Abstract

To conventionally drill, complete and produce horizontal and multilateral wells with zero skin is not an easy undertaking. Much attention has been given to "best oil field practices" in both drilling and production processes, but the reality is that formation damage will often have to be addressed sometime during the life time of the well.

Formation damage due to drilling and scale formation is common problem in carbonate reservoirs that can be re-mediated using chemical means. The success of any treatment however, requires a complete understanding of the problem and a solution that will address the majority of the damage. The solution evolves with time and experience. Extended reach intervals with variable permeability complicate the process. Reservoir and fluid characteristics, cleanup fluid chemistry, and operational considerations must always be considered.

The acidizing experience and an improved understanding of how to effectively treat long and heterogeneous intervals in carbonate formations is rapidly evolving in Saudi Arabia. Water injectors, oil producers and wastewater disposal wells have been treated and are reviewed in this study. These wells have different configurations including: vertical, horizontal, extended reach, and multilateral with open hole and cased completions. Several acid placement and diversion techniques have been applied and a specialized treatment package was developed based on the latest coiled tubing and chemical diversion technologies. Laboratory studies, lessons learned and specific design guidelines from both successful and less than expected well treatments are highlighted in this work.

Introduction

Various materials and methods have been applied to enhance acid diversion across long treatment intervals. Each technique has some merits, and the basic goal of all is to temporarily shut-off higher permeability sections in order to force the stimulation fluids into the lower permeability sections. As with many situations however, excess can cause undesirable results.

Solid bridging materials have successfully been used in acid wash applications, however they can be difficult to handle operationally, and over-treatment can prematurely cause plugging when not desired. Particulate-based systems can only be considered to provide near wellbore diversion.

In "bullhead" treatments, diversion systems are generally effective for relatively small intervals. The main point is that leak-off into lower sections must be achievable to enable the acid to flow into them. Otherwise, the acid will only treat the upper sections, typically just below the last casing shoe. The acid dissolution rate must also be considered. For example, regular hydrochloric acid will react very rapidly with carbonate formations, especially at high bottom hole temperatures; spending itself on the formation before reaching the desired placement. Emulsified acid has a dissolution rate nearly 15 times slower than the regular acid. This means the acid can travel farther along a wellbore and penetrate deeper into the formation before spending.1,2

Coiled tubing is the ideal placement method for matrix acidizing of long, horizontal, open hole intervals in carbonate formations. Pinpoint placement of acid is possible, and the leak-off rate is very low. Basically, the acid will react in the vicinity, relative to the position of the end of the coiled tubing. Another benefit of the continuous pipe is the ability to pull-out of hole without the need of breaking joints on surface (as in the case of drill pipe). The closed system is also better suited for well control when positive annular wellhead pressure is expected at the surface.

Polymer-based systems are easier to handle than particulates and diversion is achieved via the formation's natural resistance to viscous flow.3 There is a tendency in the industry however, to overtreat formations with polymers, whether as "Hi Vis" LCM pills while drilling or as a diversion pill in an stimulation treatment. Later on, the production engineer has the daunting task of trying to make a well that is severely damaged with a deep penetrating gel.

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