Low risk, high rewards

In the competitive stimulation market, new technologies must not only outperform conventional technologies, but more importantly do so more quickly and more cost effectively.

In the pursuit of stimulating wells and turning to more untapped horizons, such as deeper, hotter wells in unconventional plays, companies are turning to advanced chemistry and materials science to bring about new solutions.

One company embracing this challenge with in-situ generated stimulation technology is StimStixx Technologies. The company generates HCI and/or HCI/HF acid in situ via a tool which is delivered to the zones of interest by e-line, wireline or slickline. The company’s acidising technology produces acid gases downhole rather than applying liquid acids through the wellbore which comes with a slew of concomitant operational and health and safety challenges.

Results to date have shown the following improvements post stimulation:

• Improved production (ranges 40 – 300% +).
• 60% reduction in cost to clients.
• 50% reduction in job time.
• 50% reduction in job footprint.
• 90%+ reduction in CO2 footprint.
• Greatly reduced safety risks in comparison to conventional liquid-based stimulations.

The tools are available in two basic types:

• Single stick tools: are utilised for short treatment intervals.
• Stackable sticks: are multiple co-joined tools utilised for longer intervals and multiple acid stimulation applications.

Unlike conventional acid deployment methods, acid is not delivered to the formation in a liquid form. Instead, hot acid and diverting gases are created at the zone of interest by the ignition and combustion of proprietary chemistry contained within the tool.

The key technical advantage of generating an acid gas downhole is that the effective permeability of a gas compared to a liquid acid can be up to 10 times higher. Therefore, the acid gas generated has the potential to precisely contact a higher surface area within the formation, allowing it to penetrate further into the wellbore, rather than encounter the internals of the well whilst travelling from the surface.

Acid gas that is generated downhole is accurately delivered to the formation in a heated state. Acid in a heated gaseous state enhances the chemical reaction kinetics, allowing the quick dissolution of pore plugging debris such as scale or reservoir materials such as carbonates. A key challenge with conventional aqueous-based acid systems is stabilising the acid during the application to minimise its interaction with wellbore tubulars. Without the inclusion of acid inhibition chemistry, the stabilisation of liquid acid is a major problem that the technology avoids. The conventional methods of liquid acid deployment create wear and tear on tubulars due to the corrosive nature of the acid, often resulting in the need for costly pickling operations. Equally, pumping a reactive liquid acid from surface can often reduce the active concentration of the acid as it reacts with other materials in the wellbore prior to it reaching the treatment area. Generating an acid gas downhole can eliminate the need for inhibition chemistry and tubing pickling, therefore preserving the acids activity and reducing the potential for washing debris from the tubing into the perforations. Equally, stimulation can be effectively conducted using lower acid concentrations.

The technology eliminates the need for swabbing (fluid removal) spent and unspent acid back from the wellbore to surface with typical flow back showing a pH of between 6 – 7. The swabbing of unspent liquid acid is conducted so as to reduce the corrosive effects of unspent acid. This reduces rig time and eliminates the cost of containment, disposal of spent acid, contaminated solids and effluent. The technology eliminates the need for bulk acid transportation, iron mixing tanks, pump trucks, swabbing trucks, shower units, coiled tubing units, flow back equipment and storage tanks associated with conventional acid jobs. This greatly simplifies the logistics, safety, cost and speeds up the acidising process and the amount of well downtime. The technology is available in both a standard HCI and a HCI/HF blend in line with conventional treatment ratios. Conventional HF applications have been phased out of use in many jurisdictions due to the safety issues associated with transportation, storage and pumping. However, there are no such safety issues associated with running this technology. The deployment of this technology only requires one- or two-man crews who work with E-Line, slickline and wireline companies to run our tools in the well bore and provide electrical power.

Case studies

North Sea

An operator had a field of mature gas producing wells in the North Sea with steadily declining production. The wells were drilled in the late 1980s in a low permeability sandstone, and some were hydraulically fractured. The decline was related to natural production decline, scale formation and fines migration into the near wellbore. The well presented here had a 5 in. production liner and a 4 ½ in. production tubing. The production liner was perforated over an interval of 155 ft. It was known that most wells in the field had accumulated scale and other deposits during their active lifetime and that the entire perforated interval was likely not accessible.

As part of a pilot project to investigate alternative stimulation techniques, the operator acidised the wells by deploying a downhole tool that generated acid gas in-situ. A primary driver for stimulating with this technique was the ability to safely stimulate the intervals with mud acid (HCI/HF) where historical treatments in offset wells in this field responded positively. Due to the higher risk to personnel when handling live HCI/HF acid on surface, treating with conventional mud acid was deemed too risky and was no longer permitted. Another secondary driver was the cost-effectiveness due to simplified logistics. Acidising conventionally would have required vessels, lifts and deck space which would have been cost-prohibitive and logistically challenging.

The tools were deployed via slickline using a CCL to determine the depth. The dispersion tube was placed directly in front of the zone of interest and once the tool was at depth, a current of approximately 1.5 A was applied from the shooting panel to the tool which ignited the chemical in the chemical carrier to produce acid gas in situ. Four main benefits of the technology were observed post the application:

• A safe manner in which to to stimulate using HCI/HF.
• Sustained well uptime and increased production.
• Simplified logistics.
• Cost-effective treatment.

The pilot project was deemed successful, and the pressure build-up rate showed a marked improvement with an increase in uptime of the well, improved flow with an overall increase in production of 38%.

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