Abstract

The using of horizontal wells is an effective exploitation way for naturally fractured reservoirs. In accordance with horizontal wells in a naturally fractured volcanic reservoir, the characteristics of damages from drilling, completion and natural blockages are discussed and the aim of acidizing is outlined in this paper. The method to determine segment length and position and treatment parameters are presented for segment acidizing. The three-dimension unsteady acid flow model, acid flow heat exchange model and acid-rock reaction model are established and a semi-numerical calculating method is utilized in order to simultaneously predict the distribution of acid velocity, temperature and concentration during acidizing treatments.

The well completion and the segment acidizing design are presented for a horizontal well in a naturally fractured volcanic reservoir of a oil field in western China.

Introduction

Horizontal wells have been widely used in petroleum industry[1][2] because they have higher economic efficiency and advantages over vertical wells such as controlling sand, delaying gas or water coning and preventing turbulent flow around the wellbore etc.. They are very attractive to be used to exploitate anisotropic reservoirs[2] like naturally fractured reservoirs, because they have more opportunities to channel through fractures as natural fractures are generally vertical and they have longer wellbores in pay zone. However, the formation damage will be more severe in horizontal wells than in vertical wells because of the longer exposure time and higher mud density to maintain the stability of bore hole.

The damage impact on the production index of horizontal wells has been investigated[3][4]. The results shows consequence matrix stimulation treatments are required to increase the flow profile across horizontal section. The use of coiled tubing is the recommended method for the matrix stimulation of horizontal wells as it can provide a necessary mechanical isolation and diversion for uniform coverage along a horizontal well[5]. The optimal rate of coiled tubing withdrawal was given by Economides et. al.[4][5] for matrix acidizing of horizontal wells. As coiled tubing stimulation can not exercise a well effect in naturally fractured reservoirs where diverters would lose their functions another acidizing method such as segment acidizing should be used.

This paper presents design method of segment acidizing and treatment consideration for horizontal wells in a naturally fractured volcanic reservoir. The provided method may be applied in other naturally fractured reservoirs as carbonate reservoirs.

RESERVOIR CHARACTERISTICS

The studied reservoir was produced by paroxysmal eruption and its rock is mainly vitric and sedimentary tuff with frequently natural fractures of middle to high angles. The fractures have width of 0.5-5.0mm according to the core investigation and are seriously filled with calcite, float stone and chlorite etc.. The reservoir is driven by dissolved gas. The other parameters of the reservoir are shown in table 1. Horizontal wells were selected to develop this reservoir after compared the economical efficiency of horizontal wells with that of vertical wells.

FORMATION DAMAGE

There are two main damage sources for horizontal wells in naturally fractured reservoirs. One source is natural blockages in fractures, which were produced while they were formed and the other is from drilling, completing, workover and producing.

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