Stimulating horizontally drilled producing gas wells is a necessity for operating companies as their output is regularly impacted by either drilling damage or lower than expected reservoir quality. Stimulation, however, presents a considerable challenge as wellbore accessibility is difficult in wells drilled in deeper formations with relatively low permeability and HP/HT characteristics.

Alternative techniques, based on hydrajetting technology, have been applied by Saudi Aramco on a number of horizontal gas producers. The results so far have encouraged the company to push ahead with work on enhancing its ability to effectively stimulate complex and difficult-to-access multilateral completions.

Conventional perforating and stimulation methods such as bullheading acid at high pressure and rate conditions and performing coiled tubing (CT) acid washes had not been as successful as expected when applied in single and dual-lateral completions.

This was not entirely surprising given the difficulty of placing treatment fluids in targeted zones needing stimulation; along lengthy openhole horizontal completions; and in the presence of thief zones, washouts, and highly heterogeneous reservoir conditions. The problems associated with initiating and extending hydraulic fracturing treatments in these wells, drilled in some of the tightest formations and perforated with conventional methodologies, led to the search for new methods of overcoming the high fracture initiation pressure experienced.

Saudi Aramco decided to field test a hydra-jetting tool for the first time in a vertical gas producer with reservoir characteristics similar to other wells where attempts to perform proppant hydraulic fracturing treatments had resulted in premature screenouts. It also was used to perform proppant and acid stimulation treatments in a number of underperforming vertical and horizontal openhole producers, with excellent results.

Breaking new ground

Hydrajetting itself is not a new technology – its advantages began to be recognized and developed in the late 1950s.

The technology applies a high-velocity stream of fluid carrying small abrasive particles to create slots along the horizontal section of a well prior to performing pinpoint acid stimulation treatments. The acid is displaced in the zones most in need of stimulation, resulting in improved well productivity.

A typical bottomhole assembly run on CT and used during the operation comprises a hydrajetting tool, a CT connector, a motor head assembly, an anchor to prevent axial movement, a multicycle incremental tool to set nozzles precisely on target, and a jetting sub with multiple nozzles.

The jetting velocity of the type of tool used on Saudi Arabia's gas fields ranges from 6,100 m/sec to 9,100 m/sec (20,000 ft/sec to 30,000 ft/sec) and exerts a pressure of approximately 4 million psi. The tool is designed to withstand differential pressure greater than 10,000 psi and has a temperature rating of about 177°C (350°F).

This chart shows a comparison of hydraulic fracturing treatments in wells with similar reservoir characteristics. Wells D, E, and F were perforated with conventional perforating guns and charges, while Wells A, B, and C were slotted using a hydrajetting tool. Well B is in both sides of the plot because, as was also the case in Well D, no adequate injectivity was reached during calibration tests, so the jobs were aborted. The plot shows that in the slotted wells the treatments were implemented per design, while the opposite was true in all of the oriented perforated wells. (Images courtesy of Saudi Aramco)

Field trials

In the first field trial, the hydrajetting technology was tested in a vertical exploration well drilled in a tight sandstone reservoir to a depth of more than 4,900 m (16,000 ft). The well required hydraulic fracturing stimulation and had similar reservoir characteristics to other nearby conventionally perforated wells. In the conventionally perforated wells, the hydraulic fracturing attempts had to be aborted when the maximum allowable downhole pressure was reached either before initiating a fracture or by screening out prematurely early into the treatment.

A hydrajetting tool was used to create long large-diameter slots using an abrasive jetting slurry, which reduced the fracture initiation pressure. The jetting-induced slots achieved deep and clean penetration with a minimal crushed zone effect as the jetting slurry cut through the formation and circulated debris out through the CT annulus. A proppant hydraulic fracturing treatment was then performed. The well flowed at a post-fracture commercial gas rate and opened the door for further technology applications.

After this test, Saudi Aramco undertook another trial on a highly deviated well drilled into a tight sandstone reservoir to a total measured depth of 4,560 m (14,960 ft). The well was perforated using conventional perforating guns ahead of a planned proppant fracturing treatment, but after conducting an injectivity test and reaching the maximum allowable surface treatment pressure of 12,000 psi at a maximum injection rate of only 4 bbl/min, the planned job was aborted.

A hydrajetting approach was used with the intention of overcoming the drilling-induced stress, reducing the near wellbore pressure effect, and improving wellbore communication to the reservoir. Eight zones were slotted and the planned proppant fracturing treatment was implemented, resulting in a post-fracture gas rate of 25 MMcf/d.

Hydrajetting technology also was applied in a highly slanted well completed with dual casing strings. A number of highly slanted cased gas producers in Saudi Arabia have been perforated using conventional perforating guns run on wireline or more recently on CT. However, both approaches have proven challenging and costly.

This chart compares pre- and post-stimulation rates and flowing wellhead pressures for wells in which hydrajetting technology was used.

When using wireline, the most common problems encountered are associated with the yo-yoing effect experienced by the cable due to the common tension changes found in the high-angle or dogleg sections, which often result in stuck or stranded cable. In deep wells, the wireline cannot reach the targeted zones, so CT is normally used. But this option is time-consuming and costly.

The hydrajetting approach was implemented in Well K, a gas producer drilled with a total MD exceeding 4,570 m (15,000 ft) and a 73-degree angle. A 27-m (90-ft) net pay zone was then slotted with a hydrajetting tool after completing 12 slotting stages in record time. A CT acid wash was performed, and the well was put in production at an initial gas rate of 20 MMcf/d with a flowing wellhead pressure of 2,500 psi.

The hydrajetting application also resulted in an estimated 30% cost saving over the cost of perforating using conventional guns run on CT.

Technical information supplied courtesy of Saudi Aramco.

Pinpoint stimulation treatment

Saudi Aramco has been so encouraged by the results so far that it is planning to use a hydrajetting tool in tandem with a fiber optic-fit CT capable of recording live pressure, depth, and distributed temperature data in a multilateral openhole gas producer.

The expectation is that by combining multiple technologies it will be easier to identify and access each targeted lateral so pinpoint stimulation treatments can be implemented. The ability to monitor the differential pressure across the jetting nozzles should help control the pumping rates better and avoid the splash-back effect that is known to occur when jetting fluid creates a cavity that directs the fluid back with high velocity into the jet body.

Significant operational efficiencies have been achieved in hydrajetting operations since its first implementation, and the technology application has now become common practice at Saudi Aramco.

Overall, the company says the application of hydrajetting technology in multiple settings has helped to effectively stimulate vertical and horizontal gas producers, with the results showing significant improvement over those achieved in offset wells with similar reservoir characteristics and treated using conventional stimulation methods.

The technology also has proven effective in achieving successful proppant fracturing treatments in tight sandstone formations where unsuccessful or disappointing results were achieved previously, and it has enhanced significantly the ability to effectively stimulate targeted zones along horizontal sections in openhole completions.