can help break down the formation around the perforation.

During the 1930s and 1940s, work in the area of shaped charges progressed in the military arena. The bazooka, with its armor-piercing charges, was one of the first large-scale uses of the technology pioneered by Henry Mohaupt and others. This technology was accepted by the oil industry in the late 1940s and early 1950s and became the most used perforating method by the mid- to late 1950s.

Alternatives to explosives also were implemented, normally with an abrasive slurry of material such as frac sand and a carrier liquid, either sand or water.^[2][3] Abrasive perforating methods are slower, require a rig, and contain several wear points in the treating equipment.

Specialty perforators

Most of the specialty methods are used for special applications and do not find widespread use. Interesting perforation applications such as underbalanced perforating, tubing-conveyed perforating, and specialty phasings have their roots in much earlier applications, often 15 to 30 years before they became popular.

·         laser

·         hydraulic punches

·         mechanical punches

·         water jet

·         combination bullet/jet guns

·         electric arc perforating

Flow path

The effectiveness of the perforating process depends on the care and design of the procedure. Because a high percentage of current wells use a cased-hole completion, the importance of the design and application of the perforating process cannot be overstated. Establishing an optimum flow path requires the execution of a number of critical steps such as:

·         design

·         quality control

·         quality control inspection

Perforations are an elemental piece of the inflow section of the well and have significant impact on the total completion efficiency.

Alternative methods

Alternatives exist to cased, cemented, and perforated completions. Openhole completions offer several options that should not be ignored in a quest for a high efficiency flow connection to the reservoir. Key to the completion process is the minimization of pressure drop across the completion, specifically the piece of the flow path from the near reservoir to the wellbore. In many cases, completion requirements extend to the need to modify the flow connection in order to:

·         reduce gas or water coning

·         access multiple layers

·         assist in placing fractures

Completion requirements also extend to other aspects that involve initial completion or recompletion of the producing interval. A careful assessment of the benefits offered by both openhole completions and cased and perforated completion methods should be conducted.

Definitions

Because many of the perforating processes deal with explosive powders and gas expansion methods, a few definitions of the specialized nomenclature are needed.^[4]

High explosives are very powerful explosives such as RDX, HMX, PYX, HNS, and others that find common use in the oil industry. High explosives are characterized by extreme energy release in a very short time, some with detonation front movement on the order of 6100 + m/s (20,000 + ft/sec). The detonation of an explosive is a chemical reaction and, like many chemical reactions, certain variables control the speed of the reaction. Peak energy generation with these materials is necessary to perform effectively and can be achieved only if they have high-order initiation. The initiation process for any explosive is critical in oilfield applications.

Gas generators are explosive materials designed to generate energy at a slower rate than the high explosives, and their primary function is to provide quick fluid volume. These materials are used for power fluids (gas drive), fracturing energy, and propulsion energy sources.

Order is a term associated with explosive firing. High order means that the high explosive has been initiated properly and reacts at the maximum speed. Low-order initiation of a high explosive fails to achieve maximum energy; the explosive may react, but the energy level produced is sharply lower than the maximum potential. In perforating charges, a low-order detonation usually means a failure to produce effective perforations, although gas pressure may rise sharply. Burning is one of the low-order reactions, usually producing gas, with no perforation possible. Low-order detonations may expand or burst guns, causing obstructions and fishing or recompletion decisions. Care in design and application of the perforating system can reduce sharply the incidence of low-order firing. Low-order detonations are caused by several factors, but temperature and poor condition of detonating cord are leading causes.

A primary explosive is an explosive that is used in initiators or other devices to initiate the explosive sequence. Primary explosives usually are more sensitive to firing (can be initiated more easily) than secondary explosives. Common locations for these explosives are in detonators (also called blasting caps) and some booster devices.

Secondary explosives are the main explosives used in charges. The secondary explosives (usually high explosives) are harder to initiate and must be initiated to get proper response (i.e., a high-order detonation).

Perforation flow efficiency is a measurement of how close flow capacity in the perforated hole approaches the flow capacity of an ideal hole of the same diameter and length. There can be enormous differences in flow rate between a perforated hole and a drilled hole of the same diameter and length. The perforation flow efficiency is a part of the total well flow efficiency. Achieving the highest flow efficiency, by perforation characteristic, by cleanup, or by a breakdown operation, is a critical step. Good perforation flow efficiency is greater than 80%.

Pressure differential toward the formation from the wellbore is overbalance. Pressure differential from the formation to the wellbore is underbalance. Fluid flows from high pressure toward the low pressure in a permeable formation. Special cases of overbalance manipulation include extreme overbalance perforating (EOP).

Phasing is the angle between the charges. The most common phasings are 0°, 180°, 120°, 90°, and 60°. Several specialty guns, offering higher density charge application and guns for sand control or casing protection, may offer phasings that increase the linear distance between the charges in a direct line along the gun body.

Shot density is the measurement of the perforations made per unit length of the gun. Normally given in either shots/ft (SPF) or shots/m (SPM), the ranges of shot density extend from 1 to 27 SPF. The most common shot densities are 4 to 12 SPF (13 to 39 SPM). Shot density requirements are a function of the completion design and the formation production requirements.

Pressure drop is a measurement of the hindrances in the flow system. Rate of fluid flow through a rock is determined by the differential pressure, the permeability of the system, the fluid viscosity, and the length and area of the flow path. To maximize flow rate, the permeability must be high. Crushed rock, debris, and other obstructions result in lower permeability and lower flow rate. In a well system, maximum production is achieved by minimizing pressure drop.