A New Development in Completion Methods- The Limited Entry Technique
Authors: Lagrone K.W. , Rasmussen J.W. 
Published: December 2007

Abstract
Shell Oil Co., in Texas and New Mexico, has experienced excellent results from an improved well stimulation method called the limited entry technique. This method has proven much more effective than any other method in treating thick pay sections and in diverting treating fluids to multiple horizons. The limited entry treatment technique is accomplished bylimiting the number of perforations in a well andproviding sufficient injection rate to require the restricted flow capacity of the perforations to divert the treatment to a greater portion of the perforated interval.
From Dec. 3, 1960, to Jan. 1, 1963, Shell Oil Co. in Texas and New Mexico has treated 363 wells by this technique. The production performance of wells treated by limited entry completions is superior to that of conventionally treated wells. Gamma-ray tracer logs indicate most of the pay is being treated even though not covered by perforations. The limited entry technique has been used successfully in treating two separate reservoirs simultaneously in dually completed wells. Results of these simultaneous treatments have been gratifying in both well performance and reduced costs.

Introduction
The efficient simultaneous treatment of multiple porous intervals in a reservoir has been a long-standing problem in well stimulation. Various methods have been used to treat multiple zones with greater or lesser degrees of effectiveness. The bridge plug and packer method is effective, but is relatively expensive. Further, the injection rates are considerably reduced, and it is sometimes mechanically hazardous. Temporary plugging agents to divert the treatment have been used with apparent success. The main disadvantage of temporary plugging agents such as moth balls or gel blocks is the difficulty in determining the proper quantity of agent required to divert the treatment. Ball sealers are often ineffective because offluid communicating behind the casing between closely spaced perforations,failure of the ball sealers to seat on the perforations andabrasion of the ball sealers allowing fluid to by-pass.
These stimulation techniques (for the purposes of this paper) are considered to be conventional treatment methods. The basic objective of all stimulation efforts is to make the best well, compatible with cost. To get an effective treatment, it is desirable to treat as much of the perforated interval as possible. Also, the treatment should be proportioned into the perforated intervals. Well performance has proven that both of these objectives can be better fulfilled by a properly designed limited entry treatment, than by conventional treatments.

Limited Entry Technique
Shell Oil Co., in Texas and New Mexico, has experienced excellent results from an improved well stimulation method called the limited entry technique. Based upon data obtained to date, this method is far superior to the other methods of obtaining simultaneous treatment of multiple zones or thick pay sections. The treatment is performed bylimiting the number of perforations in a well andproviding sufficient injection rate to require the restricted capacity of the perforations to divert the treatment to a greater portion of the perforated interval.
The first limited entry treatment in this region wad performed in Shell TXL M-3, TXL Tubb field, Ector County, Tex., following a review of a paper by Murphy and Juch of Compania Shell de Venezuela. From Dec. 3, 1960, to Jan. 1, 1963, 363 limited entry treatments have been performed in many different reservoirs (see Fig. 1). No mechanical difficulties have been encountered that can be attributed to this method of treatment. Treatment failures due to "sand-outs" have not been increased by this method. Treatments have been successfully performed in carbonate, sandstone, conglomerate and chert reservoirs. These reservoirs range in depths from 3,100 to 9,500 ft. with bottom-hole pressures varying from 1,000 to 3,600 psi.

Basic Theory of Fracturing Process

Conventional Treatment
The simultaneous treatment of multiple porous intervals by conventional methods is depicted in Fig. 2.
JPT
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