Characterizing Curing Cement Slurries by Electrical Conductivity
- K.R. Backe (Norwegian U. of Science and Technology) | O.B. Lile (Norwegian U. of Science and Technology) | S.K. Lyomov (Norwegian U. of Science and Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2001
- Document Type
- Journal Paper
- 207 - 207
- 2001. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 1.11 Drilling Fluids and Materials, 5.1 Reservoir Characterisation, 1.6 Drilling Operations, 4.3.1 Hydrates, 1.14.3 Cement Formulation (Chemistry, Properties), 1.8 Formation Damage, 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology
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Electrical conductivity is a parameter that can be used to monitor the entire hardening process of oilwell cement slurries. The theoretical relationship among conductivity, porosity, cement chemistry, and ion content is discussed. The theory is confirmed by experiments; the decline in the slurry conductivity is primarily a function of porosity decrease and, thus, the degree of hydration. The applied results show that the electrical conductivity of curing slurries reflects strength development and that rapid hydration will reduce the risk of gas migration.
The main purposes of oilwell cements are to fasten the casing to the borehole wall and to seal off the rock formations. Knowledge about the entire hardening process of oilwell cement slurries is important for successful cementing operations.
Several methods exist to test cement slurries. The ubiquitous API tests1 include procedures for finding density, free water, fluid loss, compressive strength, thickening time, rheology, and gel strength. All these tests are important for composing a successful cement recipe, but most of them consider only one or a few points of time during the setting process, or only the time period before the start of the hardening process. Thus, no continuous description of the entire setting process is obtained.
The only procedure that has won some acceptance for tracking the entire setting process is the ultrasonic cement analyzer2 (UCA), which estimates the cement's compressive strength from the sound velocity through a cement sample. The UCA can be used throughout the entire setting process, over several days or weeks. However, it is relatively cumbersome and rather impractical for field use; therefore, there is a need for simpler test methods.
There are no reports in the literature of electrical conductivity measurements for characterizing oilwell cement, but many researchers have applied the method to concrete and other cement applications. Initially, electrical conductivity was used for finding the initial set and for tracking the rest of the curing process.3-7 Later works include water-content assessment of fresh concrete,8 influence of additives,9,10 and corrosion risk of concrete reinforcements.11,12 More recently, several publications have appeared on complex impedance,13-16 which may be related to the cement microstructure. Work on predicting hydration and conductivity by computer modeling also has been presented.17
The works of the previously mentioned researchers have shown that conductivity measurements are simple, robust, and useful in monitoring the entire hardening process. Thus, the method should be well suited as a laboratory test method for optimizing the composition of field slurries, on-site quality control, and cement waiting time.
The organization of this paper is as follows. We first introduce electrical conductivity and discuss the theoretical background required to understand the method. We then present the experimental work needed to ascertain that the method can be used on oilwell slurries. Finally, practical results are discussed. Some additional results can be found in another paper.18
The principle of measuring conductivity, in which alternating current is transmitted through the cement slurry by two metal electrodes, is shown in Fig. 1. When the voltage drop over the electrodes (U) and the current through the sample (I) are known, the conductivity (s) can be calculated as follows.
where =the distance between the voltage electrodes, and A=the cross-sectional area of the sample. If the geometry of the cement sample is more complex, Eq. 1 is not valid unless the geometric factor /A is replaced by an experimentally found constant, G. The simplest way to find this constant is by calibrating the measurement cell with a fluid that has a known conductivity.
In a cement slurry, only the pore fluid contributes to the flow of electrical current. Archie19 investigated the relationship between the conductivity and the porosity of rocks saturated with conducting water. Archie's law may also be applied as follows to cement slurries.
where F=the formation factor, sc=the conductivity of the cement, sf=the conductivity of the pore fluid, f=the porosity (expressed as a fraction), and a and m=constants. The constant a is generally considered to be unity because the formation factor should be one at 100% porosity. The constant m is usually called the cementation factor and increases with increasing tortuosity. Serra20 uses the name "tortuosity constant" for m. Archie found that the exponent m varied between 1.8 and 2 for consolidated sandstones and that it appeared to be approximately 1.3 for unconsolidated laboratory sands. For unconsolidated dispersions, it has been shown theoretically that the exponent is 1.5.21,22 Later work23,24 has confirmed the results of Archie, and, generally, a value of approximately 2 is used. On the other hand, the modeling work of Bryant and Pallatt25 produced an m equal to 3.2 at porosities of less than 10%.
The constants in Archie's law may not be valid for hydrating cement, in which the slurry initially goes from being an unconsolidated liquid suspension toward being a substance with an emerging matrix and decreasing porosity. Not much work is presented on Archie's law and cement. McCarter and Puyrigaud8 used conductivity to estimate the water content of fresh concrete after 30 to 90 minutes, and by recalculating their data, m was found to be 1.42. The 28-day mortar data of Tumidajski et al.26 gave an average m of 2.15, and their cement paste data for up to 29 years produced an exponent of 3.21. However, the data of Christensen et al.14 and Coverdale et al.17 do not conform to this behavior. Their results, together with our findings, are discussed later in the paper.
Electrical current is transported through the slurry by ions. Thus, the conductivity is controlled by the ion concentration (c), the number of charges per ion (z), and the equivalent ionic conductivity (?). The electrical conductivity (s) of an aqueous solution can be calculated as follows by summing the contributions for ion j.
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