Mobilization of fines can severely damage a well's productivity and is one of the more costly sources of well damage identified to date. Termed fines migration, the phenomenon is complex but governed largely by mineralogy, permeability, salinity and pH changes as well as drag forces created by flow velocity, turbulence and fluid viscosity1–10.
Over the years various mixtures containing hydrogen fluoride (HF) acid or its precursors have been used to dissolve fines and increase permeability in sandstones. However, it has been noted in considerable prior literature11–21 that these treatments can result in relatively shallow penetration due to the rapid spending of HF. Frequently the treatment is limited to a few inches in the formation beyond the wellbore region and this allows future fines re-invasion into the near well bore. Also fines acidizing alone is merely treating the symptom rather than the underlying cause and it can be assumed that the duration of the benefit is limited to how long it takes for fines to be released and begin to restrict flow again. Thus, a concern for operators is the fact that the benefit of HF acid treatments for fines can be short-lived. Other treatments were developed to fix fines in place21–22 however these systems have not been widely used.
The optimum treatment for fines would integrate techniques to dissolve fines, enlarge pore-throat size to diminish near wellbore particle bridging, and finally to "lock-in-place" those fines that are not removed by the treatment. Furthermore, a long-term, integrated solution must address the mechanism of fines migration so that a comprehensive approach to the problem may be employed.
This paper reviews fines damage mechanisms and presents a fluid system to correct fines damage and prevent fines migration in a single treatment, while keeping acidizing risk at a minimum. The new system uses an organo-phosphonic acid (HV) to generate and moderate HF for deep penetration and an organosilane fines-fixing agent (FFA).
The issue of fines migration is complicated because of the diversity of fines chemistry and mechanisms of fines damage. Fines can nearly run the gamut of clastic mineralogy but can be classified in two basic categories 1) clays and 2) non-clays (quartz, amorphous silica, feldspars, zeolites, carbonates, salts and micas) which are all common sandstone constituents. Because the source of fines is so prevalent there are numerous fields around the world that suffer from damage due to fines migration. Although the Gulf of Mexico and California have received the most study, it is clear that fines damage is a global problem and endemic to hydrocarbon production. Moreover, the unique problem of fines migration can be chronic and have major implications to entire field macro-economics. Such is the case in Lake Maracaibo, Venezuela. The combination of fines damage and water production has had a major impact on the ability of these generally low pressure reservoirs to hold production over time. Fines also have other negative side effects such as acting as nucleation sites for organic precipitation and stabilizing emulsions. Although it is somewhat difficult to diagnose, the symptoms of fines damage are typically a rapid production decline and on occasion associated fines production to surface. It is also possible to confirm fines damage with back flow test. If production increases afterwords, then fines damage is suspected. One of the best ways to diagnose the problem is with core flow testing that evaluates critical velocity or water sensitivity in an attempt to simulate past well events.
According to Stanley et al11, in the case of sand control completions, production may start out with low skin factors and then gradually increase as migrating fines accumulate in the completion or in the near wellbore formation. Reservoir rocks act as depth filters due to their complex and tortuous permeability. One can think of a gravel pack as a cartridge filter in front of the depth filter. In a filtration treatment, the "dirtiness" of the unit can be observed by a pressure increase on the in-flow line. The can be simarily observed in a well. To maintain production drawdown pressure must be increased when fines damage the well by making the filter (gravel pack) dirty. A gravel pack is difficult to clean because fines can be concentrated in the pack, thus resulting in rapid spending of HF acid. It is critical for live HF acid to reach the gravel-formation interface, where polymer residue or sand mixing may be a factor in trapping fines.