Assessing Formation Damage From Migratory Clays in Moderate Permeability Formations
- Paul R. Howard (Schlumberger) | Jerald J. Hinkel (Hinkel & Associates LLC) | Nita Moniaga (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium and Exhibition on Formation Damage Control, 15-17 February, Lafayette, Louisiana, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.6.1 Open hole/cased hole log analysis, 1.4.3 Fines Migration, 5.4.1 Waterflooding, 5.1.1 Exploration, Development, Structural Geology, 1.8 Formation Damage, 5.1 Reservoir Characterisation, 6.5.4 Naturally Occurring Radioactive Materials, 3 Production and Well Operations, 1.6 Drilling Operations, 1.6.9 Coring, Fishing
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As new reservoirs are developed and formations are exposed to various fluids, clay swelling and fines migration can cause serious damage. Clay stabilizers are generally added to mitigate these problems. However, the performance of formation stabilizers is not always properly assessed in the traditional analyses of formation samples. In general, laboratory testing with formation material is performed to define the best additive and concentration. Incorrect choice of stabilizer additives can result in severe damage from swelling and mobile formation clays. There appears to be no standard method for conducting formation stabilizer evaluations, therefore, it is difficult to make meaningful comparisons. Test methods based on the capillary suction time (CST) test or hot rolling lack any theoretical basis and may not show subtle effects.
A new test protocol incorporates several significant improvements over the traditional core flow methods. It considers important factors such as the critical salt concentration and the critical flow velocity. A key improvement is that the proposed method closely mimics pumping and cleanup operations actually encountered in field operations. The new test method is able to distinguish performance differences based upon both stabilizer chemistry and concentration.
Testing results from a moderately sensitive Berea core yielded a surprising finding: well-known organic stabilizers were only slightly more effective than de-ionized water, whereas inorganic salts were quite effective, even at low concentrations. Further, confirming findings by others, our results showed that the Berea core used in this study required no stabilization at 270°F.
As new reservoirs are developed and formations are exposed to various fluids, clay swelling and fines migration can cause serious damage. Damage from migrating clays can be quite extensive and clay stabilizers are generally added to mitigate these problems but the performance of formation stabilizers is not always properly assessed in the traditional analysis of clay effects in formation samples.
One method commonly used to evaluate a formations sensitivity to water is the capillary suction time (CST) test. This test was developed in the 1970s to evaluate dewatering of sludges (Scholz 2005). It was adapted to the oil field originally to evaluate water-sensitive (clay-rich) shale formations for drilling. The test has shifted to evaluating water sensitive reservoirs, both to anticipate the sensitivity of the formation and to evaluate potential clay stabilizers. The test is commonly performed by preparing a slurry of sized reservoir material (typically 20/40 mesh), adding it to the test fluid, and pouring the slurry into the CST test cell. Water is drawn out of the slurry by capillary suction into the filter paper. The flow is limited by any ?filter cake? of material formed on the filter paper and swelling change or dispersion of clay materials in the fluid phase (water). These can be released from the surface of the formation particles and form a filter layer to resist water movement into the filter paper.
The CST test has several limitations when evaluating reservoir rock-treatment fluid interactions. There are several effects related to the simple test method. The test will obviously be most sensitive to fine particles. Different studies can use quite different sizing techniques for CST samples from the grinds in this study (200 mesh) to the very fine grinds used in others (Weaver et al. 2011). This can lead to very different results for the same materials (Berea sandstone cores). Additionally, poor control of size distribution can lead to variability in test results. Typically several replicates are run, but results can vary widely from test to test.
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