TESTS SHOW ORGANIC CLAY STABILIZERS PREVENT PERMEABILITY LOSS

Rick Behenna
Halliburton Energy Services
Duncan, Okla.

Laboratory tests on sandstone cores show that organic clay stabilizers (OCS) are as effective at preventing permeability reduction from freshwater exposure as commonly used potassium chloride (KCI).

Although organic clay stabilizers are widely used, only limited laboratory testing has confirmed their effectiveness at preventing permeability reduction attributable to a formation's water sensitivity. Existing test results on one class of OCS indicated satisfactory performance as a KCI replacement; however, the test procedure included only short cores and the simulated brine used for testing was sodium chloride (NaCl) at 5% concentration.1

The recent work included a series of short core and long core laboratory tests made at various salinity levels and OCS concentrations. The OCS tested is a high-concentration, non surface active, ammonium salt in a aqueous fluid.

BACKGROUND

Water sensitivity of sandstones can result in rapid and severe permeability impairment if freshwater displaces saline connate water from pore spaces of formation rock. Reduced permeability can decrease the production potential of a well.

The occurrence and magnitude of permeability damage varies among formations. Some formations exhibit virtually no water sensitivity but others show more than 100 fold permeability decreases upon contact with freshwater. The degree of susceptibility to freshwater damage is a complex combination of factors involving:

Clays present in the formation
Location of clay particles within the pore network
Initial formation brine composition.

Formations are said to be at their "critical salt concentration" when the salinity of formation brine is at a level such that any further salinity reduction will initiate permeability damage.

To counteract the effects of freshwater sensitivity, operators commonly apply salt solutions of 2% KCI to stabilize clays that will be exposed to aqueous fluids during drilling, completion, workover, or stimulation. Because KCI must be batch-mixed and have a negative environmental effect, alternatives to KCI are attractive, especially when large KCI volumes may be required.

As an alternative to 2% KCI, a variety of temporary organic clay stabilizers are used, in liquid form, at much lower concentrations than 2%. OCS compounds contain comparatively little chloride ion compared to KCI, and can be added "on-the fly" during a fracture treatment, saving mixing time and eliminating on site batch mixing equipment.

SHORT-CORE TESTING

The short core tests used Ohio sandstone and Berea sandstone cores of 2.54 cm diameter and 3.5 cm length.

The Ohio sandstone contained 2 5% illite clay and 0.5 2% kaolinite and chlorite clays.

Absolute permeability ranged from 1 to 1.5 md. Samples showed moderate freshwater sensitivity typical of many formations where water sensitivity is not a primary concern, but where the hydraulic fracturing base fluid contains a salt solution as a precautionary measure.

The Berea sandstone contained 5 10% kaolinite clay and 2 5% illite clay. Absolute permeability ranged from 210 to 395 md. Berea sandstone is a standard for laboratory core flow tests, and its extreme water sensitivity is well documented.2

The damage in Berea sandstone results from the release of migrating type kaolinite clay platelets. Exposure to freshwater induces the release. Clay particles subsequently lodge in pore throats of the rock, drastically reducing permeability.

Test procedures (Table 1) were the same in all short-core tests.

Figs. 1a and b show short-core flow test results in Ohio sandstone after exposure to 1 pore volume of freshwater and 1 pore volume of 0.2% OCS solution. Results are in terms of the ratio of permeability after exposure to test fluid compared to the stabilized permeability of the assumed formation brine before exposure to test fluid (K/Ki).

Fig. 1a shows that core permeability remains essentially unchanged after exposure to freshwater if the formation brine is 10% NaCl. But permeability decreases by 10% if the brine is 0.5% NaCl.

Fig. 1b illustrates that a 0.2% OCS solution is adequate protection at any salinity level up to 10%.

Figs. 2a and b show the more dramatic responses of Berea sandstone to freshwater. When the assumed formation brine is 0.5% NaCl (Fig. 2a), exposure to 1 pore volume of freshwater reduces permeability more than 50%. However, limited freshwater exposure has little effect if the formation brine is 10% NaCl.

In Fig. 2b, the 0.2% OCS solution holds damage to 8% in 0.5% NaCl brine and no damage in 10% NaCl brine.

Based on short core tests, a 0.2% OCS solution prevents damage in Ohio sandstone, but does not completely prevent damage in Berea sandstone under worst case conditions. Analogous, but more rigorous long core tests were performed to substantiate these findings.

LONG CORE TESTING

The Hassler cell (Fig. 3) for long core tests, allowed permeability monitoring in three sections of the core. Sections K1, K2, and K3 designate these regions.

Flow in the forward direction simulated flow of formation brine toward the fracture face. Reverse flow simulated leakoff of fracturing fluid from the formation face into the formation.

Testing was similar to short cores (see Test procedure box); however, test fluid was injected in the reverse direction, the core was shut in, and then the core was allowed to set for 15 20 hr before forward flow injection of the simulated brine.

Test results support short-core test findings that 0.2% OCS solutions provide adequate protection in Ohio sandstone.

Figs. 4a, b, and c are the results of three long core tests on Berea sandstone samples. Fig. 4a indicates the response after freshwater exposure with test fluid injection in a reverse direction.

Region K3, by being at the beginning of all flows in the reverse direction, experiences severe damage because it is exposed to more flow, 2.43 pore volumes. To pass 1 pore volume through Region K1, 2.43 volumes are passed through Region K3. Region K2 has less damage because it is exposed less to freshwater, 1.44 pore volumes. It is also likely that some salt has remained in place, reducing damage.

In Region K1 it is likely that a significant concentration of salt remained in place while only 1 pore volume passed through the region. The forward injection left some solids from the brine on the core face while reverse injection flushed solids off. When forward injection resumed, the absence of these plugging solids increased permeability in Region K1.

Fig. 4b confirms short core test findings that OCS solutions of 0.2% were insufficient to protect Berea sandstone permeability. Fig. 4c shows that protection is provided by, a 0.6% solution of OCS.

The long core tests validated results from the analogous short core tests, but provided more detail about the severity of damage within the core and the depth of damage protection that could be expected. A summary of the results from these tests is as follows:

Ohio sandstone core samples were moderately sensitive to water, while Berea sandstone was extremely sensitive. This variation makes the two core types well suited for judging effectiveness of clay stabilizers.
A 0.2% OCS solution, i.e., 2 gal/1,000 gal, provide adequate permeability protection in Ohio sandstone at brine salinity levels down to 0.5%.
OCS solutions of 0.6%, or 6 gal/1,000 gal, provide adequate protection to a depth of 15 cm in Berea sandstone, under the worst case conditions such as a formation brine at the formation critical salt concentration.
Composition of formation water plays a significant role in OCS function. Formations with relatively high salinity levels are less susceptible to damage than those containing relatively fresh brines.
Test results indicate that the required OCS concentration can vary. Laboratory tests should be performed to determine optimum OCS concentration.

REFERENCES

Himes, R.E., and Vinson, E.F., "Environmentally Safe Salt Replacement for Fracturing," Paper No. SPE 23438, SPE Eastern Regional Meeting, Lexington, Ky., October 1991.
Khilar, K.C., and Fogler, H.S., "The Existence of a Critical Salt Concentration for Particle Release," Journal of Colloid and Interface Science, Vol. 101, No. 1 September 1984, pp. 214 24.