A Rapid, Rigsite-Deployable, Electrochemical Test for Evaluating the Membrane Potential of Shales
- Talal M. Al-Bazali (Kuwait U.) | Jianguo Zhang (BP America) | Martin E. Chenevert (U. of Texas at Austin) | Mukul M. Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2007
- Document Type
- Journal Paper
- 205 - 216
- 2007. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 1.8 Formation Damage, 1.7.1 Underbalanced Drilling, 1.6 Drilling Operations, 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State, 2.4.3 Sand/Solids Control, 5.1 Reservoir Characterisation, 4.3.1 Hydrates, 4.6 Natural Gas, 1.11 Drilling Fluids and Materials, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3.2.6 Produced Water Management, 1.2.2 Geomechanics, 5.4.10 Microbial Methods, 1.10 Drilling Equipment
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The membrane efficiency of shales is usually measured in the laboratory using pressure-transmission techniques that can be very time consuming and require shale core samples that may not be available. These tests also require special high-pressure equipment and cannot be conducted at the rigsite.
This paper presents a quick and relatively easy method for obtaining the membrane efficiency of shale cuttings (or cores) through electrochemical-potential measurements. The electrochemical test measures the voltage drop across shales that are in contact with fluids of different salinities. The measured voltage drop is used to calculate the shale's ion selectivity, which reflects the shale's ability to restrict ion flow (membrane potential). Data are presented to show the influence of ion type and ion concentration, shale permeability, and cation exchange capacity (CEC) on the ion selectivity. It was found that the shale membrane efficiency is well correlated with the membrane potential. The membrane potential is shown to be proportional to the ratio of the CEC and to the permeability of shales. A higher CEC/k ratio correlates very well with higher ion selectivity. The rigsite-measured ion selectivity can be correlated with the spontaneous-potential (SP) curves in the well.
The rapid determination of shale membrane efficiency using shale drill cuttings allows chemical interactions between shales and water-based fluids to be taken into account in wellbore-stability calculations without the need for shale core samples.
When two aqueous solutions with different ionic concentrations are separated by a porous medium that is permeable to cations and anions, an electrical potential develops. This is because of differences in ionic mobility (diffusion rate). Generally, smaller ions have higher mobilities than larger ones and multivalent ions have a lower mobility than monovalent ions (because of their larger diameters when hydrated). Differences in ionic mobility can be attributed to the hydrated-ion size for each type of ion. Sodium ions have a larger hydrated-ion size than chloride ions. Thus, chloride ions move faster than sodium ions, and as a result, a diffusion potential develops across any interface across which a concentration gradient is imposed.
The magnitude of this diffusion potential (or liquid junction potential) depends on the difference between the transport number of cations and anions and the ratio of the ionic activities on both sides of the membrane as follows:
where t + and t- are the transport numbers of the cation and anion, respectively, in bulk solutions, R is the universal gas constant, T is the absolute temperature, F is the Faraday constant, and a 1 and a 2 are the ionic activities.
When two solutions of different concentrations are separated by a shale, a modified diffusion potential develops because the shale allows cations to pass while restricting anions. Therefore, shales are considered to be ion-selective membranes that selectively allow cations to move through while restricting anions. Since the diffusion potential is a measure of the shale's ability to restrict anion movement, it can be used to deduce the shale's membrane efficiency.
Background and Past Work
Ion-selective membranes yield a modified diffusion potential, whereas nonselective membranes yield a diffusion potential. If the ion-selective membrane completely blocks the passage of the co-ions, the charge transport is entirely because of the counter ions. This type of membrane is referred as a perfect ion-selective membrane. The maximum possible potential difference, that can be generated by a perfect ion-selective membrane is given by the Nernst expression as follows:
The Nernst equation is a special case of the liquid-junction expression, Eq. 1. For this type of membrane, the co-ion transport number is zero while the counter-ion transport number is unity.
Shales contain negatively charged clay particles. The negative charges on the clay surfaces tend to facilitate the movement of positive ions, and restrict the negative ions (Lomba et al. 2000). Sherwood (1995) pointed out that ion exchange plays an important role affecting not only the rates of transport of ions, but also the mechanical and swelling properties of the shale. van der Zwaag (1995) measured electrochemical potentials across shale samples under different loads and atmospheric conditions. The main objective of his experimental work was to determine shale membrane efficiency through the evaluation of ion-transference numbers.
Jin and Sharma (1994) presented a formula that related shaley-sand conductivity to the membrane-potential. In their work, they showed that membrane potential measurements can be closely correlated with the CEC of the rock. Lomba et al. (2000) conducted experiments to evaluate membrane efficiencies of native shales by measuring the electrochemical potential across the shale samples. The experiment involved placing shale samples between two fluids of different concentrations. The electrical-potential difference was measured and then converted to a membrane efficiency. They showed that the composition of the interstitial pore fluid in shales plays a determining role on the electrochemical potential and, in some cases, the behavior of the shale is close to the expected behavior of a perfect cation-selective membrane. They developed mathematical models that estimate shale membrane efficiencies and modified diffusion potential on the basis of the shale and drilling-fluid physical parameters. It is important to note that the membrane efficiency calculated by Lomba et al. (2000) was based on comparing the actual diffusion potential of shale to the maximum expected diffusion potential.
While the previously mentioned studies measured the diffusion potential of shale, no attempts were made to relate the measured diffusion potential to the membrane efficiency. In addition, the factors that control the diffusion potential (ion selectivity) of shale were not investigated thoroughly.
In this work, we measured diffusion potential (ion selectivity) of shales following the same test matrix used for the membrane-efficiency test. This is done to correlate the ion selectivity of shale to its membrane efficiency so that our electrochemical test can be used to infer shale membrane efficiency. Moreover, we have studied the dependence of the ion selectivity on ion type and concentration and on shale permeability and shale CEC. We believe that these are the most important factors that control the ion selectivity and membrane efficiency of a shale.
|File Size||2 MB||Number of Pages||12|
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