The permeability damage that certain reservoir rocks undergo when infiltrated by fresh water is well recognized in the petroleum industry. Two causes of damage, swelling of clays and plugging by rearrangement of indigenous particles, result from inherent properties of some rocks and can be determined by laboratory analysis. The presence, cause and magnitude of water sensitivity in most reservoir rocks can be recognized by a combination of the following techniques:Single phase permeability measurements with gas and a series of brine of decreasing salinity on extracted core plugs to determine if permeability varies with salinity.X-ray diffraction analysis to detect swelling clays (montmorillonite, mixed-layer clays and certain types of illite).Physical swelling tests to determine the actual swelling capacity of indigenous clay minerals.Microscopic examination of rock thin sections to determine the distribution of clay minerals relative to the pore system. Typical analyses are given for reservoir rocks than arenot sensitive,water sensitive from swelling clays andwater sensitive from particle plugging. Introduction Reservoir rocks that are susceptible to permeability damage during drilling, completion, stimulation and workover operations are well known in the petroleum industry. Permeability damage around a well bore can prevent the detection of oil zones in a wildcat well or lower productivity of completed wells. In secondary recovery operations, a damaged reservoir may mean the difference between economic success or failure. Research and development work on secondary and tertiary oil recovery processes must consider the susceptibility of a reservoir rock to permeability damage in evaluating the recovery process.A whole host of physical and chemical processes are responsible for reservoir damage. Some types of damage occur because of the particular mineralogical and textural properties of the reservoir rock; other types of damage occur as a result of a particular production operation.The causes of damage that are related to rock properties are:1. swelling of indigenous clays that constrict the pores;2. dispersion of indigenous, non- swelling particles, rearrangement of the particles during fluid flow, and plugging of the pore system; and3. a combination of swelling and dispersion; slight swelling promotes loosening and mobility of fine particles.Causes of damage that originate from a specific production operation are:1. invasion of solid particles from drilling mud or injected fluids;2. invasion of solid particles from oil well cementing operations;3. precipitation of salts from the reservoir brine in the zone of relatively lower pressure around the well bore;4. water block or emulsion block;5. direct or indirect effects of bacteria; and6. completion practices not suitable for a specific reservoir.Any porous medium, regardless of its inherent properties, could undergo permeability damage by conditions in the second category. The first category of reservoir damage, however, is possible only in rocks that are inherently water sensitive as a result of their mineralogy and texture. In either case, reservoir damage produces the same symptom: reduced well productivity. Before the damage can be repaired, or before damage to a similar reservoir can be prevented, the cause must be determined.The analytical methods presented in this paper were developed to recognize water sensitive reservoir rocks. The philosophy of developing this type of analysis is as follows. If damage is suspected in a reservoir, core samples can be analyzed for water sensitivity. If the analysis indicates sensitivity, then production operations can perhaps be modified to prevent additional damage. If the results of the analysis indicate no sensitivity, attention can then be focused on the production operations rather than on the reservoir rock. Present Investigation Investigation of water sensitive reservoir rocks was begun at the Marathon Oil Co.'s Denver Research Center six years ago. JPT P. 813^