ABSTRACT: During the previous 20 years, numerous experimental studies have documented the dramatic weakening of water free North Sea chalks when exposed to water. Recent laboratory compression tests provide compelling evidence that water weakening also occurs in poorly-consolidated, weakly-cemented, detrital, siliciclastic (e.g., quartz and feldspar) rocks as well as chalks. A series of experimental studies was undertaken to identify the mechanism of water weakening in rocks, including many triaxial compression tests and uniaxial strain compression tests with fluid injection. Experimental evidence indicates water weakening requires the presence of chemically active water molecules and intimate chemical contact between liquid water and the rock matrix. One model that appears to be consistent with experimental results is "stress-enhanced solubility" of matrix grains subjected to high grain-to-grain contact stress. Quantitative models indicate this to be a viable water weakening mechanism.
The consequences of water weakening are many, including a huge increase in rock compressibility, loss of strength, and reduction in matrix permeability. Consequences of water weakening for reservoir management include increased reservoir compaction and sea floor subsidence, reduced well bore stability, and increased casing failures. The water weakening of detrital rocks is less dramatic than observed in chalks, but it may easily account for the often observed well bore instability, production of solids, and casing failures that frequently accompany water breakthrough in hydrocarbon production wells. Water weakening also may contribute to poorer hole quality frequently observed in boreholes drilled with water base muds as compared to boreholes drilled with oil base muds.
People have observed and studied the weakening effects of water on glass, rocks, and other ceramic materials for many years. Many mechanisms have been proposed to account for the water-weakening phenomenon. Suggested mechanisms include reduction in surface free energy and intracrystaHine gliding (Boozer et al. 1962), dissolution and pressure solution (Newman 1983), capillary forces (Brignoli et al. 1994, Papamichos et al. 1996), and bond rupture at crack tips (Parks 1990). Recognizing and identifying the mechanism(s) of water weakening of sedimentary rocks is an important first step toward developing technology to avoid or mitigate the effects of water weakening.
