Figure 1. Because carbonate rocks have high mechanical integrity and high acid solubility, acid jetting can effectively create holes without drilling.
A new coiled tubing (CT) technique creates lateral holes (tunnels) by jetting acid into the rock, which creates new hydrocarbon pathways and increases near-wellbore porosity and permeability. The recent introduction of this novel technique to the Mara field in the western part of Lake Maracaibo in Venezuela has changed the cost-benefit equation for stimulating production from the field’s significant carbonate formations.

Matrix acidizing has been the traditional stimulation technique in this field, but results were below expectation due to limited acid penetration even when sophisticated retarded acids were used in conjunction with pulsating tools. Acid fracturing is another option, but treatment cost increases dramatically because of the challenges of maintaining fracture pressure and achieving effective diversion. Nearby fields in Western Venezuela have used this technique in similar formations in openhole or cased completions with short-lived production increases that soon drop to those obtained from matrix acid jobs, at a cost several times higher.

For recompletion or remediation, one alternative to fracturing is to drill a new lateral or extension from the mother bore. Carbonate reservoirs can be drilled and completed using CT and conventional drilling or underbalanced drilling techniques, but this approach can be costly for remediation because it may involve both drilling and cleaning up any skin that forms, or the use of very light foams in low-pressure reservoirs.

Carbonate rocks have two useful properties: mechanical integrity and high solubility in acids. Typical solubility of carbonates in hydrochloric acid is greater than 95% and often exceeds 99.5%. These characteristics are well suited to the technique of pumping acid through coiled tubing and a jetting nozzle to create stable holes without drilling (Figure 1). This proprietary technique creates lateral holes, or “tunnels,” by dissolving the formation. Acid not consumed in hole construction leaks into the rock, creating “wormholes” that enhance flow while extending contact with the reservoir or existing natural fractures.

By adding orienting and indexing tools to the bottomhole assembly, we can construct a mother bore with up to four bore holes per elevation at different depths, each containing smaller fractal sub-conduits resulting from acid leak-off and rock dissolution. In theory, the technique can be used to “create” every 20 ft (6 m), 4- to 20-in.-diameter tunnels in a wagon-wheel pattern around the well bore. (Hole diameters are average and vary with the job program and rock’s acid dissolution rate.)

This increases reservoir contact by distributing inflow across an enormous surface area. Because the boreholes are not drilled with mud, there is no filter cake and no risk of formation damage.

Figure 2. Unlike traditional and CT underbalanced drilling, acid tunneling stimulates the formation as it creates holes, typically resulting in negative skin.
Field results
We have used the acid tunneling technology to treat nine wells in the Mara field; all have responded favorably. In cost and time, acid tunneling compares favorably to alternative stimulation options. This is especially true when considering resulting production improvements.

For example, one well stimulated with a conventional matrix acid stimulation achieved less than half of its expected oil production, with a payback time of 10 days. Two months later, an acid tunneling procedure doubled production to the expected level, with payback in just 6 days.

Acid tunneling can use less acid for deeper stimulation than comparable alternatives. In a recent treatment, for example, we created nine tunnels in 480 ft (146.3 m) of gross pay in two formations, with total tunnel length of 390 ft (119 m). The treatment enabled this older well to resume production above its expected potential.

Considering 32% HCl, average acid consumption was 43 gal/ft of tunnel created or 39 gal/ft (zone height), for an average cost of less than US $750 per lateral foot created for this project. Assuming rigless operations for comparison purposes, a matrix acid treatment in this field would have a similar cost, and an acid frac would cost at least double. Drilling one lateral with a CT unit to an equivalent accumulated length of all acid tunnels would also double the cost, adding an additional 25% more if done underbalanced.

More importantly, the tunneling technique offers much more opportunity for reservoir contact (Moss et al., 2006, SPE 103333). Matrix acidizing is effective only to remediate damage; in an undamaged well the maximum productivity increase that can be expected is 1.5 times. The productivity increase for an acid tunneling job with similar dimension is 3.5 times!

To compare with overbalanced drilling operations, drilled holes are usually damaged and chemically stimulated individually after drilling, using enzyme/acid combinations through a special CT tool. Theoretically, after this treatment the lateral can produce similar results to acid tunneling, but at a much higher cost.

Wells drilled underbalanced are not typically stimulated because they should have no skin damage, thus their production theoretically is 20 to 30% lower than an acid tunnel-created well bore (Figure 2).

Finally, acid fracturing results are known to outperform matrix acidizing but not always cost-effectively. Its ultimate effect on production remains limited to acid penetration and conductivity after fracture closure. New techniques and chemicals have been developed for acid fracturing in massive openhole carbonates effectively; one, known as MAST (Purvis et al, 1999, SPE 55614), has been successful in producing long-term results.

That said, acid tunneling is self-diverting, and tunnel length is limited only by the length of the CT that can safely negotiate the tunnel. The jetting nozzle selectively aims the acid in a desired direction to create a primary tunnel, and secondary dissolution carries unspent acid into the created well bore — effectively performing matrix acidizing of the tunnel. Therefore, for the same acid volume, the technique has the potential to outperform matrix acidizing and acid fracturing by factors of six and four, respectively.

Reaching the reservoir
Experience in the Mara field has shown that acid tunneling achieves best results with an accumulated tunnel length between 1 and 2 times Re (reservoir drainage radius).
Individually, the longer each tunnel the better; 60- to 100-ft (18- to 30-m) tunnels are realistic and effective. The best option is to have two opposite tunnels per elevation and at least one elevation every 25 ft (7.6 m).

For example, in the most recent treatment performed as of this writing, we treated a well with 445 ft (135 m) of drainage radius, creating 12 tunnels in 555 ft (169 m) of gross height (190 ft or 58 m with high resistivity), for a total accumulated tunnel length of 790 ft (241 m) and an average individual tunnel length longer than 60 ft (18 m).

This treatment used 1,440 bbl of acid, or about 110 gal/ft of formation height and 29 gallons of 32% HCl per foot of tunnel created — roughly half of what was required in the first acid tunneling treatment (demonstrating the evolution of the technique). The resulting production exceeded the well’s estimated potential and allowed it to flow naturally — just the second well in the field to do so. (The first was also an acid-tunneled well.) Payout was achieved in about 11 days.

All of the wells treated with acid tunneling to date have been openhole completions; cased holes would require opening a window. In some wells, this may be economical because of the opportunity to significantly improve production. Furthermore, once the casing has been bridged, the technique can create a useful sidetrack without damage to the formation.

Although the technique has not yet been used in any horizontal completions, most of the wells treated to date have had deviated trajectories, and only two have been vertical wells. We use BJ’s proprietary CIRCA CT modeling software to determine whether we can safely and effectively reach a desired depth and achieve useful tunneling.

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