A recurring theme in this column is well stimulation, as it is such a crucial aspect of oil and gas production enhancement. Another recurring theme is that “old” or older technology can become forgotten, or passe, but can also become “new” again.
One such general area of well stimulation application is the delivery of stimulation technologies to distances deep in the formation, whereby production from new wells as well as existing wells can be enhanced beyond that possible with an initial completion or with a remedial stimulation treatment. There are opportunities that especially exist in hydraulic fracturing, including in unconventional and shale formations, and in stimulation of wells completed in carbonate formations.
Hydraulic fracturing. Of course, hydraulic fracturing in new wells and refracturing in older wells, where applicable, are “long distance,” deep into the reservoir, stimulation methods themselves. But what about additionally delivering production enhancement additives via fracturing to improve well production response? There are two general methods that are designed to do so.
One is with proppant additives, such as lightweight proppants added to the overall proppant loading that are designed to extend effective propped fracture length and thereby increase fracture conductivity. Two examples are SUN OMNIPROP ®, from Sun Specialty Products, and CARBOAIR, from CARBO. SUN OMNIPROP is an advanced thermoset nanocomposite bead with near-neutral buoyancy, high strength, and high temperature stability. CARBOAIR is a high-transport, ultra-low-density ceramic proppant developed for slickwater (low-friction reducer fluid) fracturing. Other examples of enhancement additives are those that control proppant flowback, such as fiber or malleable and oddly shaped solid materials that can keep proppant “trapped” within the fracture, maintaining greater conductivity over time.
The other general method that has merit, but which has seemingly diminished in interest, is the use of proppant-conveyed production enhancement chemistries. In the early 2000s, the former BJ Services (now Baker Hughes), developed their SorbTM line of products. These were developed with proppant-size, highly porous, diatomaceous earth as a substrate infused with inhibitor chemistries, such as for scale, salt, paraffin or asphaltenes. Sorbs are added to proppant stages in low percentages, so that they are delivered just as deep as the propped fracture extends, and then slowly release inhibitor during production, providing early time and longer-term control of unwanted depositions. CARBO also markets their GUARD line of products, which are engineered ceramic proppants similarly infused with chemical inhibitors, such as for salt or scale control.
A newer addition to this category of proppant-delivered chemical treatments is XLGUARD from Hallux Talon. XLGUARD utilizes the same engineered, porous ceramic proppant, to deliver proprietary, slow-release, polymer-specific enzymes throughout the full proppant packed fracture to accelerate gel-based frac fluid cleanup and enhance production.
Perhaps utilizing enzymes that generate acid in low concentration would be a creative option for additional, deep stimulation of carbonate-containing, unconventional formations. Also, combining multiple proppant-delivered chemistries should be a greater consideration, which could be an option to enable more productive wells and possibly even smaller fracture designs.
Carbonate stimulation. Standard stimulation of wells completed in carbonate formations is either by conventional, propped hydraulic fracturing, acid fracturing without proppant, or matrix acidizing. With acid fracturing, high-strength acid, hydrochloric (HCl) or HCl–organic acid blends are pumped at high rates above fracturing pressure. The acid is typically viscosified one way or another, to slow reaction and control leakoff into the matrix formation during injection, thereby propagating and extending the created fracture. Acid etches the walls of the created fracture, so that when the fracture closes, asperities generated along the fracture wall support a highly conductive fracture channel connected to the wellbore.
With matrix acidizing of carbonates, acid, also often viscosified or retarded chemically, is simply pumped below fracturing pressure, creating branched wormhole-like channels into the formation several or more feet.
Stimulation methods in carbonate formations have the same objective – creating infinitely conductive, small-diameter channels as deep into the formation as practically possible. But what about creating large-diameter tunnels? There happens to be an existing technology that does just that and has a history of success—acid tunneling—which was developed earlier this century and gained popularity in Latin America, initially with successes in Venezuela, then in southeast Asia and the Middle East, where massive open-hole completions in carbonate formations are prevalent. It was developed by the former BJ Services, as their StimTunnel technology, which is still available through Baker Hughes, but now somewhat forgotten.
Acid tunneling is a method for effectively targeting placement of HCl acid into open-hole carbonate formations, and creating tunnels that can vary from a few feet to perhaps 60 ft or more. The longest tunnels created with this method have been over 120 ft. The technology utilizes a special tool, conveyed by coiled tubing, to initiate entry and then extension into the formation, all the while jetting the highly reactive acid at high pressure to dissolve away rock and, in effect, “drill” a large, wide-open tunnel.
Acid also leaks off into the rock matrix during the operation, creating secondary channels or “branches” that feed into the created tunnel. A memory inclinometer gauge also can be included in the tool string to track tunnel trajectory and orientation. After one tunnel is created, the coiled tubing and tool can be pulled out, and re-entered multiple times at different locations to create additional tunnels. Acid tunneling is a clean process, since water, dissolved calcium chloride salt, and carbon dioxide are the only byproducts of the acid reaction in carbonate rock. There is no need for large footprint surface equipment, either. A coiled tubing unit, acid tanks, and acid pumping unit are all that are needed.
These “long distance” delivery methods for enhancing well stimulation and production are not universally applicable, but they are applicable for more than they are given consideration. In multi-zone fracture completions in unconventional wells, minor improvements in well clean-up or extended inhibition of scale or salt deposition can be significant with respect to production rate enhancement, especially early in well life. And in open-hole carbonate completions, acid tunneling should not be forgotten, as an already proven stimulation method.
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