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Abstract

In previous publications we have introduced methods for the matrix stimulation of horizontal wells, damage characterization and removal, and economic criteria for the evaluation of the job effectiveness. The characteristic shape of damage, which is neither radial nor evenly distributed along a horizontal well, would impact the effectiveness of a matrix stimulation treatment. Full removal of damage is probably never practical because of thief zones and a marked inability to divert the stimulation fluids. Also, the large volume of acid to be injected in horizontal wells will likely create significant corrosion problems. Therefore, partial stimulation and perhaps partial completion, by deliberately leaving segments of the horizontal well unperforated, should be contemplated. By balancing the expected well performance against the fluid volumetric coverage and the length of the open segments, the optimum stimulation and completion design can be evaluated. The net present value (NPV) is used u the optimization criterion.

In this paper a cue study of a 1600-ft horizontal well in a 160-acre drainage area in a sandstone reservoir is presented. Based on expressions for original and posttreatment skin effects, which are presented in explicit form, an economic evaluation of several stimulation and completion design options is carried out. A general recommendation on the proper matrix stimulation in horizontal wells is derived based on the results of this cue study.

Introduction

Horizontal wells, frequently spanning several thousand feet are likely to be damaged and in need of stimulation. Causes of less than expected performance from horizontal wells include inappropriate reservoir selection (thick formation with low vertical permeability) wrong well trajectory (e.g., normal to the minimum permeability) and, especially, unstimulated damaged wells.

Frick and Economides have presented analytical expressions for the skin effect of a horizontal well. This skin effect characterizes the shape of damage which is likely to be in the form of a truncated elliptical cone with the larger base near the vertical well section. The aspect ratio of the ellipse depends on the vertical to horizontal permeability anisotropy. The same authors also presented analytical expressions for the posttreatment skin effect. In the cue of sandstones the posttreatment skin effect. In the cue of sandstones the stimulated zone was presumed to be of the same general shape u the damage zone. For carbonates, where stimulation depends more on reaction kinetics and less on flow through the porous medium, the stimulated zone was assumed to be cylindrical, although this assumption is currently under scrutiny.

Bullheading stimulation fluids into a horizontal section is unlikely to be a successful stimulation treatment since much of the fluid would be spent laterally, where damage has been removed and the impediment to flow has been reduced. Therefore, diversion, such u chemical or mechanical (provided by coiled tubing) or both has been proposed and is in use in the industry.

A semianalytical solution of the performance of a partially completed horizontal well has suggested that a large portion of the theoretical open-hole undamaged flowrate can be realized with several perforated segments that are separated by unperforated intervals. This is a particularly useful finding because completion and stimulation become the object of optimization and, in practical terms, allow a variety of option in the hands of an operator. One extreme would be the full removal of damage along the entire well trajectory.

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