Friday, December 8, 2017

Perforation Strategy

Perforations form conduits into the reservoir that not only allow hydrocarbon recovery, but influence it. Each of the three main types of completions - natural, stimulated and sand control - has different perforating requirements. In the natural completion (in which perforating is followed directly by production) many deep shots are most effective. In stimulated completions - hydraulic fracturing and matrix acidizing - a small angle between shots is critical to effectively create hydraulic fractures and link perforations with new pathways in the reservoir. And in gravel packing, many large-diameter perforations effectively filled with gravel are used to keep the typically unconsolidated formation from producing sand and creating damage that would result in large pressure drops during production.

To meet the broad requirements of perforating, there many perforating guns and gun conveyance systems. Optimizing perforating requires selection of hardware best suited to the job. A good place to start, therefore, is with the basic of perforating hardware.

The Language of Perforating

There was a time when describing the perforation operation defined the perforator: running through-tubing guns, shooting casing guns or tubing-conveyed perforating (TCP).  





 The two broad categories of guns are exposed and hollow carrier guns. These can be used in two types of perforating operations : through-tubing , in which guns are run through a production or test string into larger diameter casing; and through-casing, in which guns are larger diameter and run directly into casing.

Exposed guns are run on wireline and have individual shaped charges sealed in capsules and mounted on a strip, in a tube or along wires. The detonator and detonating cord are exposed to borehole fluids. These guns are exclusively through tubing and leave debris after firing. 

Perforation-Reservoir Interactions 

Flow efficiency of a perforated completion and stimulation success are determined mainly by how well the perforation program takes advantage of the reservoir properties. The program includes determination of two main factors:


  • The proper differential between reservoir and wellbore pressure (The usual preference is for underbalance, meaning wellbore pressure is less than reservoir pressure at time of perforating).
  • Gun selection, which determines penetration tunnel length, shot phasing, shot density and perforation entrance hole diameter. The relative importance of the different components of shot geometry varies with the completion type.


The main reservoir property that affects flow efficiency is permeability anisotropy from whatever cause - in sandstone, typically from alignment of grains related to their deposition; in carbonates, typically from fractures or stylolites. Shale laminations, natural fractures and wellbore damage, which can cause permeability anisotropy, are considered separately because they are so common. In most formations, vertical permeability is lower than horizontal. In all these cases, productivity is improved by use of guns with high shoot densities. 

Natural fractures are common in many reservoirs and may provide high effective permeability even when matrix permeability is low. However, productivity of perforated completions in fractured reservoirs requires good hydraulic communication between the perforations and fracture network. To maximise the chances of intersecting a fracture, penetration length is the highest priority, with phase angle second. Shot density is less important because fractures form planes and increasing density does not increase contact with a fracture system. In fractured formations, a popular gun configuration uses 60 degree phasing with 5 spf. A Schlumberger version of this gun has a large change that penetrates 30 inch (76 cm) into the standard API test target.

An important geometric consideration of a perforation is how deeply it penetrates -whether it reaches beyond the zone damaged during drilling or connects with existing fractures. The penetration of various shaped charges is documented in surface test and in test under stress with API targets.

Penetration in surface tests is different than under stress in the well. Unconfined compressive strength of test targets is a minimum of 3300 psi, representing only low-strength reservoir rock (reservoir rock strength ranges from 0 to 25,000 psi). To estimate depth of penetration into a rock of arbitraty strength under a given stress, data measured at unstressed surface conditions have to be transformed. Because rock penetration data exist for only a few combination of charges, rock strength and stresses, a semiempirical approach is used that combines experimental data with penetration theory.

Schlumberger calculates penetration change caused by formation stress using experimental data for three generic charge designs after first calculating the change due to formation strength at zero stress. These data provide transforms implemented in the SPAN Schlumberger Perforating Analysis program.


The SPAn program consists of two modules: penetration length calculation and productivity calculation. In the penetration module, perforation length and diameter estimates are calculated under downhole conditions for any combination of gun, charge and casing size. 

Another influence on flow efficiency is formation damage, usually considered in the context of skin, and index of flow efficiency related to properties of the reservoir and completion. Skin comprises a variety of influence : flow convergence, wellbore damage, perforation damage, partial penetration (perforation of less than the toal height of reservoir) and the angle between the perforation and bedding plane. The goal is to design perforations that minimize skin and therefore maximize flow efficiency. 


Formation damage is caused by invasion of mud filtrate and cement fluid loss into the formation, creating a zone of lower effective permeability around the wellbore. Extending the perforation beyond the damaged zone may reduce this skin significantly, enhancing productivity. But even for perforations that do penetrate farther, the wellbore damage zone reduces the effective tunnel length.

During perforating, a "crushed zone" of reduced permeability is created around the perforation. In laboratory experiments, the thickness and permeability damage of the crushed zone are influenced by all variables to varying degrees: the type of shaped charge, formation type and stress, underbalance and cleanup conditions. Pucknell and Behrmann found that permeability near the perforation is reduced because microfracturing replaces larger pores with smaller ones. The current rule of thumb is to assume a crushed zone 1/2 inch (13 mm) thick with permeability reduced by 80% to 90%. 

 

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