Inadequate productivity of perforated wells has been a major concern since the introduction of cased completions. Petrobel company, operating in the Belayim fields of Sinai, Egypt, and responding to an unpredictably fast decline in field production, launched an initiative in 1996 to identify the under-performing perforated wells in order to plan remedial actions. Since the vast majority of the wells were under artificial lift, this involved the integration of data from various sources. The study revealed that the majority of wells in the field were in fact suffering from excessive damage. Wells were then categorized based on the value of their completion factors (the ratio of actual to theoretical productivity indices), and the potential sources of damage were identified for each category. This was accomplished using techniques such as nodal analysis and skin modeling.
One of the most successful techniques used to cure the damage was the re-perforation of the damaged wells with deep-penetrating perforating charges. Until the use of this technique, and even with attempted improvements in other operating parameters, field performance had continued to fall short of theoretically predicted results. This paper highlights the origin and development of formation damage in the Belayim fields, and how this damage was attributed to different damage mechanisms using a novel combination of nodal and damage analysis techniques. It also presents some cases of dramatic productivity enhancement due to the use of these techniques and the introduction of deep penetrating perforating.
Inadequate productivity of perforated completions has always been a major concern for oil companies. Many wells have been abandoned prematurely due to reaching their economic limits too early. Formation damage can be defined as any barrier within the confines of the near wellbore reservoir or wellbore completion interval that restricts the natural production of formation fluids. In the following, potential sources of damage during various well operations are outlined.
A) Drilling Damage
Drilling damage results from the invasion of the formation by drilling fluid particles or by drilling fluid filtrate. The depth of particle invasion is usually small, ranging from 1 to 12 inches (Gatlin, 1960). The relative sizes of the fines and the pore throats are the primary factor in determining how much formation damage will occur. External bridging will occur if the median particle size is larger than 1/3rd of the mean pore throat size, causing little or no damage. Particle sizes in the range of 1/3rd to 1/7th of the mean pore throat diameter will result in shallow invasion but is the most damaging type of particle invasion. Finally, particle sizes less than 1/7th of the mean pore throat diameter are usually small enough to readily flow through the pore throats without causing significant plugging (Van Poolen, 1966).
Drilling mud filtrate will invade the formation to a greater depth than drilling mud particles, with depths of invasion of 1 to 6 ft being common (Hassen, 1980). Increased water saturation around the wellbore, emulsion formation, and clay dispersion are some of the formation damage mechanisms that can take place as a result of drilling fluid invasion. The volume of fluid lost prior to the formation of the filter cake (spurt loss) is related to the permeability of the formation. If adequate bridging agents are not present, particularly in high permeability formations, the entire fluid may be lost during the spurt period. Once an effective filter cake is formed, constant pressure filtration occurs, and the thus much lower rate of leak-off is controlled by filter cake permeability (Hassen, 1980).