Acid stimulation, or acidizing, is the oldest technique for stimulating wells. It predates hydraulic fracturing by over 50 years. The first acidizing treatments were in 1895, when the Standard Oil Company used concentrated hydrochloric acid (HCl) to stimulate brine wells producing from a carbonate formation at their Lima, Ohio, refinery.
These treatments were very successful, but acidizing of oil wells did not proliferate until the early 1930s, when acid corrosion inhibition chemistry was discovered from an 1845 steel industry patent. With that, acidizing wells drilled in carbonate rock took off, and many companies quickly started acidizing services, including the Halliburton Oil Well Cementing Company (HOWCO), the Byron Jackson Company (later BJ Services), and the newly formed Dow Well Services (Dowell).
In the late 1930s, Halliburton introduced “Mud Acid,” a mixture containing 12% HCl + 3% hydrofluoric acid (HF) for removing drilling mud damage. That formulation is still standard, as are other HCl + HF and organic acid + HF mixtures, formulated more scientifically, from the early 1960s into the 1980s.
For decades, creative advancement in acidizing has taken a back seat to hydraulic fracturing, even with acid fracturing (without proppant) in carbonates, a method often more effective and less expensive than conventional, propped fracturing. But there are opportunities for new applications, especially in unconventional and deepwater completions.
Unconventional horizontal completions. In unconventional wells, “acid” is really only HCl. Its uses are essentially limited to carbonate scale removal in existing wells and acid “spearheads” to break down perforations in new wells to reduce frac fluid and proppant stage injection pressures.
But what other potential acid applications could there be in long, horizontal, multi-zone fractured completions? One that has merit is in restimulation. In that regard, refracturing is on the rise, but mostly as a repeat of what was employed initially, with standard propped fracturing.
However, many unconventional wells are completed in formations containing significant carbonate content. So, why not restimulate with fluid that reacts with carbonate? An opportunity is with acid-generating formulations of mild pH that enable deeper penetration of reactive fluid into the formation. Fluid Energy Group, for one, provides stimulation fluids that are organic compounds complexed with hydrogen chloride. Most are near neutral in pH but generate strong HCl acid at downhole temperatures. Such solutions can be pumped below fracturing pressure, to stimulate existing fractures, as well as enter microfractures, opening new flow channels into main fractures, enhancing production.
And how best to apply reactive fluid systems across long horizontal completions with as many as hundreds of fracture entry points? One option comes from Comitt, with their Excite System, a fluid placement tool through which stimulation fluids can be injected at sufficiently high rates into pre-selected sections along the horizontal interval.
Deepwater sandstone completions. Acidizing deepwater oil and gas wells is generally successful, such as with HF acid for removal of sandstone formation fines that cause plugging near wellbore. But treatments are based on long-standing procedures with multiple steps and acid formulations originally developed for onshore wells with shorter production intervals. There is a need for simplified procedures specific to very deep, often high-temperature wells completed in deep water.
Apart from cost, there are impediments to taking greater advantage of sandstone acidizing potential. Wells with high water production are avoided. Multi-step HF acidizing procedures can be very time-consuming, leading to operational challenges. Well production impairment is often due to multiple causes, including fines, scale, and organic (wax or asphaltene) deposition. But acid can only address fines (HF) and carbonate scale (HCl), not barium sulfate scale or organic deposition.
However, these concerns can be addressed with more creative solutions. HF treatments can be simplified, using single-step formulations, eliminating the multiple acid preflush and overflush stages that are included before and after HF injection stages, respectively, in standard designs. Certain combinations of HF with organic complexing agent or scale inhibitor chemistries can be pumped in a single step and can further be combined with chemical additives to reduce water production while stimulating hydrocarbon production. Such formulations, absent of strong acid, are also more easily inhibited against corrosion.
Simplified HF acidizing procedures also can be successfully combined with other treatments to remove acid-insoluble scales and organic deposits. Including solvents that remove wax or asphaltene in an acidizing procedure is common practice. But when an acid-insoluble scale, such as barium sulfate, is present along with fines plugging, which is the most common production impairment mechanism in sandstones, HF acid is ruled out, and fines are not treated.
High pH dissolvers that can at least partially remove barium sulfate are not compatible with acids, especially HF. But successful treatments in the Gulf of Mexico, using a high pH dissolver followed by HF to remove sulfate scale and fines, have been pumped. The two treatment steps are separated by an innocuous spacer fluid stage. Modification to a moderate pH dissolver and a mild pH, single-step HF formulation would be an even more desirable simplification.
There is considerable unrealized potential with acidizing. So, there is opportunity for those in drilling, completion, production and stimulation roles to explore past and present acidizing chemistries and methods, and to think creatively about new applications in old and new wells, conventional and unconventional, onshore, and offshore. With increased knowledge and imagination, exciting possibilities are out there.
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