Gravel packs completions are widely used today on unconsolidated formations worldwide, as to provide sand control, and, as such, behaves as a filter. A filter is subjected to be plugged by fines, (heavy) hydrocarbons, scale depositions, among others, which may result in a considerable drop of production, or even total lack of.
The synergism of coiled tubing rotating, cyclical jetting tools, and adequate acid systems has demonstrated great results in Northeast of Brazil. The cyclical jetting tool acts to disaggregate plugging particles from the gravel, and the acid acts to chemically dissolve acid soluble plugging materials. The acid system utilized is quite unique and simple. The method consists of pressure cyclically jet solvent and a small amount of a specially designed acid system, to clean up the gravel pack. No acidic flushes are used ahead or after the acid system, because the acid system is a high pH one (2 - 3), and uses an organophosphonic acid to control the dissolution of carbonates, and, therefore, controls the precipitation of insoluble fluorides. This way, release of calcium ions is less likely to occur. The main acid system is hydrofluoric (HF) based, generating HF on a controlled fashion, using much less hydrochloric acid (HCl) when compared to regular HF acid systems.
This paper describes three gravel packed wells treated with this single step acid system. These wells were very similar, and used the same acid recipe However, the first well was treated with a rotating pressure cycle jetting tool, and the remaining two wells used a different pulsation tool. All wells showed production increase, but the first well yielded much better results compared to the last two. Results from these three treatments will be presented and compared as towards the effectiveness of coiled tubing tools deployed.
The three (3) wells under discussion were drilled in the same offshore field, at the Potiguar/Ceará basin, Northeast Brazil. Petrobras started to develop this field in early 80's. Wells from this field have a tendency to produce sand even during drilling. The formations are unconsolidated turbidities, and the permeability in some wells may be as high as 2.5 Darcies. Wells in this field are completed with gravel packed screens. The first well treated, A-3, had a steep production decline and had dropped to zero a few months prior to the treatment. Due to the success of A-3's treatment, the operator decided to apply similar treatments to two additional wells in the same reservoir. However, as the original jetting tool used on A-3's treatment was not available at the time required to perform the additional 2 well treatments, it was decided to run a different jetting tool. The way each tool works will be generally described later in this paper.
The treated formation has viscous oil in place, with approximately 40 cp, with strong potential to form emulsions with water. As the screens on these wells were aged, there were some concerns about damaging them during exposure to acid and possibly uncontrolled jetting forces.
The oil production decline on wells from this field is attributed to fines plugging the gravel and screens, and deposition of heavy hydrocarbons from the viscous oil in place. During the planning phase, concerns over corrosion issues, as well as mechanical damage to the aged screens of the candidate wells were raised and dealt with. The acid sytem choosen was one only requiring small volume, with no need for preflush or postflush, contact time between the acid and the screen would be reduced, therefore minimizing corrosion. The tools chosen were sensitive to pump rates, a factor which could influence the amount of energy impacting the screens.