Biot critical frequency applied to description of failure and yield of highly porous chalk with different pore fluids

Abstract

Injection of water into chalk hydrocarbon reservoirs has led to mechanical yield and failure. Laboratory experiments on chalk samples correspondingly show that the mechanical properties of porous chalk depend on pore fluid and temperature. In case of water-saturated samples, the concentration and nature of dissolved salts have an effect. Water has a significant softening effect on elastic properties of chalk as calculated from wave data, and the softening increases with increasing critical frequency as defined by Biot. The critical frequency is the highest frequency where elastic wave propagation is controlled by solid-fluid friction. The reference frequency is thus a measure of this friction, and we propose that the fluid effect on mechanical properties of chalk may be the result of liquid-solid friction. We reviewed 622 published experiments on mechanical properties of porouschalk. The data include chalk samples that were tested at temperatures from [Formula: see text] with the following pore fluids: fresh water, synthetic seawater, glycol, and oil of varying viscosity. The critical frequency is calculated for each experiment. For each specimen, we calculate the thickness to the slipping plane outside the Stern layer on the pore surface. For electrolytes, the thickness of this layer is calculated based on Debye-Hückel theory. The layer reduces the porosity available for fluid flow. We find that the Biot critical frequency based on pore scale data can be used to explain effects on the macro scale. We find that the effective yield stress and also the effective stress of failure in tension as well as in compression are log-linearly related to log reference frequency. This opens the possibility to predict yield and failure under reservoir conditions from mechanical tests made under laboratory conditions. It also opens the possibility of predicting the effects of water flooding on chalk stability.

Comparative study between Botucatu and Berea sandstone properties

“…This means that the calcium presents a "shielding power" of particle charges greater than sodium. Similar increase of permeability with the increase of ionic strength was also observed by (Andreassen and Fabricius, 2010) for highly porous chalk. The observed results are in agreement with the relationship between the thickness of the double layer (Debye length) on colloidal particle surfaces and the dissolved salts in the bulk solution.…”

Quick, Easy, and Economic Mineralogical Studies of Flooded Chalk for EOR Experiments Using Raman Spectroscopy

“…Injection of seawater-like brines is one of the most successful Improved Oil Recovery (IOR) methods on the Norwegian Continental Shelf (amongst many others: [1,2]). The mechanical strength of chalk is weakened by seawater at reservoir temperatures, and as a consequence, compaction and loss in porosity occur, affecting the oil recovery factor of carbonate fields [3][4][5][6][7][8][9][10]. It is important to understand how fluids interact with rocks, because textural and chemical/mineralogical changes in the pore space affect the way water will adsorb and expel oil from the rock [3,8,[11][12][13][14][15][16][17].…”

Static and dynamic effective stress coefficient of chalk

“…Andreassen and Fabricius (2010) show that failure in a rock occurs at lower stresses if it is saturated with fluids having lower kinematic viscosity. As air has higher kinematic viscosity (approximately 15 × 10 −6 m 2 ∕s) than brine (approximately 1.2 × 10 −6 m 2 ∕s), dry (air saturated) rock will behave stronger than brine-saturated rocks.…”

“…The fluid effect on the strength of rocks can be characterized by Biot's (1956) critical frequency, f c ¼ ðφηÞ∕ð2πρ fl kÞ, which is calculated from porosity (φ), liquid permeability (k), fluid density (ρ fl ) and viscosity (η). The higher the critical frequency, the stronger is the rock (Andreassen and Fabricius, 2010). The effect is more prominent in low-permeability rocks, such as chalk, as fluid flow is strongly affected by the specific Table 2.…”