A widely acknowledged problem in gravel packing is plugging of wire wrapped screens and slotted liners Clays, drilling mud, cement, formation sand and pipe dope are all known plugging contributors.
Field tests and laboratory evaluations have demonstrated that an acid soluble screen protector material may be utilized to prevent plugging as well as replace wash pipe and to serve as temporary blank pipe or casing.
Plugging or partial plugging of wire wrapped screens and slotted liners is a common problem detrimental to the achievement of maximum productivity. Full scale tests conducted by Chevron have demonstrated dramatically that the wedging of gravel and other material in the openings of a slotted liner during gravel packing can result in severe loss of flow capacity. Field experience has also shown that circulation of fluids through a slotted liner can cause severe plugging with formation sand that has been mobilized while running the liner into the wellbore or by subsequent circulation past an open hole section.
In steamed wells, the problem of a plugges liner may be compounded by the creation of thief zones caused by preferential cleaning of liner slots opposite higher permeability zones. This results in preferential and incomplete treatment of the production preferential and incomplete treatment of the production interval.
In order to circumvent these problems an acid soluble coating was developed. This acid soluble coating has an advantage over the previous temporary coatings in that the dissolving mechanism is controlled by the operator and is not dependent on bottom hole conditions. The coating is insoluble in oil or gas and water, but will soften and weaken with long term exposure to water. Coatings made of wax or polymers require accurate knowledge of the bottom polymers require accurate knowledge of the bottom hole temperature for removal of the coating.
The acid soluble screen protector material is composed of a mixture of inorganic magnesium salts and water. The mixture is usually applied as a thick paste to the exterior of a screen or liner. Once paste to the exterior of a screen or liner. Once applied, the paste will harden in 4 hours to a compressive strength of approximately 6,000 psi forming a coating to protect the screen from plugging or damage.
The acid soluble screen protector material may also be used as a replacement for wash pipe. A screen or liner is coated leaving a 10'-15' section on bottom uncoated. Using a crossover tool hook-up, gravel can be circulated until the lower blank screen is covered. When this blank section is covered, a pressure increase will be observed. Then the tool can be lowered into the squeeze position and the gravel pumped into the formation. After the gravel pack is complete, 10–15% hydrochloric acid can be spotted on the inside of the screen and allowed to soak for 30 minutes. The acid may either be squeezed into the formation or reversed out.
FIELD CASE NUMBER 1
The acid soluble screen protector material was used as a temporary casing in a Louisiana offshore well. A 300' open-hole gas well was to be completed. The upper 100' of this formation was shaley, water sensitive and very susceptible to fluid loss. In order to complete this well, an external casing packer was used to isolate the upper portion of the reservoir. (see Figure 1) Using port collars and a combination tool hook-up, the lower zone was gravel packed with a slurry containing 15 pounds of gravel packed with a slurry containing 15 pounds of gravel per gallon of fluid. After a sand-out, the excess per gallon of fluid. After a sand-out, the excess slurry was reversed out by pumping down the annulus between the acid soluble screen protector coated 4-1/2" screen and the combination tool.
The combination tool (washing tool) was raised to a position inside the acid soluble screen protector coated upper screen and an attempt was made to hydraulically wash off the coating. A pressure of 2,000 psi failed to remove the coating.