WOR vs. Np for the Loma Alta Sur field CDG pilot.(Images courtesy of Tiorco)

Marcit gels, also called “bulk gels,” are high polymer concentration gels designed for reservoirs with extreme heterogeneities such as natural or induced fractures, fissures, and other multi-Darcy permeability anomalies. Marcit gel technology, developed by Marathon Oil Co., involves crosslinking mechanisms that form a continuous intermolecular network of polymer molecules.

Colloidal dispersion gels (CDGs) are named for the nature of the gel solutions. CDGs are suspensions of individual bundles of crosslinked polymer molecules, or colloids. A solution of separate gel “bundles” is formed in which there is a mixture of predominantly intramolecular and minimal intermolecular crosslinking reactions. CDGs provide residual resistance factors that are several times the residual resistance factor of un-crosslinked polymer, which is particularly useful in heterogeneous matrix reservoirs.

CDG technology was developed by Tiorco.

Used separately and in combination, these polymer gels have been applied to increase oil recovery in mature waterfloods.

Polymer gels at work

One of the applications of this technology took place in the Golfo San Jorge (GSJ) basin in southern Argentina. GSJ is the most prolific basin in Argentina, having produced more than 3 billion boe and approximately 3 Tcf of gas since the initial discoveries in 1907.

The Comodoro Rivadavia formation, a normally faulted fluvially dominated sandstone, is the most important hydrocarbon reservoir in the El Tordillo field, and was the objective of the sweep improvement pilot.

El Tordillo was discovered in 1932 and was operated by YPF, Argentina’s national oil company, from 1932 until 1991. The Consortium El Tordillo, in which Tecpetrol is a partner, was appointed operator in 1991. The field lies on the north flank of the San Jorge Basin in the Province of Chubut, approximately 31 miles (50 km) southwest of the city of Comodoro Rivadavia. More than 1,200 wells have been drilled in the 45-sq-mile (117-sq-km) field.

The pilot area of approximately 80 acres consisted of two adjacent patterns with 10 offset producing wells.

In November 2005, two candidate injection wells received 15,000 barrels and 12,000 barrels of Marcit gels, respectively. Both treatments were “bullheaded” into four to five layers of the Comodoro Rivadavia formation at polymer concentrations ranging from 1,500 ppm to 4,000 ppm.

Seven months after the Marcit gel treatments, the operator began a CDG pilot in the same two patterns, injecting a total CDG pore volume of approximately 18% (about 500,000 bbl) in the targeted layers of the Comodoro Rivadavia formation. CDG injection began July 1, 2006, and was completed April 30, 2007. A chemical injection plant was connected at a point upstream of the two injectors so that both wells could be treated simultaneously, with the flexibility to vary the rate and polymer gel concentration in each well.

Based on well tests every one to two weeks, the operator monitored oil and water production in each of the 10 offset producing wells. Two additional producers in the vicinity of the first line of producing wells also showed a positive response. As expected, the oil response in each of the producing wells was not uniform in timing or magnitude. No new producing wells were drilled in the vicinity of pilot area, and no workovers were performed on neighboring injection wells.

Figure 1 shows actual oil production versus the projected oil production based on the exponential decline for similar production wells. The operator estimates incremental oil reserves of approximately 314,400 bbl (50,000 cu m) in incremental oil reserves as of December 2007. Current incremental oil production is about 189 bbl/day (30 cm/d).

Note that Figure 1 includes extended periods of production interruption due to normal mechanical problems with one or two producers. Removing these events would result in additional incremental reserves.

In a second case, polymer gel technology was applied in the Loma Alta Sur field, operated by Repsol-YPF, in the province of Mendoza in Argentina’s Neuquén Basin. The producing interval in the Loma Alta Sur field is the Grupo Neuquen, which is divided into four stratigraphic sequences: V, VI, VII, and VIII. Each sequence contains multiple sand packages consisting of lenticular channel-fill deposits of medium- to fine-grained sandstones that are typical of a meandering river depositional environment.

Exploitation of the Loma Alta Sur field began in 1990 with the LAS-X1 well. Today, the field includes 72 producing wells with an average depth of 1,476 ft (450 m). The Loma Alta Sur structure is a north-northeast to south-southwest anticline approximately 2 miles (3.5 km) long and 0.6 miles (1 km) wide with dipping flanks of 20° to 25°.
Tiorco implemented the CDG pilot in the LAS-58 injector, an irregular 57-acre pattern with six “first line” producers and four “second line”
producers. Water injection began in 2002. Injection was followed by rapid water breakthrough resulting from the combined effects of reservoir heterogeneity and 30 cp oil at reservoir conditions.

CDGs were implemented in two phases in the LAS-58 pattern. In Phase 1, 186,000 bbl of CDG were injected between July 14, 2005, and Feb. 2, 2006. Phase 2 included a 192,200 bbl CDG injection between April 11 and Oct. 31, 2007.

No significant workovers were performed on the LAS-58 injector or any of the offset producing wells during the CDG injection phases. No new wells were drilled in the LAS-58 drainage area.

During the Phase 1 and Phase 2 CDG injections, there was no polymer breakthrough noted at any time. The WOR trend before the Phase 1 CDG was projected to an economic limit (WOR of 50). Based on a parallel trend line projected from the latest data point to the same economic limit, the operator estimates total incremental oil of 389,968 bbl. This represents the actual incremental oil production as of October 2007 and the projected oil production until the economic limit.

The current evaluation represents 2.9% of the original oil in place in the LAS-58 drainage area. The ultimate incremental oil realized is almost certain to increase with time, particularly due to the expected effect of the Phase 2 CDG pilot.

Traditionally, aluminum is used as the CDG crosslinking agent. In the Loma Alta Sur field, however, trivalent chromium was the multivalent ion of choice because of the chemistry of the injection water. Chromium (III) gels have a long and successful history in gel applications, but at minimum polyacrylamide polymer concentrations of 0.3% (3000 ppm), forming a three dimensional molecular network termed “bulk gel” described above. The unique feature of the Loma Alta Sur CDG pilot is the use of chromium triacetate at polymer concentrations of 0.03% to 0.06% (300-600 ppm).

The changing face of IOR

Chemical IOR has historically been applied in relatively new waterfloods. These case histories, however, demonstrate that polymer gels can significantly improve oil recovery efficiency in mature waterfloods.

The availability of brine and temperature-tolerant gels provide the opportunity to design fit-for-purpose gel applications based on the rock and fluid properties of a given reservoir.

Operators in Argentina receive a net price that is about 45% of the average world oil price due to special federal taxes on crude oil sales. The development cost for each of these case histories is approximately US $4.00 per incremental barrel.

The El Tordillo pilot has been significantly expanded, and the operator of the Loma Alta Sur field is evaluating additional CDG treatments.

Editor’s Note: The case histories described above are summarized from SPE 113320 and 113334 presented at the 2008 SPE Improved Oil Recovery Symposium held in Tulsa, Okla., USA, April 19-23, 2008.

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