The Effect of Water Chemistry on the Laboratory Compression and Permeability Characteristics of Some North Sea Chalks

Abstract

Summary Chalks or chalklike carbonates have been reported to fail when returned to or near in-situ reservoir stress in the laboratory. Failure of chalks during reservoir production tests has also been reported. This paper presents laboratory results showing that the compaction behavior of the chalks is dependent on the water chemistry of the saturating fluid. The chalks were found to be strong and stable under hydrostatic stress when saturated with oil. Injection of water into the oil-saturated chalks, at stress, caused immediate compaction. The weakening of chalks by water increased the compressibility values four to eight times the oil-saturated values. The chalks resisted failure when stabilized with synthesized formation water containing calcium/bicarbonate ratios equal to the calcium/ bicarbonate ratio present in the formation-water analysis. Permeabilities of the chalks saturated with oil and brines are also presented. Introduction The failure or yielding of high porosity >25 %) chalks or chalklike carbonates during the application of stress in the laboratory has been extensively reported in the literature. Meigh and Early and Carter and Mallard found that the strength of air- or oven-dried outcrop chalks was two to three times the strength of water-saturated chalks. Blanton studied both dry and oil-saturated outcrop chalks from the North Sea area and from the U.S. gulf coast. He found no significant difference in strength between dry and oil-saturated chalks. However, he found that, although the ultimate strength increased initially with increasing hydrostatic stress, the chalks eventually collapsed at higher hydrostatic stress. Marek used a simulated reservoir brine to obtain similar results on chalklike carbonates. Owen reported that brine-saturated chalks from the Ekofisk field in the Norwegian North Sea failed when subjected to only a few hundred pounds (- 1500 kPa) of pressure in a compressibility apparatus. The weakening of calcite rocks by water has also been described by Rebinder and Likhtman, Boozer et al., and Rutter. The reported chalk failure has not been restricted to laboratory tests. Voll and Holman reported that chalk flow and casing collapse had been observed in the Valhall field, Norwegian North Sea, and cited one example where 1,500 ft (457.2 m) of produced chalk was found inside a test string. This paper presents a summary of laboratory results obtained on the compaction and permeability characteristics of some chalk samples from the Danish North Sea. The studies were conducted on core material from wells in the Goren, Cora (renamed Tyra), and Dan fields. The investigation was done to confirm the reported failure of the chalks following their return to simulated reservoir stress in the laboratory. It was postulated that, even if the chalks were weak, they should be able to withstand the stresses they had been subjected to in the reservoir before removal, unless they were weakened by the recovery or testing process. Compressibility tests conducted by a service company on a suite of samples from the Gorm field showed that most of the samples failed at stresses much less than the anticipated reservoir stress. Laboratory Tests Core-Material Preparation Core plugs 2 1/8 in. (5.40 cm) in diameter and about 3 in. (7.62 cm) long were cut from water-base-mud core material from Well DNN-3, Gorm field; Well DNE-4, Cora field; and Well DNM-8X, Dan field. The samples were cut either parallel with (vertical), or perpendicular to (horizontal), the coring axis of the submitted full-diameter core. Kerosene was used as drilling fluid to cut the samples, which were extracted in toluene to remove hydrocarbons and water, vacuum-dried, and encased in a flexible Teflons sleeve. The air permeability of each sample was measured before the samples were saturated with kerosene to obtain the PV. JPT P. 976^