Mechanical Properties Of Chalk Research Articles

Impact of acid wormhole on the mechanical properties of chalk, limestone, and dolomite: Experimental and modeling studies

Matrix acidizing is a widely applied stimulation process in carbonate reservoirs to remove damaged parts of the formation and enhance wellbore productivity. Few studies considered the impact of acid on the deterioration of mechanical rock properties considering different carbonate types. Assessment of the wellbore stability and the sustainability of the wormhole structure depends on the mechanical rock properties after acidizing. In this study, three rock lithologies (chalk, limestone, and dolomite) were treated with acid at different injection rates to establish acid efficiency curves. The acid was injected in a coreflooding setup until wormhole breakthrough, indicated by zero pressure drop across the sample. The rock petrophysical (i.e., porosity, permeability) and mechanical properties were measured before and after the treatment. XRD was used to estimate the mineralogical compositions of each rock type. Then, the impulse hammer technique was implemented to measure the rock surface hardness at ambient pressure. Also, acoustic measurements were used to quantify the dynamic Young’s modulus and Poisson ratio at high confining pressures. To estimate the deformation in the samples with wormholes under varying confining stress, a finite element model was constructed. Results indicate that the acid treatment significantly reduces the rock surface hardness (by 47 to 77 %), while Young’s modulus decreases by 18 to 33 % at ambient stress. The treated rock samples showed higher Young’s modulus under high confining stress. Chalk samples were the most impacted by the treatment, while dolomite specimens were the least. The finite element numerical model indicates that rock deformation is more significant in chalk samples, especially at high confining stress that acts perpendicular to the wormhole.

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

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.

The influence of CO 2 gas and carbonate water on the mechanical stability of chalk

The mechanical strength of high porosity chalk is affected by using carbonate water as EOR (Enhanced Oil Recovery) fluid. The water-weakening effect is enhanced due to increased dissolution of chalk in the presence of CO 2(aq). In this study, the mechanical properties of chalk were compared with and without CO 2(aq) present in the injected water. By use of a standard triaxial cell, series of samples with and without pre-flooding at confining pressure of 0.5–1.0 MPa were hydrostatic loaded up to yield point at room temperature or 90 °C. Then a major part of the samples were exposed to subsequent creep and flooding phases at increased stress levels of about 2 MPa above yield point. In all cases flooding experiments were performed without any backpressure and with flooding pressure varying between 0.2 and 0.8 MPa. Injection of pure CO 2 gas into water-saturated chalk resulted in only marginal weakening of the chalk material. At stress levels below the yield point, the chalk exposed to carbonate water becomes considerably weaker than chalk flooded with pure Eq. Water. At a given flooding pressure of carbonate water, an increase in temperature to 90 °C improved the chalk stability. Flooding with CO 2 saturated modified seawater, which contained an enhanced concentration of sulfate, increased the strain significantly due to the precipitation of CaSO 4·2H 2O(s). The preferential oil-wet cores at S or = 0.62 experienced an increased strain of 20% compared to the water-wet test series during flooding with CO 2-saturated Eq. water. Increasing the confining pressure by a factor of 10, strain values of about 10 times higher or even more were observed when flooding with carbonate water. The experimental observations are discussed in terms of chemical and pressure dissolution of chalk.

Deformation and yield in high porosity outcrop chalk

Mechanically high porosity chalks behave as frictional materials, but with an end-cap reflecting pore collapse failure. Shear failure between the grains seems to be the basic failure mechanism, not only in compression tests at low confining pressures where shear bands are formed, but also in pore collapse where distributed shear failure will occur in the material. Also tensile failure seems to be initiated by a shear failure mechanism, and tensile strength can be predicted from the compressive yield criterion (Mohr-Coulomb). The mechanical properties of chalk are strongly dependent on the type of fluid in the pores. Water saturated chalk is considerably weaker than dry or oil saturated chalk. Chalk and fluids may interact through capillary forces and through surface physical/chemical reactions. Capillary forces will always be present when two immiscible fluids are present in the pore space. But such effects can only partly explain the experimental observations. There are however indications that the water weakening effect might be caused by repulsive forces generated by dipole-dipole interactions in the very narrow grain contact areas.

Mechanical Properties of Chalk with Emphasis on Chalk-Fluid Interactions and Micromechanical Aspects

Mechanical Properties of Chalk with Emphasis on Chalk-Fluid Interactions and Micro- mechanical Aspects — The paper addresses the problem of water weakening effects in chalk. Using miscible fluids, capillary forces can be eliminated. Experience with water and methanol has revealed that methanol saturated chalk is considerably stronger than water saturated chalk. A study of water-methanol mixtures has shown that the transition is a gradual one, only dependent on the composition of the fluid. The changes in mechanical properties can be accounted for by repulsive forces at the grain contacts. The strong interaction between the highly polar water molecules and the chalk surface may give rise to charged or polarized layers which will develop repulsive forces when two surfaces are in close contact, like in the grain contact region.

Programme of laboratory, pilot and full-scale experiments in tunnel boring : O'Reilly, M P; Roxborough, F F; Hignett, H J Paper to Tunnelling '76 conference, London, 1–5 March 1976, P287–299, disc P311–317

The paper reviews research being carried out at TRRL aimed at improving the performance of tunnelling machines. From early investigations into the cutting and mechanical properties of chalk, further studies followed on reconstituted chalk and also full-scale trials with a 5 M diameter tunnelling machine. Laboratory tests were also carried out on bunter sandstone and magnesian limestone. In addition, preparations for monitoring the performance of A tunnelling machine in a coal mine are at an advanced stage. /TRRL/