An Economic, Field-Proven Method for Removing Fines Damage from Gravel Packs

Abstract

It's a fact of life—many gravel packs plug up. As time passes, skin factors go up and production rates go down. Much of the money that was spent on creating a sand control completion with minimum skin may eventually count for nothing. In order to prolong the effectiveness of the gravel pack (GP) and as a added benefit reduce the need for work overs, a method was developed in the Far East to remove these fines and dramatically reduce the skin factor. The method employs relatively small volumes of a specialized HF (hydrogen fluoride) acid system, designed to penetrate into the GP and very near wellbore area. The HF acid system uses an organo-phosphonate (HV) acid to control the production of HF and to control and drastically reduce potentially damaging secondary and tertiary precipitates. The system will be referred to as HV:HF Acid in this paper. This HV:HF Acid system can be applied to any well with a screen and/or GP that has damage caused by the migration of formation fines and/or drilling mud particles. It is not limited by the type of screen or gravel nor by the length or number of sets of screens. The method has potentially enormous implications throughout the world. Any well with a GP or screen could potentially benefit from this method of treatment, including long horizontal intervals, which up to now have been very expensive to treat, due to the larger volumes of acid normally recommended. The results of this HV:HF Acid method will be illustrated by 3 case histories from the Far East, including horizontal wells. The observations from these treatments allow us to conclude that this method is highly successful, cost effective, and applicable to a broad spectrum of well conditions. The reduced stimulation volumes provide better economics, so that even long sections and horizontal wells can now be stimulated at reasonable costs. Introduction Around the world there are literally thousands of gravel packed wells which could benefit from a reliable and cost effective method for removing damage caused by fines. Many of these wells start out with relatively low skin factors and then gradually show an increase in skin as the GP, a.k.a. "the filter", collects migrating particulates. In other areas of the oil industry where filters are used (hydraulic systems, preparation of completion fluids, engine lubrication etc), filters are cleaned or replaced as they get plugged. Replacing a GP is a risky and expensive process which involves the often problematic well killing operation. Alternatively, cleaning the GP has the potential to be a much more cost effective solution. There are inherent problems associated with trying to remove fines damage from the matrix of a GP. Firstly, the fines themselves are usually composed of either quartz particles (silica), silicates and alumino-silicates (clays and feldspars) or, more commonly, a combination of these. Because the GP is filtering out some of these particles, they are generally much more concentrated in the GP area than they originally were in the formation. The only method available for removing these fines—short of doing a work over—is to use some kind of hydrogen fluoride (HF) based acid system, typically referred to as "mud acid" (as these acid systems were first developed to remove formation damage caused by drilling muds). The use of such acid systems to dissolve silica, silicates and alumino-silicates is sometimes poorly understood and can often result in the production of several different types of precipitates. Indeed, the process of HF acidizing has been described as an effort to remove more damage than the damage created by the treatment itself.1 Instances of HF treatments actually reducing production (i.e. creating more damage than they remove) are unacceptably common.

Successful Sandstone Acid Design Case Histories: Exceptions to Conventional Wisdom

“…In both the non-gravel pack and gravel pack cases cited 16,17 , what was found to be successful (and unsuccessful) is opposite of what was found in the non-gravel pack fines migration case example in the Gulf of Mexico.…”

Rotating Jet Technology Improves Stimulation Fluid Placement to Effectively Treat Horizontal and Gravel Packed Wells - Lab Model Tests and Global Results are Reviewed

“…The original HV:HF Acid System was first described in 1996 10 and has since been used on over 1000 wells worldwide for matrix stimulation and nearwellbore damage removal. Numerous case histories for this HV:HF Mud Acid system are available 1,2,5,6,9,[11][12][13] . Below is a summary of the main advantages of the system for treating fines damage in GP wells: 1.…”

Matrix Acid Stimulation - A Review of the State-Of-The-Art

“…Due to their lower inherent corrosivity, they are also a much safer option to use in the clean-up of gravel packed wells, since there is a greatly reduced risk of damage to screens. (8,9,20) One new system combines a powerful iron complexing agent and a scale inhibitor to eliminate the risk of iron precipitation, a common cause of treatment failure. Results obtained with this new system in diverse and very challenging mineralogies were extremely impressive and this has been borne out in the field (21) .…”