Coiled Tubing Technology

Acid tunneling with coiled tubing significantly increases production

A technique to construct dendritic wells enhances surface area and subsequent production in carbonate formations.

Victor Espina, Melvi Guerrero and Omar Colmenares, PDVSA; Jose Diaz, Phil Rae and Gino Di Lullo, BJ Services Company

Carbonate formations form about 35% of the world’s petroleum reservoirs, yet these same reservoirs contain an estimated 60 to 70% of the world’s traditional reserves. They are found in most oil producing provinces, but are particularly common in the Middle East, Central Asia, the United States, Canada and Northern Europe. Important carbonate reservoirs are also found in Sumatra, Borneo, China, Brazil, Venezuela and elsewhere.

Like sandstones, carbonate reservoirs can be drilled and completed using conventional techniques. However, drillers often overlook a combination of interesting properties of carbonate rocks: their mechanical integrity and their high solubility in acid. Typical carbonate solubility in hydrochloric acid is greater than 95% and can often exceed 99.5%. This characteristic lends itself to some novel techniques for constructing drainage holes in such formations. Using a suitable nozzle to jet acid with coiled tubing (which also eliminates the need to make/break connections), it is possible to create holes without the use of a drill bit, Fig. 1. This article describes the jetting process and presents a case study where it was used in a field in Venezuela.

Fig. 1. An acid-jetted hole in carbonate rock.

 BACKGROUND

By adding appropriate orienting and indexing tools, multiple drain holes can be constructed by rock dissolution. Furthermore, due to the mechanical rock properties, the holes can be left “barefoot,” with no casing or slotted pipe. This is significant because it simplifies operations, reduces cost and provides maximum contact between the reservoir and wellbore.

Ultimately, it is possible to construct a well consisting of a mother bore with multiple “daughter” boreholes at different elevations, each with smaller, fractal sub-conduits resulting from acid leak-off and consequent rock dissolution. Such a configuration—a series of multiple branching, progressively smaller conduits—resembles the roots or branches of a tree and, for this reason, we refer to it as a “dendritic well.”

This type of well exhibits interesting behavior compared to a conventional well. With numerous laterals and a multitude of wormholes and expanded pores, a dendritic well provides greatly increased reservoir contact and, therefore, the possibility of higher productivity by distributing inflow across an enormous surface area.

The technique can be used to stimulate old, new, vertical, and/or horizontal wells, especially with openhole completions in formations greater than 50 ft thick. The length of the resulting tunnels is controlled by the length of the CT pipe, while tunnel diameter depends on nozzle pressure, acid strength and volume, and how many CT passes are made. Theoretically, up to four tunnels can be acid etched every 15 to 20 ft in four different directions from a vertical main borehole. The jetting process presumably creates smaller branches off of the main lateral, as evidenced by leak-off of acid beyond what would be expected from construction of the main laterals, thereby increasing near-wellbore porosity and permeability. Most probably, the tunnels are elliptical with dimensions of 3 to 4 in. wide and 10 to 20 in. high.¹

For the same volume of acid, acid tunneling outperforms a matrix acid job by a factor of six, and an acid frac by a factor of four, although such results cannot be guaranteed in all cases, Fig. 2. Furthermore, because the well is not drilled with mud, there is no mud filter cake and, therefore, little risk of formation damage.

Fig. 2. PI comparison of acid uses: matrix acid job vs. acid-tunnels vs. acid fracturing.

Mara field in the western part of Lake Maracaibo in Venezuela has about 95 active and 300 inactive wells of greatly varying production levels. One well, drilled in 1997, was producing about 100 bopd (16 m³/D) with an electric submersible pump, when the operator, PDVSA, sought a recompletion.

After initially trying a small-scale field experiment in this well, just to prove that the concept would work, PDVSA was prepared to try this new, patented hole-construction/stimulation technique in a full-scale field trial. The goal was to construct a dendritic well by creating horizontal tunnels at different depths in the wellbore, instead of conventional acid stimulation techniques such as matrix acidizing or acid fracturing.

The treatment involves pumping acid into the carbonate formation through coiled tubing and a specially designed jetting assembly, to create roughly 8.5-in.-diameter lateral holes, or “tunnels,” by dissolving the formation. (Tunnel diameters can be significantly larger or smaller, depending on acid dissolution rates.) The technique increases the formation drainage using much less acid than conventional acid stimulation treatments because of the selectivity of the process.

The process uses a special jetting tool at the end of coiled tubing. When the tool reaches the desired tunneling kick-off depth, acid pumping is initiated and the internal pressure on the coiled tubing activates a 15° knuckle to aim the tool into the formation, Fig. 3. To initiate the tunnel, about 50 bbl (6.4 m³) of 15% HCl is jetted. To lengthen the tunnel, weaker acid, typically 10% HCl, is used.

Fig. 3. Lateral, acid-jetting tunnel construction can be done in carbonate rocks with inherently high mechanical integrity and high acid solubility. A bent (knuckle-jointed, pressure-activitated) sub and special BHA does the �drilling.� Leak-off can create root-like dendritic patterns.

Typical tunnel lengths vary between 4 and 60 ft (~1 to 18 m), depending on the mineralogy and whether oil or gas is present. PDVSA has been successful in jetting tunnels nearly 100 ft (~30 m) long.

 CASE HISTORY

For logistical reasons, work could not be performed at night. So, the stimulation/tunneling required four days (about 48 hours of operations).

The job began with pulling the submersible pump and drilling an 8.5-in.-diameter, 53° deviation hole to reach a 6,840-ft (2,084-m) MD. Well DM-154 was dead after removing the pump, since there was no reservoir pressure. The resulting openhole section covered three significant formations: Maraca, at a 5,530- to 5,630-ft MD; Lisure, 5,630- to 6,170-ft MD; and Apón, 6,170- to 6,820-ft MD.

Using this acid tunneling technique, 15 distinct tunnels were created, the longest of which extended 25 ft into the formation and the shortest just 1 ft, as shown in Table 1. The combined length of the tunnels was a little over 150 ft, spanning an interval of 828 ft (252 m), which was the difference in measured depth between the bottom and top tunnels.

TABLE 1. Summary of tunnels created in Well DM-154

 RESULTS

It came as an unexpected and pleasant surprise that, after completing the operation, the well commenced natural flow to surface. Unassisted production after this unique process rose to nearly 300 bopd (48 m³/D) and 7 Mcfg/D (198 m³/D) through a 3/4-in. choke with a 600-psi (41 bar) surface pressure, almost triple the initial assisted oil production. This was the only naturally flowing well in the field at the time.

This technique can be deployed in fields that have carbonate, openhole wells; cased wells require opening a window but, provided this can be done at reasonable cost, there is no reason not to apply this approach in cased wells, also. We see considerable application for the technique, particularly, for example, in the US, Canada, Ecuador, Venezuela, the Middle East and numerous locations in Asia.

Since the initial job, PDVSA has performed 89 tunnels in 9 wells, Table 2. We have constructed 3,596 ft of tunnels using 15 and 10% HCl at an average efficiency of 0.35 ft/bbl of tunnel created, with the longest tunnel reaching 89 ft. All wells have responded favorably, and PDVSA has decided to use this technique to replace other acid stimulation techniques in this field.

TABLE 1. Summary of acid tunneling jobs performed to date

As these treatments continue, more about the process dynamics are learned, and various aspects of the jobs are being fine-tuned, including acid strength and volume, BHA configuration, pump rates, etc. Significant improvements in the technique can still be made to increase penetration rates, acid consumption and, ultimately, well performance. For example, Fig. 4 offers a snapshot of production results from an acid-tunneled well (DM-163).

Fig. 4. A production plot for Well DM-163 shows good performance with only 7 acid-created tunnels totaling 138 ft in length. The frequency measurement is a function of the electric submersible pump.

Through these results and other successes in this field, the viability of this technique was demonstrated, and we believe that it has applications in many fields, worldwide.  

¹   Moss, P. and L. Portman, P. Rae, G. Di Lullo, �Nature had it right after all�Construction a �plant root�-like drainage system with multiple branches and uninhibited communication with pores and natural fractures,� SPE 103333, SPE Annual Technical Conference and Exhibition, San Antonio, Texas, Sept. 24�27, 2006.