Segment Acidizing Design of Horizontal Wells In Naturally Fractured Reservoirs
Authors: Gu F. , Guo J. , Fan X. 
Published: March 2010

Abstract
Using horizontal wells is an effective exploitation method for naturally fractured reservoirs. In accordance with horizontal wells in naturally fractured reservoirs, the damage characteristics from drilling, completion and natural blockages are discussed in this paper. The method to determine the length, position and acid volume of each segment is presented for segment acidizing. The three-dimensional unsteady acid flow model, heat exchange model and acid-rock reaction model and solving method are developed in order to simultaneously predict the distribution of acid velocity, temperature and concentration during acidizing.
The well completion and segment acidizing design results are presented for a horizontal well in a naturally fractured volcanics reservoir. Satisfactory productivity was obtained after the segment acidizing. The developed models and method are also suitable for the acidizing design of a horizontal well in a naturally fractured carbonate reservoir.

Introduction
Horizontal wells have been widely used in the petroleum industry(1,2) for their higher economic efficiency and many advantages over vertical wells. They are also effective to exploit anisotropic reservoirs(2). Horizontal wells have more opportunities to channel through fractures than those of vertical wells while they are used in naturally fractured reservoirs. However, the formation damage of horizontal wells is more severe than that of vertical wells because wellbore exposure time to mud is longer and mud density would be higher to maintain the stability of a wellbore.
The influence of damage on well productivity of horizontal wells has been investigated(3, 4). The results show that consequence matrix stimulation treatments are required to increase the flow profile across the horizontal section. The use of coiled tubing is a recommended method for matrix stimulation of a horizontal well as it can provide a necessary mechanical isolation and diversion for uniform coverage along a horizontal well(5). The optimal withdrawal rate of coiled tubing was given by Economides et al.(4, 5) for matrix acidizing of horizontal wells. Because coiled tubing stimulation could not exercise effectively in naturally fractured reservoirs where diverters would lose their functions, alternative acidizing methods should be attempted. Segment acidizing is one of these methods. In the treatment, packers are used outside the casing to seal each segment and then acidizing is taken one segment after another. The sealing positions have to be determined before completing.
This paper presents a design method for segment acidizing for a horizontal well in a naturally fractured reservoir. It includes the determination of the position and acid volume of each segment and the prediction of acid penetration. Case design results are presented for a horizontal well in a naturally fractured volcanics reservoir.

Damage Characteristics
There are two major damage sources for horizontal wells in naturally fractured reservoirs. One is natural blockages which were produced while the reservoirs were formed and the other is from operations, such as drilling, completing, workovers and production. Generally, natural fracture blockages are randomly distributed in reservoirs and around wellbores. Nevertheless, the operation damage depends on the time and the fluid type of operating and the formation permeability.